^/. ni7 SAND78-0080 \X Unlimited Release ' UC-21 Г Electron Bea Progress Report April ThroughMS B ber1977

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ELECTRON BEAM FUSION' PROGRESS RETORT APRIL THROUGH SEPTEMBER 1977

- NOTICE - This repurt wai prepared as an ipnnsnied by the United States Gottri i Neither the United Statci nar the Untied Slat Energy, nor any of Ihelt employees. n,u »ni (ч their ciniliacioM, suhcuniiaclon, or their employe", makei any warranty, eipreu «r implied, чг iHiinid in> le?a! lutnluy i'r Jcipuniiilily l<4 ihcic^rjo.rratfiltleneii •ч useftilnets »f an) infmnution. jppjrjILi. pimJut! in pttKeu Jisdntcd. IH repreienii Ihat iti uie would n>r infringe pnnttly i>*ned г Rim

Directorate of Physical Research 5200 Sandia Laboratories Albuquerque, New Mexico 87115

JOVLT.iiXT IS ЦШЛЖТЕ;TVHВ 0,,14- 3-4 ' CONTENTS

Page

INTRODUCTION П

TECHNICAL OVERVIEW (G. Yonas) 11

General Discussion 11 Pulsed Power 12 Particle Beam Source Development 12 Target Interaction 13 Applications.. 13

MANAGEMENT OVERVIEW (G. W. Barr) 14

EBrA Fiujtiut Status 14 Personnel and Operating Budget 20

PULSED POWER 21

OVERVIEW (T. H. Martin) 21

EBFA-I (T. H. Martin, D. L. Johnson) 21

PROTO II 21

Summary (T. H. Martin) 21 Accelerator Description (T. H. Martin, D. L. Johnson, D. H. McDaniel) 23 Twenty Nanosecond Mode Operation (T. H. Martin, D. L. Johnson, D. H. McDaniel). . . 25 Varying Pulse Width Mode {T. H. Martin, D. L. Johnson, D. H. McDaniel) 26 SF-6 Trigatron Switching (T. H. Martin) 26 Prepulse Prevention (J. P. VanDevender, T. H. Martin) 27

MITE 27

Summary (T. H. Martin) 27 Solid Dielectric Capacitor Testing (D. L. Johnson) 28 Blumlein (D. L. Johnson, J. P. VanDevender) 28 Strip Lines (D. L. Johnson, J. P. VanDevender) 29 Diode CE. L. Neau) 29

POWER FLOW - SELF MAGNETICALLY INSULATED TRANSMISSION LINES 29

Introduction (D. H. McDaniel, J. P. VanDevender). 29 Computational Results (K. D. Bergeron, J. W. Poukey, D. H. McDaniel, J. P. VanDevender) 30 Sandia Triplate Experiment (D. H. McDaniel, J. P. VanDevender, D. L. Johnson) ... 30

MAGNETIC FLASHOVER INHIBITION (MFI) WITH SELF-FIELDS (J. P. VanDevender) 34

MAGNETIC FLASKOVER INHIBITION USING AN EXTERNALLY APPLIED MAGNETIC FIELD (E. L. Neau, D. H. McDaniel) 35

OPTICAL CALIBRATOR (E. L. Neau) 37

S CONTENTS (cont)

Page

VOLTAGE MONITORS (J. P. VanDevender) 38

SUPERFAST Z-PINCH (J. P. VanDevender) 38

EBFA DATA ACQUISITION FACILITY (W. B. Beyer) 40

HYDRA, PROTO I, PROTO И - ОЛТЛ ,'• :Q!J1SITI0N FACILm (К. В. Boyer) 41

IMAGE PROCESSING 0*. B. Boyer) 41

REFERENCES 43

THEORETICAL RESEARCH 45

OVERVIEW (J. R. Freeman) 45

FUSION TARGET PHYSICS 46

Single-Shell Fusion Pellets for EBFA with Electron or Ion .Ablative Drivers СМ. А. Гл vnay) 46 Multiple-Shell Fusion Pellets for HKFA (M- A. Sweeney) 48 Voltage Shaping with Electron Beam Fusion Targets (M. Л. Sweeney, H. J. Clauser). . 49 Proto II Magnetic Targets (A. V. Farnsworth, Jr.) 52 Magnetic Target Preheat Calculations (L. Baker) 52 Hydrodynamics of Power-Deposition Driven Expansions (A. V. Famsworth, Jr., M. M. Widner, M. J. Clauser) 54

REACTOR AND STANDOFF RESEARCH • 56

Plasma Channels for RcB Propagation (L. Baker, J. R. Freeman, M. M. Widner) .... 56 RF.B Combination (T. P. Wright). 57 Electron Deposition in Thin Tr>r;.;ets (M. M. Widner, E. J. T. Burns, S. A. Goldstein, J. N. Olsen, J. W. Poukev, J. P. Quintenz, J. Л. Halbleib, Sr.) . .' 59 Preliminary Studies of iil;ist loading at the First Wail of an Electron Beam Reactor Chamber СМ. Л. Sweeney, M. M. Widner) 63

TOWER FLOW THEORY AND DIODE PHYSICS 67

Magnetic Insulation of Vacuiri Transmission Lines - The Short Line Problem l"K. D. Bergeron, J. W. Pouke>', D. H. McDaniel, j. 1', VanDevendfг) 07 Magnetic Insulation of Vacuum Tr.ji'.smission Lines - The Short Pulse Problem (} . 0. Bergeron, J. W. Poukey) 70 Double Sheaths in Plasmas fK. 0. dergeran, T. P. Wright) 72 Electron Emission from Cat>,i.de PI. ;nas in Relativistic Diodes (K. D. Bergeron). . . 74 Theory of Radial Powc:' Flow (J- »'• Poukev. K. D. Bergeron) 75

DIODE CODE DEVELOPMENT AND APPLICATION' 76

Variable Zoning Diode Code (J V. i.Minvn. 76 Foil Anode Effects (J. Vf. Pouipy, J. P. Qiiintcn;) 77 Proto II Ojode Studies (J. V Ош'Пеп,-, :. W. :\;iJe; 80

6 CONTENTS (cont)

Page

Los Alamos PHERMEX Diode Simulation (J. P. Quintenz) 83 Large Diode Simulation (J. P. Quintenz, J. W. Poukey) 83 Hermes II Ion Diode Studies (J. W. Poukey, J. P. Quintenz) 83 Angara Cathode (J. P. Quintenz) 84 Parapotential Cathode Simulation fJ. P. Quintenz) 84

PARTICLE BEAM SOURCE DEVELOPMENT AND THEORY 86

Collective Ion Acceleration fC. L. Olson) 86 Calculations of Ion Beam Neutralization (J. W. Poukey) 87

REFERENCES 89

PARTICLE BEAM SOURCE DEVELOPMENT 93

OVERVIEW (G. W. Kuswa) 93

IMPEDANCE CHARACTERISTICS OF HEATED REB DIODES (D. J. Jolinson) 93

ELECTRON BEAM PINCHIN'G FROM DISCRETE LARGE DIAMETER CATHODES (S. A. Goldstein) .... 96

Diagnostics and Apparatus 97 Experimental Results 99

Conclusions 100

REB PROPAGATION AND COMBINATION IN PLASMA CHANNELS (P. A. Miller) 104

Pinch Formation 104 REB Injection Into Channels 104 REB Transport in Plasma Channels 105 REB Overlap 107 Summary Ill A HIGH RESOLUTION TIME RESOLVABLE FLASH X-RADIOGRAPHY SYSTEM (J. T. Chang) Ill

HIGH POWER MAGNETICALLY INSULATED RADIAL DIODE (G. W. Kuswa) 112

Principles of Operation 112 Diagnostics and Results 113

Direction of Future Research 117

HIGH INTENSITY PULSED LINEAR ION ACCELERATORS (S. Humphries, Jr.) 118

SLOT REFLECTOMETRY TOR DIAGNOSING DIODE PLASMAS (C. W. Mendel) 121

TARGET INTERACTION 125

OVERVIEW (A. J. Toepfer) 125

BEAM-TARGET INTERACTION MEASUREMENTS ON PROTO I (L. P. Mix) 125

Irradiation Symmetry Measurements 126 Energy Deposition Measurements 126

7 CONTENTS (cont) Page Theoretical Calculations 128 Summary 130 ELECTRON DEPOSITION IN THIN GOLD AND ALUMINUM FOILS HEATED BY A RELATIVISTIC BEAM (E. J. T. Burns) 130 Introduction 131 Experimental Procedure 131 Results 13S Conclusion 139 MEASUREMENT OF THE TEMPERATURE OF A THIN FOIL USING OPTICAL AND VUV DETECTORS (E. J. T. Burns, J. N. Olsen, S. A. Goldstein) 146 IMPLOSION PHYSICS (F. С Perry) 149 SCINTILLATOR-PHOTOMULTIPLIER RESPONSE TO A SIMULATED REB GENERATED THERMONUCLEAR NEUTRON PULSE (R. J. Leeper, J. T. Chang) 152 Thermonuclear Neutron "Signature" 154 STABILITY OF MULTIPLE SHELL ICF CAPSULES (A. J. Toepfer, W. J. Tiffany) 156 NEUTRON PRODUCTION FROM ADVANCED REB FUSION TARGETS (J. T. Chang, M. M. Widner, A. V. Farnsworth, Jr., R. J. Leeper, T. S. Prevender, L. Baker, J. N. Olsen) 158 TARGET PREHEAT EXPERIMENT (J. N. Olsen) 163 REFERENCES 166

APPLICATIONS 169 REP-RATE PULSE POWER. TECHNOLOGY STUDIES 169 Overview (K. R. Prestwich) 169 Development of a 350-kV, 300-J, 100-pps Test Facility (M. T. Buttram, G. J. Rohwein) 169 Low Voltage Rail Gap Switch (G. J. Rohwein) 169 High Voltage Switch (M. T. Buttram) 171 Multichannel Surface Discharge Switch Experiment (G. J. Rohwein).... 174 Spark Gap Gas Flow Experiments (G. J. Rohwein) 175 High Voltage Transformer Development CG. J. Rohwein) 178 Marx Generator Design Studies (M. T. Buttram, K. R. Prestwich, A. W. Sharps). ... 178

LOW CURRENT DENSITY DIODE STUDIES 179 Overview (1С. R. Prestwich) 179 Cathode Plasma Instability Studies (M. T. Buttram, C. W. Mendel) 179 Emission Enhancement Experiments (R. S. Clark, J. J. Ramirez) 186 Emission Suppression Experiments (R. S. Clark) 188 LEGS, A Test Facility for Large Area, Microsecond Diode Studies (M. T. Buttram, K. R. Prestwich, A. W. Sharpe) 191

• CONTENTS (cont) Page RADIAL ELECTRON BEAM LASER EXCITATION (J. J. Ramirez) 193 Total and Spatially Resolved Energy Deposition Measurements 193 Radial Diode Impedance Characteristics. , 195 Hibachi - Anode Investigations 199 AN INDUCTIVE GENERATOR TOR FUSION POWER GENERATION (M. Cowan) 200 Introduction 200 Plasma Armature Code Development 20] The 1-m Magnet 201 SYSTFMS STUntES (j. L. Mitchiner, S. С Varnado) 201 Overview 201 Parametric Analysis of a REB Fusion Reactor 203 Parametric Analysis of a REB Hybrid Reactor ... 206 Conclusions 209 REFERENCES 210 PUBLICATIONS 213 PRESENTATIONS 215

9-10 INTRODUCTION

TECHNICAL OVERVIEW «-«в wEarr-:- ющв ю-» General Discussion

The key milestones achieved during this re­ porting period are as follows:

1. Detailed measurements and further analysis of magnetic thermoinsulation targets which support the conclusion that the neutron production observed is of thermonuclear nature and not due to ion acceleration.

2. Continued measurements and analysis of beam г ipagation in preformed channels to de- fji. quirsnents for Proto II, hbFA, and eventually reactor experiments.

3. Operation of Proto II at the 8-TW Fig. 1. Particle Beam Fusion Program power level thus demonstrating most of the options. pulsed power components for EBFA.

4. Initial tests of the EBFA module ОПТЕ) Success in electron beam focusing with with preliminary data on the critical aspects •> 1-U diodes on Proto I together with beam of ;nagnetically insulated vacuum transmission propagation in plasma channels and demonstra­ lines. tion of enhanced electron deposition have led to the adoption of this combination as 5. Completion of theoretical and experi­ our base line approach. The need for complex mental electron beam focusing and target targets involving a central ignitor, such as studies on Proto I demonstrating 5 to 10 a magnetically insulated region, became more MA/on2 and power densities of ~ 5 TO/g and clear as our analysis of simple targets came ~10 to 15 TW/g in thin Au and Al foils, to an end. respectively, in agreement with theory which predicts enhanced deposition due to multiple With the definition of an experimental and electron interactions. theoretical program at NRL to pursue the self- magnetic field approach to electron suppression 6. Demonstration of the high rep-rate in ion diodes, we were able to narrow our test facility with over 10' shots yielding attention at Sandia to use of external fields. data on switch reliability and lifetime. This diode approach for light ions has also been adopted for a multistage ion accelerator 7. Substantial progress on the construc­ which would be necessary for medium or heavy tion of the Electron Beam Fusion Facility ions. This similarity in the light ion and indicating that the completion schedule can heavy ion programs further simplifies our pro­ be met and also completion of EBFA prelimi­ gram. A revised option matrix is shown in nary design involving 36 separate modules. Fig. 2.

As a result of these steps we have been able Developments in these areas are summarized to narrow our option matrix leading toward a below. candidate driver system for an experimental power reactor. The diagram from the previous progress report showing these options is reproduced here to indicate this narrowing process (Fig. 1).

II .топ» i i i .»™m ! Theory

Target theory during this period emphasized the preheat and scaling aspects of magneti­ cally thermoinsulated targets and the effects of beam stagnation on enhanced energy deposi­ tion in thin foils. Predictions for magnetic targets continue to be encouraging with the goal of 107 to 109 DT neutrons from Proto II Fig. 2. Option matrix as of October 1978. remaining as a conservative estimate. The importance of both electrostatic аи." magnetic effects in enhanced deposition in thin foils Pulsed Power was predicted and compared with experiments on Hydra and Proto I. The detrimental effect Proto II became fully operational during this of scattering on enhanced deposition in high-Z period following completion of switch testing foils was shown to be due to radial diffusion and modification. The accelerator has operat­ of the beam at v/y values - 1. This analysis ed over a wide range of pulse durations from also indicated that higher v/y beams should 20 ns to 80 ns producing up to 8 TW at the achieve substantial enhancement with factors input to the diode. This wide range of pulse proportional to I/Гд. Tn particular, we durations is easily achieved simply by vary­ expect that a \>/y = 3 beams with a radius of ing the water switch arrangement. The switch 1.5 mm should achieve a deposition enhancement behavior is now as initially expected and factor of ~ 6 in a 6-y thick plannar Au foil. beam focusing work is beginning. The five­ On the other hand, the enhancement factor is fold power increase sought in EBFA will predicted to not be readily detectable for require either substantial improvements in currents of SO to 75 kA which have been obtain­ vacuum breakdown if the Proto II disc vacuum ed typically in the Hydra pinch. One way to feed approach is to be used, or the use of improve the enhancement factor is through separate modules and magnetically insulated reduced scattering with low-Z foils and theory vacuum lines must be followed. At this time, indicates that with V/Y of only 2 and the use the latter approach has been adopted with of a 6-um Al foil that an enhancement factor the final design definition planned for of ~ 2 to 3 should be expected. Experiments January 1978. The MITE apparatus became on Proto I have supported these predictions. operational during this period and as a result of theoretical target studies wbich Further work on the use of channels for beam have indicated the need for higher energies propagation emphasized the development of a in longer pulses, we decided to adopt the use 1-D MHD code to define channel conditions and of the 40 ns Proto II-type strip lines instead to provide a design tool for scale up experi­ of the shorter pulse Blumleins originally ments. In addition, beam overlap analysis considered for EBFA. and reactor chamber overpressure calculations continued to indicate the viability of this This use of longer pulse should permit highe- •nproach to solve the stand off problem. This efficiency operation with a projected EBFA i.riTiple channel approach may also be useful output of ~ 1 MJ at a peak power of 30 TW. i'o' the guiding of light ion beams and this Initial vacuum insulation experiments on МГК .(•ssibility is under consideration also. The indicated energy losses for azimuthally non­ diode code was improved to handle nonuniform uniform lines as opposed to a coaxial line and and nonplanar diodes and this code is in use the need for a "closed traplate" design was in design of diodes for Proto II and EBFA. defined. Two-dimensional particle in cell simulations for long magnetically insulated lines were encouraging in that the only losses Particle Beam Source Development appear to exist at the pulse front with effi­ cient and stable energy propagation during Beam focusing studies emphasized aspects of the major part of the pulse. At the end of ion suppression and methods for improvement this reporting period the hardware to fully of beam reproducibility and focusing effi­ test the long modified triplate approach on ciency using heated anodes and segmented MITE was on order and tests are planned for cathodes. Miltiple beam research emphasized the end of the year. detailed diagnosis of preformed channels,

12 initial beam combination experiments on Hydra, surface of an imploding target was obtained. and design of a 12-beam Proto II experiment. Neutron signature experiments using a new Ion experiments using external magnetic field pulsed neutron source and analysis showed suppression of electron flow were carried out that the heavily shielded scintillator on Hermes indicating large electron leakage photomultiplier in use could readily differ­ and also showing the need for a symmetric entiate between the long pulse beam-target power feed which is available with the two- neutrons commonly produced in high current sided machines such as Proto I and Proto II. diodes and the short pulse neutron pulses Ion diode hardware for these accelerators expected from valid implosion produced thermo­ was designed and fabrication of a Proto I nuclear conditions. experiment began. The use of external fields in a multistage accelerator for medium and heavy ions was studied analytically and prom­ Applications ising theoretical results have led to the initiation of an experiment to test these Operation of the 30 kW average power rep-rate concepts. Further diagnostic development test experhiient began during this period with continued with substantial improvement in a data collected on system reproducibility and pulsed X-ray source for use in flash X-radio- lifetime. Individual experiments in excess graphy. of 5 x 1.0s shots were limited by the lifetime of the high current switch electrodes and lack of adequate flow rate in the output Target Interaction switch to provide for arc quenching. Redesign of these components is underway to improve Experiments using preheated fuel and magnetic performance and lower flow requirements. UV thermoinsulation with exploding pusher spheri­ illumination of the output switch was found cal targets continued. In order to further useful in achieving stable operation at low investigate the nature of this neutron output rep-rates but was found to be unnecessary at a comprehensive series of null experiments, rep-rates of 20 to 30 pps. Additional studies as well as detailed neutron time-of-flight on hydrodynamics of flow in various switch experiments, tended to confirm our original geometries are underway to improve arc quenching interpretation of these neutrons as being a re­ and electrode cooling with the lowest possible sult of implosion and not due to ion accelera­ flow rates. tion. Experiments with a glass shell filled with ОТ gas as well as separate experiments to An economic model has been employed to define diagnose preheat conditions were tvgun during optimum design parameters for the high effi­ this period. Enhanced deposition experiments ciency and low cost particle beam driver on Hydra and Proto I were completed during approaches. The basic difference from similar this period showing that due to the limited laser analyses is that a relatively low gain focusing used in deposition experiments on pellet (< 100) has applicability here. Our Hydra (J <_ 106 A/cm-) and the marginal values analysis has included both pure fusion and of v/y available on Proto I, substantially hybrid cases and indicates that pellet fabri­ enhanced deposition in thin planar high-2 cation cost plays a strong economic role as foils could not be obtained. On the other long as the (driver efficiency) x (focusing hand, by employing Al instead of Au this efficiency) is greater than - 20 percent for enhanced deposition effect was observed as a pure fusion reactor and ~ 10 percent for a predicted. The two-beam irradiation of spheri­ hybrid reactor. If the pellet fabrication cal high-Z thin pellets on Proto I was carried cost is <_ $0.10, then the pellet gain can be out showing good symmetry of irradiation could <_ SO for pure fusion and as low as 3 for hybrid be achieved with sufficient care but this reactors (blanket gain of 25). If the pellet experiment was discontinued due to the low cost can be $0.01, then the gain 3 pellet is deposition obtained. These results indicated compatible with a gain 10 hybrid blanket. This the validity of the two-beam irradiation study has clearly identified the need for real­ approach, but showed the need for higher istic estimates of the mass production costs currents such as that available on Proto II. of pellets. If these costs can be as low as Optical diagnostic techniques were demonstrated a few cents per pellet, then economically for use with open sided imploding targets and attractive hybrid scenarios can be envisioned these diagnostics will be employed in future for low cost and efficient particle beam studies to investigate the growth of Rayleigh- drivers which could be readily built after Taylor instabilities. The first evidence of the net energy gain goal has been moderately pusher breakup (fluff formation) of the inner exceeded. This type of economic model can

13 also allow one to create a scenario for the the basement has been completely enclosed. evaluation of technology and its impact on The hexagonal pit and tunnels to the mechani­ power cost. An arbitrarily chosen evolution cal pimp room and beneath the cortrol room of technology is hypothesized showing how are completed. The 2 ten-ton capacity improvements of various kinds can lead to re­ platform lifts have been installed. It duced power costs. The numbers shown (on the can be seen that several sections of the high right of Fig. 3) give the similar laser pro­ bay laboratory walls have been erected and jections for a pure fusion reactor based on that the high bay concrete door frames зге in high gain pellets. place. The entire laboratory building is to be constructed of precast concrete, double As technology improves with mass producton tee sections, both walls and roofs. The methods and the unit costs jre decreased, laboratory is expected to be completely en- the cost of power would decrease with time clc.ved by Tbanksgivin.i; 1977 so that the which is in contrast to the situation that uiside finish work can be accomplished during exists today where the cost of ever scarcer Che winter. At this time, everything still fuel drives the cost of energy up with appears on schedule for acquiring beneficial time. occupancy of the High Pay Laboratory to start the installation of the EBFA tank on the scheduled Milestone Mo. 3, i.e., August 11, MANAGEMENT OVERVIEW 1978.

EBFA Project Status A "Baseline" design configuration of EBFA-I was established during July 1977 as a result This is the first full reix>rting p'.riou for of the progress made in the research areas of the EBF Project since the start of const ie. power flow and beans generation/transport durir tion at the EBFF site on January 21, V~" the reporting period. The elements of the and the initiation of the engim-erinj . :"ign EBFA "Baseline" design which are based on of the EBFA-I on January 4. 1977. techric.nl considerations were stablished pri­ marily as a project managemen. tool lo faci­ The construction activity at the EBI-'F site litate planning, tracking, and reporting. has progressed at a rlightly slower thai: This allows the J'BF Project to make realistic anticipated rate betwoop March and October estimates of the schedule, cost, and manpower 1977. An aerial pho'v:;. aph showing the necessary to support the project "End of status of construct i- ^ i of October 2;!, Construction" date. The "Baseline" approach 1977 is shown in Fig. ". However from tnis does net preclude necessary changes to the photo, one can see that rhe entire floor of overall design of F.BFA, however, if a change the laboratory building including The trenches occurs after the scheduled design freeze of covered with gratings m the- high bay portion a particular element, a delay in the end of has been completed. Ac this po'ttt -he con­ construction and an increase in total cost tractor, R. E. McKee, Ire. i's ahead of the may be the result. contracts construct'i/ ^!i'..л ! : - 'Jo. 1 of having the laboratory building fir. :• completed The EBFA will consist essentially of 36 in­ by November 3, 1977. The process oil tank dependent particle beam accelerators arranged storage farm can be seen nearing couplet Ion in a radial configuration within a single on the southeast corner of the laboratory integrated tank structure. A preliminary building. Each trnk has a capacity с Г design sketch of the EBFA-I "July 77 Baseline" 250,000 gallons of t.T".:'sformer oi„ for ".he is shown in Fig. 6. For discussion purposes, electrical insulation o' ti.e e:i»i•••;>- storage a single accelerator line can be divided into section of EBFA. The ungraded -.Ьг.МТ «.ill four separate sections: (1) the energy storag require 430,000 gallons oi transformer ""1. section, (2) the pulse forming section, (3) The EBFA will also use approximately 335,00(! the power flow section, and (4) the front-end gallons of deionized writer with a "-r-sis-i- experimental section. The energy storage vity < 1 Mi/cm. The deicnized water system section consists of a 112 kj Marx generator will be designed by the А/Г., i>. С Kroger module, solid dielectric intermediate storage and Associates with the main storage tar..':; capacitors, and a triggered gas switch sub­ located due south of the two oil storage merged in transformer oil for electrical tanks. In. the photo, the southeast со Tier •nsulation. The energy is transferred on of the laboratory which comprises the corananu through a high voltage barrier into mechanical equipment room and d.'.ode i repara­ the wat-.r insulated pulse forming section, tion room on. fh."; first level ant. main pumps •'us e.'is.ists of back-n-back Blumleins used for the oil and '.-r-.-r tгаяsfor ;;;. stems '>•; о sbape the pube to 20-ns FKHM and a set

M RES Dillon

Pdletgain «50 -»Ю0 900 | Pellet cost •$.30 ->$K> $.10 I System efficiency1 •10% ->30» 2% | Pulse rat» »5Hz -*ЮНг 15Hz 3 ж .Л0

MO CPF"«X K>Z2M* Ome

ng. Rt'B Fusion Experimental Power Reactor Performance.

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PeHetyin «3 -> 20 Pellet co*t *$r -»$.K> Swtem efficiency * 10% -»зох ftfcerate =2Hz ->K)Hz

Fig. 4. REB Hybrid Experimental Power Reactor Performance.

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r: #4^%^ ^-з^ьЖ* 'W*-W3

Fig. S. FJectron Веши fusion Fnciljty Site - October 1977. approximately 18 inches from the center of the pellet chamber. The tightly pinched electron beam then penetrates a thin anode foil and follows a low-density preionized channel in the background gas to the pellet. The low- density channel is established by exploding a wire between the anode foil and the pellet just prior to the arrival of the electromagne­ tic wave at the cathode. The central portion of EBFA is being designed using a modular philosophy which will allow for the easy insertion of alternate front- end concepts. Several different pellet irradiation concepts are presently under in­ vestigation. The design freeze for the front- end concept is presently scheduled for July 1978.

The overall design of EBFA-I is proceeding on or slightly ahead of schedule. A major mile­ Fig. 6. EBFA-I preliminary design. stone in the engineering design was reached during June with the final design of the EBFA tank structure approved by the PBF Program of transformer lines designed to natch the personnel. The detailed fabrication drawings impedance of the output load. The Magneti­ were being prepared with a target date of cally Insulated-Vacuum Transmission Line September 1, 1977 to start the procurement (MI-VTL) concept of power flow is used to procedure. However in August, the results of transmit the energy the last 18.S ft. into MITE engineering evaluation tests of the solid the central or front-end section of the dielectric capacitors to be used as the inter­ accelerator where the electron beams are mediate stores indicated a limited life expec­ generated. The MI-VTL concept is presently tancy at the EBFA-I operating voltage. It w.is being investigated using the pulsed power decided at this time to use water capacitors test bed in Area V, called MITE, and by a for the intermediate stores based on the high research contract with Physics International. reliability and excellent performance charac­ Preliminary experimental results have been teristics experienced with the Proto II water encouraging. Also, theoretically, the MI-VTL capacitors. It was decided to locate the power flow concept is scalable to much higher intermediate stores on the water insulated power levels than EBFA-I. This makes it a side of the H. V. barrier in the pulse forming principle candidate for EBFA-II, the 2 MJ section instead of in the oil insulated energy accelerator proposed to be built in the storage section of the accelerator. With this 1982-1983 time frame, which will be used to decision, the intermediate store could be con­ achieve net energy gain with a standoff capa­ figured as an open, flat plate capacitor using bility. This approach circumvents the insula­ the deionized water in the pulse forming sec­ tor flashover problem, which would require a tion for electrical insulation and energy major improvement in breakdown strength of storage, thereby simplifying the capacitor the insulator surface to allow extrapolation geometry. This configuration also eliminates of the Proto II approach to higheT power the possibility of gas trapping and electrical levels. The design freeze for the EBFA power breakdown. flow concept is scheduled for December 1977. The intermediate stores decision resulted in The most promising front-end concept for the a slight reconfiguration of the EBFA tank to generation, focusing and transport of the enlarge the pulse forming section and reduce electron beam for pellet irradiation is the the energy storage section accordingly. This Mdtiple Electron Beam (MEB) configuration. caused a one-month slip in starting the pro­ Ihe №B concept involves using the central curement action on the EBFA-I tank structure. conductor of the Ш-VTL pulse charged nega­ A "Request for Quote" was sent to six prospec­ tive with respect to ground. Thirty-six tive bidders on October 25, 1977 with a pre- individual electron beams are generated from bidders conference scheduled for November 7, two inch diameter cylindrical cathodes located 1977 at Sandia. The bids will be opened in

17 early January 1978 with a contract to be award­ section now consists of a flat plate inter­ ed in February 1978. Installation of the EBFA mediate storage capacitor, a triggered gas tank structure in the High Bay Laboratory switch, a flat plate transmission line-type Building at the EBFF site is scheduled to pulse forming section and a transformer start on August 11, 1978. section to the vacuum interface. The power flow section consists of the MI-VTL immersed Research results in the areas of target phys­ in water (for radiation shielding purposes ics and beam interaction led to a reevalua- only) with the pellet irradiation candidate tion of the basic parameters for the EBFA-I based on the MEB concept. The EBFA tank during this period. It was decided to change structure parameters are presented in Table the pulse length of the accelerator from 20 ns I. to 40 ns in order to improve the overall system efficiency and energy output. Based on preliminary experimental results obtained in TABLE I the MITE accelerator on this longer pulse length, the beam energy should increase from EBFA Tank Structure Parameters 500 kJ to 1.0 MJ with the total power decreas­ ed from 40 TW to approximately 30 TW. Also, 1. Energy Storage Section - Oil Insulated during these initial longer pulse experiments 100' O.D. x 64' I.D. x 14' high-mild steel on MITE, it was determined that improved Oil Capacity: I = 300,000 gal.; system efficiency could be achieved with the II = 430,000 gal. proven flat-plate Proto II-type pulse forming lines. 2. Pulse Forming Section - Water Insulated 64' O.D. x 40' I.D. x 15.5' high-stainless The redesign and cost analysis of the Proto II steel Marx generator connecting hardware for use in Water Capacity: I = 135,000 gal.; EBFA was completed in June 1977. A total re­ II = 225,000 gal. design was undertaken to reduce the per unit cost of the hardware to be used in EBFA by 3. Power Flow Section - Vacuum/Water pursuing alternate mass production techniques. 40' O.D. x 12' I.D. x 15.5' high-stainless The present plan is to use 36 of these rede­ steel/aluminum signed Marx generators in EBFA initially and Water Capacity: I = 65,000 gal.; to double the number of generators in the . II = 110,000 gal. upgraded configuration, EBFA-II. Procurement of a sufficient amount of this redesigned 4. Front-end Pellet Chamber Modules - connecting and hanging hardware to build a Vacuum/Pressure single, full-scale EBFA Marx generator mock- 8' diameter - aluminum up was initiated in June. The engineering evaluation of this mock-up will include the mechanical fit of the assembled unit, the The EBFF/EBFA Installation Schedule has been quality of manufactured hardware, the handling established and is presented in Fig. 7. This characteristics of the entire modular unit, and schedule consists of many parallel activities finally, the electrical characteristics of the that must be accomplished to meet the June 30, redesigned Marx generator. All aspects of the 1980 operational date. ' It is anticipated that procedure will be checked out to verify that a two-shift assembly operation will be started the specialized handling equipment, presently on EBFA as soon as the tank structure is being designed, will satisfy the assembly/ complete. This schedule has helped to estab­ transportation/maintenance plan which has been lish the manpower needed to perform the entire developed for the 36 EBFA Marx generators. EBFF/EBFA assembly.

In sunmary, during the reporting period the EBFA-I design experienced several changes dictated by the results of the various research efforts and the engineering evalua­ tion tests on the single EBFA module. The present EBFA-1 configuration consists of 36 identical modules as before with the energy storage section consisting of 36, 112 kJ terx generators housed in a single annular tank. The deionized water filled pulse forming

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l-'ig. 7. EBFF F.BFA Installation Schedule. Personnel and Operating Budget The operating budget for FY'78 is as follows:

Man Years Total K$ Source Development 38 Target Research 24 Applications 10

TOTAL 72 8700

20 PULSED POWER

OVERVIEW stacked or unstacked have been demonstrated to operate. This provides a wide range of Options for EBFA-I were narrowed as data became voltages and pulse lengths. The Marx genera­ available from Proto II, MITE and power flow tors and intermediate storage capacitors are

research. The solid dielectric capacitors pro­ operating well in Proto II and SF0 switching posed for intermediate stores have been elimi­ difficulties have been resolved. A variety nated for EBFA because of low reliability. of pulse durations has been demonstrated on Water capacitors based on data from Proto II Proto II which could accommodate a wide variety and Hydra will be used on EBFA. These capaci­ of EBFA fusion experiments. The disc-shaped tors will be located inside the main water accelerator is operating well and has several tank which will allow more flexibility in operational advantages over concentric co­ pulse outputs and switching. The discovery axial line type of accelerators. Magnetic of a new prepulse prevention scheme has insulation is being investigated and will be allowed stacking of strip lines to provide used to alleviate the vacuum interface flash- the same energy store as Blumleins at the same over problem. Combining the above, the pre­ diameter and to provide better efficiency. sent EBFA design is shown in Fig. 1. The The EBFA pulse duration and energy were in­ 30.5-m-diameter tank will contain 36 modules. creased to 35 ns and 1 MJ to provide a more Each module will contain a Proto II-type Marx, optimum output from 40 to 30 TW. This allows an intermediate storage capacitor, a strip elimination of one pulse forming stage. line pulse forming network, transmission lines, Improved SF-6 switching data from Proto II a single diode and a 4-m-long magnetically shows that present parameters are adequate insulated transmission line leading to a common for EBFA. A switch jitter of 3 ns with reli­ e::perimental chamber. ability exceeding 0.986 was demonstrated. Changes since the last report have occurred Proto II has achieved the design output and is primarily in the intermediate storage capaci­ now a user oriented accelerator. Several de­ tors which have been changed to water energy sirable features of the disc accelerator were stores located inside the main water tank. proven. This was done to provide better capacitor reliability and to provide a faster charge Initial magnetic insulation experiments on a time for the pulse forming lines. The pulse 1.5 m-long-triplate show small energy and forming lines will be strip lines with an power losses. Theoretical understanding of output pulse length of 35 ns at the half power magnetic insulation was greatly enhanced and points. agreement between projections and experiment were obtained. It is expected that the longer As found from the Proto II data included in pulse chosen for EBFA will simplify the long this report a rather modest decrease in magnetic line application. The 7-m line will peak power can result in a large increase be tested during the next report period. in output energy because another pulse com­ pression stage can be eliminated. Subsequently Magnetic flashover inhibition results from the output parameters of EBFA were modified to Ripple and Omen promise very low inductance increase the energy output from 0.5 MI to 1 MJ close in diodes for Proto II - EBFA-type total and to modify the power output to 30 TW accelerators and could allow greater power peak. The projected outputs are 2 MV and 15 densities than are now being achieved. MA but these values could be modified pending future results from MITE. EBFA-I PROTO II Results from Proto II and MITE along with modified target requirements have reshaped Summary EBFA-I. Proto II has met the original design objec­ The EBFA design can be chosen from a wide vari­ tives and is being used for experimental ety of hardware. Strip lines or Blumleins either studies.

21 ; •-•'•:>^;Й-;-Т

i>V* uf^* fVjiM i

hi'i':: V.-.T. 14= Accel-" '.Vit.h the completion of the pulsed power test­ perturbing the equapotential lines in the ing, the accelerator can now be scheduled diode area. New developments in MFT reported and used for a variety of experimental pur­ elsewhere show even lower inductances are ples including power flow. Proto II is now possible. the world's most powerful (~ 8 TW) low-imped­ ance {- 1/8 fl) accelerator. •Varying pulse durations - Proto II has shown that several pulse widths are possible. P'-oto II has proven to be, for its size and Changing the pulse duration is possible with­ complexity, a highly reliable and easily out dumping the water and can be accomplished "inintair.ed accelerator. The new flat disc in less than four hours to provide 80, 60, 45, design had some variance from conventional or 20 ns outputs. On conventional accelera­ accelerators but provided a multitude of tors pulse width variation is difficult if advantages. The following pulsed power pro- not impossible over this range. • '.:-.: have been greatly reduced by the design: •Untriggeri. i water switching - Data from •'; ! VM\) air in the water causing voltage Proto II shows 3 ns (rater switching jitter nrejkdjwns • Once the water is placed in the and indicates that this low value could be ac^leiator and the bubbles eliminated, switch further decreased. Untriggered water switch­ gap f iia::...ing, monitor calibration and servic­ ing should extrapolate to far larger accelera­ ing ii; JC performed from the top of the lines. tors. ;Te /at or : ' removed only for major changes in ••..vel^rator configuration. Accelerator Description • Var;jbie output voltages and impedances •• ?!'.••> else type line transfoliners were success- Reference 1 has an in-depth description of :"ui <-.:wi 'asily adjusted. This eliminates the Proto II, a later paper2 detailed the slower need for a separate set of large expensive pulsed power data. Л photograph of Proto II close rolerar.cc stainless steel cylinders to is shown in Fig. 2. Eight Marx generators, „•range the conventional accelerator output tested to 1 HI, are located in a annulus impedance. formed between a central 7,9-m-diameter tank and the outer 13.4-m-diameter tank. The •Mult L.iU Marx generator operation and re- tanks are 2.8 m high. Low jitter 400 kV 1 iabilir." - The Proto II Marx's are continuing trigger units synchronize the eight Marx -a ope:ate with very low jitter (~-10 ns) and generators and tests indicate less than 20 ns '?•••• .jrefires. The design provides safety jitter for the entire Marx and trigger system. Matches for the generator which connects After triggering, the Marx generators transfer the Ъ.гх either to a dump resistor or the their energy to 16 intermediate water dielec­ rare-! c.-.paoitors. This fact allows the Marx's tric storage capacitors which are then switched to ':.-' tested prior to every experiment to through the eight 1.2-m-diameter high voltage .!Sve:tain if they are running properly. This feedthrough located around the inner tank wall : procedure checks the higher failure components. by S!t trigatrons. The triggered SF6 switches Fearing the experiment, the Marx's are connected provide time synchronization for the accelera­ to the load for only a fen1 seconds after being tor and transfer the energy into the first rhbrsoc thus further narrowing the time window water line in about 300 ns. A series of rod 's.-it', a prefire could damage the experiment. switches (16 to 80) then discharge the first water line into the second water line which in turn self-breaks through a sharp edge • Wat --у intermediate storage capacitors - ' e v.ntci capacitors have proven very relia- switch in typically 150 to 200 channels and '-.•<.- af.ei initial problems with gas bubble provides a short rise time pulse into the ;.,:.'nerritioti. Intermediate storage capacitors radially com'erging transmission lines. . >r a vital portion :f any multiple channel Circumferential interconnections between the wster switching accelerator. first water lines average voltage and time differences between the water capacitors and SF^ switches. The radially converging lines • Low diode i:\ducttuice - The Proto II diode join to two bad;-to-back low inductance diodes inductance is about 6 nh per diode or a total which provide two oppositely directed 1-m- i.achine diode inductance of 3 nh. The diode diameter ring electron beams. The lines also has with..t-iod a voltage of 2 MV which results provide a transformer with a 0.125 ohm input it: ар. in-iutance of 1.5 rih/MV. The natural and a 0.23 ohm output for this series of tests. diode gr:v:mg inherent in the concept elimi- Tne output of the transformer section attaches .;at::- tit' aeed for field shaping and greatly

23 ^я&

Fig. 2. Proto II accelerator. to the back-to-back diode that has been des­ a voltage is applied across the second switch cribed. This diode was designed to have which is a sharp edge blade. A photograph an inductance of 4.1 nH hithin the insula­ of the channels obtained is shown as Fig. 4. tor envelope. The effective inductance, determined by short circuit shots in all pulse modes, was found to be within 10 per­ cent of the design value. The short circuit ^hots provided a cross-check of the monitor calibration, calculated inductance, and effective output impedance of the accelera­ tor. The electron beam was generated with a 1-m-diameter razor blade cathode and a 24 segment prepulse switch, consisting of 1.27 cm by 1.27 cm cylinders and was installed at a 1.32 m diameter. This increased the effective diode inductance by l.S nH and reduced the prepulse by a factor of S for the 50 ns pulse mode as shown by the traces in Fig. 3.

PI PREPIH.SE

Fig. 4. Photograph of Proto II switching arc channels.

Note the large number of channels per radial section that were obtained. As expected, the switch rise time is limited by the water switch resistive phase and the total switch jitter. Voltage monitors are located oppo­ site line 1, line 2, and the entrance to TIME (M) the radial transmission line. The voltage on I TIMl ntPWIE line 1 rises to 2.1 MV in 240 ns, then the г A-K тпнк rod switches break down and charge line 2 in about 40 ns and subsequently cause a prepulse voltage at the transmission line input. After Fig. 3. Proto II prepulse voltages. line 2 switches the output peaks at 1 MV or 3.9 TW in 12 ns with an energy of 60 kj per side. The pulse then passes thru the trans­ Twenty Nanosecond Mode Operation mission line transformer sections where the impedance changes and the voltage is increased Sixteen sharpened rods are used between the by 1.4 before impinging on the.diode. Each first water line and the second water line in diode has four В monitors and V monitors. this mode of operation. The small number of These are integrated, averaged then compared switching points provides inductive isolation to the opposite diode. The data provides between the two lines and exchanges the line power, energy, and the inductively corrected energy in a limped constant fashion. The voltages for the electron beam. The power energy transfers with a typical voltage of and energy are shown in Fig. 5.

V = V0 (1 - cos tot)/2 waveshapes with a zero to peak of 70 ns. When line two is charging,

25 \ PI OUTPUT РП CURRENTS

S SO-

TIME (ill) ь

Fig. 5. Proto II output power. Proto II output energy. TIME (in)

The generated pulses arrive at the diode in a slightly asymmetric manner. They average to Fig. 6. Proto II diode currents. provide the diode current and voltages. A typical combined current trace is shown in Fig. 6 for monitors 1 m from center and 90° Varying Pulse Width Mode apart. The currents and voltages average out for a small diameter cathode but a large dia­ Due to the three pulse forming networks and meter cathode probably emits preferentially. the large number of water switches, several If the arrival time differs by a spread of different pulse lengtlis can be obtained from it, then the apparent rise time increase to this type^of accelerator. The simplest PFN is the diode will be ~ At/3. Therefore, a described'- and consists of dumping the water spread of IS ns will add S ns to the diode capacitors into a load. Approximately 2 TO rise time. The spread originates primarily peak was generated by charging water line 1 in the SF6 switches and first set of water and using 1, 3, or 5 rod switches to shape switches. An analysis of the switch data the pulse. In this mode, 80, 60, or 35 ns is shown in Table I. The switching appears pulses can be obtained. The line 2 switch is to operate as predicted. The larger spreads shorted to the transmission line during these are caused when one or two gas switches fire tests. A summary of the normalized accelera­ early. The cause of these early firings is tor outputs in this mode is shown in Fig. 7. being investigated.

SF-6 Trigatron Switching TABLE I While Proto II achieved its output parameters Proto II - Switch Data the SF-6 Trigatron switches exhibited a spread up to 50 ns or a jitter of about 16 ns on la Predicted several experiments. On Proto II this time Spread Deviation Deviation spread was reduced to about one-half by the ns ns ns voltage averaging on line 1 and its water switches. For instance, the 16 ns SF-6 jitter SF Trigatron 2S.0 6.8 5.0 6 would correspond to a jitter of 8 ns in the water switching. A program was initiated to Untriggered Rod 10.5 3.2-6 2-6.4 improve the SF-б switch performance during Switch September.

26 extend through the holes and face points on PBsig a auTPtji vs РОКИЙ fiust i/г wrorH the opposite pulse forming line (PFL). The capacity from the PFL to the output line is reduced by approximately 90 percent and hence the prepulse is lowered by the same fraction. The energy formerly stored in that capacity and formerly dissipated in the switch, is now available for use in the accelerator. After the main pulse has passed through the ground planes, the gap between the holes and the switch electrodes breaks down and protects th( pulse forming network from the pulse reflec­ tions from the diode.

GttHMD FUNIS

^>'^^ \

PUIS* IENBTH (HI)

Fig. Proto II output vs. рокег pulse one-half width.

Precise timing measurements showed that the trigatron switch spread was caused by a few switches breaking down slightly before the Fig. 8, Cut away views of water dielectric trigger signal arrived. • A few of the eight switches are shown without (a) and switches would break and then the rest would with (b) the prepulse reducing trigger as expected. The switches were oper­ ground plane. ating at about 0.8 reliability. The SF-6 gas was checked and found to be acceptable. Three other probable causes were electrode The tests on the Ripple accelerator show that damage, electrode material, and arc debris. the prepulse can be reduced to approximately A series of tests showed that all played 6 kV into the transmission line for a 3 MV a part in the early firing. New electrodes charge on line 2 (the worst case for prepulse). of 304 stainless steel were installed and the switch reliability was > 0.986 for an An additional, water dielectric prepulse extended series of experiments on Proto II. switch based on the same concept can reduce Subsequent user experiments have proven that to approximately 2.5 kV. Vacuum flash- this initial data and the SF-6 switch opera­ over prepulse switches or plasma erosion tional level should be adequate for EBFA. switches in the diode can further reduce the prepulse to < 500 "'. These types of switches have been des .;ned for Proto II Prepulse Prevention operation and are beim fabricated.

The suppression of prepulse on super power accelerators is a major problem. A new- MITE approach for preventing the prepulse from reaching problematical levels has been Summary developed and tested on the Ripple accelera­ tor. A ground plane with several holes in The №TE (Magnetically Insulated Transmission it is inserted between the electrodes in the Experiment) is a one moiule (1/36) version of water dielectric switches as shown in Fig. 8. EBFA. Because of the ir utiple diode approach The switch electrodes from the output line proposed for EBFA, a te t bed was needed to

27 test several of the modified Proto II compo­ reach a peak voltage of 2.5 MV in 70 ns. At nents and the vacuum insulated line transport that time, the knife edged Blumlein switches efficiency. The MITE pulser uses the same launch two pulses down the water-insulated type of Marx as Proto II which charges solid transmission lines to the vacuum envelope. dielectric intermediate storage capacitors. Monitors at positions 1, 2, and 3 as well The intermediate storage capacitor initially as at the vacuum envelope are used to measure charged back-to-back water insulated voltages and arrival times of the pulses. Blumlein transmission lines. The solid di­ Measurements of pulse simultaneity showed a electric capacitors were investigated as an mean difference of 3.8 ns. alternative to the water insulated interme­ diate energy storage capacitors. These new capacitors offer some distinct advantages . SIHGIE CHAHHEl SWITCHES over the water capacitors: no polarity effect as in water; compact packaging; no , WIIIEB TMNSMISSIQ4 LIME deionized water system. The primary un­ certainty is the capacitor lifetime. Life­ time tests conducted during this report period indicated a reliability problem and are detailed below. /—i г Solid Dielectric Capacitor Testing ' ILUMLEIN SWITCHES Testing of the solid dielectric intermediate storage capacitors has continued on MITE. Four simultaneous capacitor failures occurred Fig. 9. MITE Blumelin pulse forming lines. after 180 shots in a voltage range of 1.5 MV to 2.8 MV. An equivalent number of shots of 109 was calculated for an EBFA operating The charging waveforms at positions 1 and 2 voltage of 2.5 MV by using the 8th power are similar to those for Proto II. The scaling law. After an additional 18 to 2.5 Blumlein output waveform at position 3 is MV shots, four more capacitors failed. shown in Fig. 10. The power waveform shown is for both sides of MITE and was determined The capacitors are made up of 123 paper- from the Blumlein output and line impedance aluminum foil windings connected in series. at that point. An examination of some of the failed capaci­ tors showed that an arc had punched through the paper insulation totally shorting some MITE OUTPUT capacitors and partially shorting others. The partially shorted capacitors can be identified by an increase in capacitance. A capacitor was defined as having failed if its capacity had increased by 10 percent.

While a Wiebull analysis of the capacitor failure may not be entirely correct due to synergistic effects, a mean failure rate of ~150 shots at 2.5 MV is predicted. On EBFA if all capacitors would be replaced after 100 shots (~ 10 percent failures) the cost would amount to ~$5000 per shot.

0 10 20 30 40 90 Blumlein TIME(n) The initial MITE Blumlein pulse-forming lines are shown in Fig. 9. The line 1 sections are pulse charged to ~2.5 MV in 250 ns. The energy stored in line 1 is then transferred Fig. 10. a. MITE Blumlein output voltage, to line 2 through self-breakdown single b. MITE Blumlein output power. channel water switches. The line 2 sections

28 Strip Lines accelerator. All four current feeds gave measured inductance values within 1 nH of The discovery of a new prepulse isolation the calculated value of 36 nH. The inter­ method has allowed stacking of the Proto II- face is now undergoing high voltage testing type strip lines for EBFA. A new set of in the MITE accelerator. pulse lines were reconfigured from the Blunlein parts and comparative exper-jnents were made. Initial results indicate more POWER FLOW - SELF MAGNETICALLY INSULATED efficient strip line operation. The strip TRANSMISSION LINES lines have a larger output current, lower voltage, and a simpler configuration than Introduction the Blumlein, A higher efficiency at lower cost make the strip line preferable for EBFA. In the Electron Beam Fusion Accelerator Outputs and pulse shapes will be obtained (EBFA)4, 36 magnetically insulated vacuum for the next progress report. transmission lines are to be used and each is approximately 7 m long. Since the transit time through the line is approxi­ Diode mately the pulse duration, the electron flow must be magnetically insulated by the The initial vacuum envelope used was a short­ self-limiting impedance of the line, which ened (22-cm insulator length) version of a is a much more stringent criterion REBA3 diode. The measured inductance of 80 than encountered in conventional load nH is much greater than the intended EBFA dominated lines.5 envelope, vacuum triplate magnetically in­ sulated vacuum transmission lines were Both coaxial and triplate transmission lines connected to the diode to allow experiments have been considered for EBFA. With mean to be performed prior to the intended diode stresses of 2.4 MV'/cm, power can be trans­ delivery. The 28-cm-wide vacuum lines are ported to within 1.1 m and 0.45 m of the spread 1 cm apart with the outer lines held pellet in EBFA for coax and triplate con­ at ground potential and the center line pulsed figurations, respectively. Consequently, positively. A 1.2-m set of vacuum lines has the triplate configuration is being pursued been tested and a set of 7-m long is on order. because of its advantageous power flow Experimental results of the lines are pre­ capability. sented in the MITE section. The magnetic insulation program has been a The intended vacuun insulator interface for cooperative venture with Physics International EBFA was designed and constructed for use Company. Experiments have been performed at on the MITE accelerator. This interface Physics International on coaxial6 and triplate7 assembly, designed using the JASON field configurations. Additional experiments are plotting code, uses standard 45° Lucite now in progress on the coaxial configuration. insulators and Gask-O-Seal grading rings. At Sandia, experimental work was undertaken The diode is sized such that no interface on coaxial lines and semiempirical theory was will fail on a given shot when 36 such inter­ developed to explain the input impedance of faces are operated in parallel in EBFA. The magnetically insulated transmission lines. insulator rings are circular in shape with Two numerical codes have been developed to internal metal field shapers which convert study power flow in long self-magnetically from the circular input geometry to the insulated transmission lines8»" and have racetrack geometry required for the magne­ been useful in interpreting the experimental tically insulated triplate transmission results. Recently, experimental work has line output. The combination of rectangular begun on the MITE accelerator'* using a 1.5-m- and semicircular geometries required a number long triplate transmission line with a geo­ of assumptions during the design process to metrical impedance of 8 ohms. The results of arrive at the calculated inductance of 36 the numerical computations and the results of nH to the beginning of the magnetically in­ the MITE experiments will be discussed in sulated transmission line. Inductance mea­ this section, and related, to the design of surements have been made on the interface EBFA. assembly using a S-ohm fast pulse generator and uniform resistive load. Four different current feed geometries were used to simulate possible current feed geometries in the MITE

29 Computational Results

Two time-dependent numerical codes have been developed to study the properties of magnetic insulation.8»" Results will first be dis­ ttt cussed for the two-dimensional, time-dependent, fully self-consistent, particle-in-cell com­ puter code. A voltage waveform is applied t. the input of the simulated transmission live, and the time history of the pulse propagation and electron flow is computed. Figure 11 shows the voltage waveform for ?. 7-m-long, 8-0. vacuum coax. The input voltage wavelon". was a trapezoid with a 10 ns rise to u J.4 Ml' level and then constant; this shape wa« chosen to approximate the MIT!." voltage pulse. The voltage pulse without the та,;тп tic in--u lation effects is shown by the dotted lino in Fig. 11. The apparent input impo lance Z = V/I of the coax for this case war ?.4 Л, MAGNf .KALI T If which is 67 percert the geometries" i:",j e.;aiK._. VblON The pulse rise time ha? sharpened, anc iae t front of the pulse ha:- beer, eroded. The average velocity of th e fv-nt is c.2 у 10's m/sec. Peak power is f.uisported '-.'it:- V- percent efficiency. - •nee ele^tr-"'1: • . e ••:•'.. 11. --D time-dependent, s imula - to the anode during *. i".-'-"- '• '-'• ' ..к "jls tion of magnetic i .; ulation. the average energy i.-t ne ei-.ct гот:-: r-. ir' ing the anode is typical. . 1 :.tri .\!t of !';•.• maximum potential ac-' -••; "re tra.".;,::a---ii.n resulting in a nearly linear voltage drop lines. Electrons in • •-" ' ."'ft region behnd along the length of t.h: tripl •• . line. The the front are confin" 'ilic.T a wi. :.': is code i,as beer, mocified to include a small relatively close 'o ': '•od' ; "hc'4 is no conductance behind the pulse front. Simple leakage o'' elect < n' .!:<: у :• !", • he consideration of poecr balancr for a line cutoff region. ~.... :.". ;.lowi- • "i the Kit!! a voltage loss yields ал expression negative inner с -. ..:.i:-.t - / ;"'.! i he t tat earner. :or the conductance per unit length of v has oeen compute.: ro- .: •' -."oitjiges ')etn .-en g = i^,- ^ . For the PI results this 1 and 1С MV and ti с L• ' , • .igrefi wi •".'': percent with that ;..•: :~ t :\i . i К ':.Milium suggests an equivalent loss current density current allowed 'i ; :• ;.,n'..'ir of 12 A/em- or 1..' percent of the possible theory. O:'.1 "s la'.-., space charge limited value for thi"- geometry. The solid dots with error Another simulation wa ae bars in Fig. i2 are the experimental values circuit analysis orfe .t i'Rr.. T':e ] IV- '.c« for the pe.-k voltage as a function of length of electron emisiun .ir'' tgnetic m :•• , tion and the open squares are the code results •isir.L 4ie! C A/cm- loss current. Hence the are introduced v...; i c-m.i •tancc :i e.|t!iva 1'nt distributed nerv.o-'l аупоче-,: azimuthal asymmetries inherent in triplate Ic-sy transmis." •• . '. .. .v rav lines ma> cause a low level leakage current dependences of the eonJ'Ji-Var.ct en to flow in the self-magnetically insulated voltage and currsr.i arc sisieirpi: triplate *,ne. Recent 4-chlorostyrene mea­ to experimental <.V :.hort lib surements of the emission pattern confirm that Hie Jos.-es are piedonii.nantly ir. the men's. The resu't;- 'h!' crde- . vel'. with the '.-•• .-± ••. ' л regions 'vith reduced П ar.d 3 fields. experiments wiTh ;n>.ir:. tag' of tr i.1 coce i.- tin г stuoy the effect.- •' -/a.: _- It i'l-id'a Triplate Experiment tion,- am. -.-iul.:f. •:.'•; pulse fr-v л iii.Jl. module of EBFA has been designed and -~ being fabricated. The module con- Recent expeг;мс •ip•31;.ti - jj'.r-: of a 7-m-long, 3-'; vacuum triplate Physics ii.anis '.ue ni " •' low inductance. Sfi nil insulator.

30 • HP PCISHS

: < II !

1 l «nd | MDNHQf U b (

FL>: 52. Voltage loss as a function of length Fig, 13. М1ТП vacuum transmission lines and for a simple triplate. diagnostics.

A 1. .5-m-lcng, 8-Q triplate was adapted to transmission line. A Faraday cup is installed the MITE pulse forming network using a modi­ in the middle of the centerline to measure loss fied RFJ-A diode with an inductance of 8 nH. current density to the side of the line. The Figure 13 shows the triplate transmission cup measured losses of between 10 Van^ and . !.;e and some of its diagnostics. The center- 2 .Van' during the pulse rise time, and much line was constructed of two lines 25.4 cm less than 4 A/an2 during the rest of the pulse. wide, spaced 5.08 cm apart and connected electrically. The outer lines were 30.5 cm The initial shots were performed with bare apart and connected electrically. The outer aluminum lines and damage was severe and lines were 30.5 an wide and were spaced 1 cm spotty. Four-chloro showed damage patterns from the inner set of lines. The lines were that were teardrop shaped (6 mm x 12 mm) terminated in a field eirj ;?ion diode in which shaped with doses in excess of 2 x 106 rads. the anode is a carbon calorimeter and the >tore uniform emission was obtained by coating cathode is 20 cm of 5 mill thick stainless the outer lines with Aerodag. The dose at the steel. Input voltage and current are measured top and bottom of the lines was 30 to 50 per­ or. each side of the diode. Б-dots are mounted cent larger than the dose on the sides. Volt­ on the input, middle, and output of the inner age, L dl/dt, and corrected voltage waveforms and outer conductors of the transmission line. are shown in Fig. 14 for a charge level of Experiments have been conducted with positive 2,4 MV on the Blumlein. ' The high inductance polarity on the center conductor. The moni­ of the diode made it difficult to calculate tors on the inner line measure total current the corrected voltage available at the input and those on the outer line measure boundary of the transmission line with the fast 15 ns current, which is total current minus the FWHM pulse. These waveforms were taken when electron current. Since B-dot monitors suffer the transmission line was running at its fton the effects of nonuniform current flow self-limiting impedance, i.e., tje A-K gap and emission, reliable data were takon only- was not drawing sufficient current to insulate after the uniformity of emission was improved the transmission line. The input impedance by coating the lines with graphite. A magne­ for this case is 4 Я at peak power. tic energy analyzer is mounted within the cnode and uses 4-chlorostyrene to record time-integrated electron energy distribution In order to obtain data at higher voltages at the load. The azimuthal symmetry of elec­ while we awaited deliver)' of the low-inductance tron loss current is measured with 4-chlo?-o tube, the switching configuration in the MITE : laced around the circumference of the inner pulse forming network svas changed from that in Ref. 4. Figure 15 shows the configuration used

31 I-DIODE VOLTAGE

' l HI 3-CORRECTED VOITACE r WMEB IRANSMISSION UNE

Pulse forming line configuration for long pulse.

llMf (n.)

Fig. 14. Voltage waveforms for short pulse operation.

to obtain the long pulse. The Blumlein switch voltage along with curves for the boundary on line 2 and the transfer switches between current necessary for saturated parapotential line 1 and line 2 were removed and a three- and minimum parapotential electron flow. The point Blumlein switch was installed on line boundary current and the total current are 1. Typical output, current, and corrected equal up to the arrow. The total current voltage waveforms and the output of the magne­ begins to rise at a faster rate and exceeds tic energy analyzer are shown in Fig. 16 for the value for minimum parapotential current' a charge voltage of 1.7 MV on the Blumlein. near the end of the pulse. The voltage The L dl/dt correction to the voltage is now decreases as the current remains high at much lower, allowing a more reliable calcula­ the end of the pulse which indicates that tion of input voltage. The input impedance the magnetically insulated line is not a of the transmission line is 4 S, and is the constant resistive load for an accelerator. same self-limiting value as that obained The current transport was excellent for with the short pulse. The magnetic energy case and the cur . t loss density was between analyzer data and the A-K gap geometry indi­ 10 and 50 A/cm2 on the top and bottom of the cate the gap impedance to be 7 Q. Hence, line and < 2 A/cm2 on the sides between plates although the current transport was 100 percent to the end of the transmission line, only a A summary of the loss current density for third of the current was flowing to the anode various experiments is shown in Table II. from the cathode. The rest must be flo*"' - The values of the losses have been normalized directly to the anode in the last 2.5< to the space charge limited current densities between the last B-dot on the cathode u ...J for the various configurations. The experi­ load A-K gap. Figure 17a shows plots of the ments above the dotted line were all for uni­ total current as a function of corrected volt­ form coaxial geometry. The values of loss age with curves for minimum parapotential current are very low and the power transport current and saturated parapotential current. was excellent for these experiments. The 2-D These curves are similar to those obtained PIC simulations have indicated no loss current in the P.I. experiments. Figure 17b shows in uniform geometries and is consistent with plots of the boundary current vs. corrected experiments. Experiments involving nonuniform

32 I-OTOE railAGE

' l ill 3-CORRECTED WHMCE

J \

20 lh 30

TIME (гч)

Ca) (b) Fig. 16. Typical electrical and magnetic energy analyzer data for the long pulse.

Fig. 17. Total current and boundary current V-I plots for a magnetically insulated line.

ЗЭ TABLE II

Loss Current Density Summary

Z Aerodag Coating Experiment Ohms Negative Electrode

Hydra Coax 11 1 0.2 No

Pi - Short 24 to 49 < 0.2 i 0.1 No

PI - Long 49 <_ 0.03 ± 0.02 No

Curved Coax 11 3.0 (1/4 area) No Hydra

PI Triplate 24 0.5 (1/3 area) Yes/No

MITE Triplate 8 5.0 to 10.0 (1/6 area) Yes

geometries are listed below the dotted line. The fraction of the anode area over which the WATER DIELECTRIC loss current was observed is also indicated. POWER FEED These experiments indicate that current is lost in the regions in which the ratio of the vacuum fields Ey/By at the anode is well above the average Ey/By in the lines. Consequently, ANODE although the loss currents in a simple tri­ plate line result in an unacceptably low trans­ port efficiency for EBFA, a line designed for VACUUM POWER FEED a nearly constant value of E/B should be much more efficient. An oval shaped or "closed ALUMINUM triplate" cross section line approximately satisfies this criterion and will be tested CYLINDER in the near future.

MAGNETIC FLASHOVER INHIBITION (MFI) WITH SELF- FIELDS

The study on magnetic inhibition of vacuum CATHODE flashover with self-magnetic fields was extended to higher electric fields on the 0.3-П Ripple accelerator. The experiments ACRYLIC indicated that the average flashover electric INSULATOR field along the insulator surface Вц is pro­ portional to the magnetic field Be perpendi­ cular to Ец, as has been predicted, and the Son flashover strength of a MFI insulator can exceed that of the standard +45° acrylic in­ sulator. Fig. 18. Schematic of Ripple diode. The radius of the aluminum cylinder i The Ripple diode used in the experiments is changed to vary the inductance or shown schematically in Fig. 18. The diode the cylinder is replaced with an was shorted with inductive shorts ranging A-K gap for a particle beam load. from 0.47 nH to 1.8 nH in most of the tests.

34 Shots were also taken with particle beam loads the insulator is then determined by water with a plasma erosion switch,!' in which the breakdown. With water dielectric convo­ impedance was initially zero and then in­ luted feeds to the diode15 and the short creased to ~ 0.1 a after the current was pulse breakdown strength of water, then established. MFI double diodes for Proto II and EBFA-I could be built with tube radii of only Tests were conducted with +45°, -45°, and 0° 0.28 m and 0.54 m, respectively, with acrylic insulators lightly coated with sili­ inductances of 0.56 nH and 0.3 nH, respec­ con diffusion pump oil. The voltage across tively. the insulator was measured with an integrated D monitor, and the currents inside and out­ However, the Ripple diode has a small area side the insulator were monitored with inte­ (45 cm') insulatOT. There is a well-known grated В probes. Flashover was determined dependence of flashover strength E on area to have occurred when the current inside the for +45° insulators with EaA"0-l so that the insulator was crowbarred to a constant value flashover strength could be degraded in a and that outside the insulator continued to larger diode. This area dependence is related increase. Typical voltage and current wave­ to the width of the probability distribution forms are shown in Fig. 19. The values of for flashover. The fact that there were more E]j and Bg at flashover for the three insu­ than ten flashover channels per shot and they lator geometries ai : shown in Fig. 20 with were uniformly distributed around the insula­ the low B0 values obtained by other experi­ tor indicate that the width of the flashover ments ДЗ probability distribution in these experiments was very small and, hence, the area dependence For +45° insulators, in which the electrons very weak. Nevertheless, a full-scale test are geometrically inhibited from striking of MFI is required to confirm the utility the insulator, the flashover strength was of the approach in superpower accelerators, tiot strongly dependent on the magnetic field. and that experiment is being planned for This result is consistent with the earlier Proto II. observation of McDaniel, et al.l4 with ex­ ternally applied Be. For -45° and 0° insu­ A second obvious use of MFI is in preventing lators the flashover strength was approxi­ restrike of the insulator during the current mately proportional to the value of Bg and rise in Z-pinches. That approach has been was approximately given by successfully tested on the Ripple accelerator and is reported under the heading "Superfast Z-Pinch."

E,, = 0.045 B6c

MAGNETIC FLASHOVER INHIBITION USING AN EXTER­ = V/d (cose) NALLY APPLIED MAGNETIC FIELD where Be is the magnetic field in Tesla, Magnetic deflection and trapping of electron с = 3 x 10" m/sec, V is the voltage across trajectories has been employed to develop the insulator in volts, d is the separation vacuum pulse power transmission lines operating between the electrodes in meters, and S is at more than 1.5 MV/cm. Theoretical predic­ the insulator angle. This value of Е;ц is tions of improvement in vacuum insulator per­ consistent with the -45° data with an exter­ formance have been developed.17,18 The prin­ nally applied Be in Ref. 14. ciple has been, experimentally, investigated as a method of improving the voltage hold off The presence of UV and X-ray radiation from characteristics of vacuum-dielectric inter­ the anode and cathode with a particle beam faces used in electron beam machine diodes for load did not degrade the flashover strength. shallow insulator angles using an externally applied magnetic field. The scaling of the flashover strength with magnetic field Be reverses the usual depen­ The experiment consists of a polymethacrylate dence of the tube inductance or the insu­ disk insulator with an area of 116 cm2 between lator radius. If the tube is made small metal field shaping electrodes which create a enough for MFI to be useful and the ratio 14° angle between the field lines and the disk of the tube height to the length of the surface. The disk is lightly oiled with DC 704 vacuum feed is a constant, then the induct­ silicon oil and subjected to 36 ns wide pulses ance varies as 1/r. The limiting radius of of up to 500-kV amplitude from the 2-P. coaxial

35 B.m B.m

Fig. 20. Eij vs. В at breakdown for A: ' 0° Ш and -45° Ш insulators and B: +45° insulators Щ• A completely shaded symbol indicates breakdown, an open one indicates no breakdown, Fig. 19. The current waveforms outside the an and a partially shaded symbol in­ insulator (-Ioyt) ^ inside the dicates late time breakdown. The insulator (IjjJ are shown at 0.56 points at В =0 are from Ref. 13 MA/div and 10 ns/div in A and B, with electron initiating points at respectively. Flashover of this the anode or cathode to remove the -45° insulator occurred upon volt­ area dependence. The highest stress­ age reversal. The voltage waveform ed points in each case were obtained is shown in С at 50 kV/div ar.-i 20 with a particle beam load. ns/div.

THOR accelerator providing mean fields at the IHDR EXPERIMENT disk surface up to 20 MV/M. A separate 20 kV, 21S ufd capacitor bank is used to provide

variable amplitude and polarity magnetic fields -гп-Эвп, H;0 PEl л211-|вп, HjO at the insulator surface of up to 1.3 Tesla. / IRMSMBSKW UNE A 155 nH inductor across the normal A-K gap is used to carry the bank current while main­ taining voltage across the disk surface. The experiment is shown schematically in Fig. 21. The inductor across the A-K gap causes the voltage waveform to department, with an L/Z droop, from the normal rectangular shape. An integrated D-dot pick-off probe is used to monitor the voltage at the disk while an inte­ grated B-dot coil, oriented to ignore the symmetric fields produced by the coaxial line, Fig. 21. Schematic of the THOR experiment. is used to monitor the current due to the non- The isolation inductors keep the symnetric breakdown across the disk surface. fast pulse out of the slow capaci­ tor bank circuit while maintaining The voltage pulse amplitude applied to the in­ a circuit for the bank current. sulator disk was raised in steps until break­ down was observed approximately 5 ns after the initial voltage peak at a mean field of 15 MV/M. First experiments with magnetic fields

36 on the order of 0.2 Tesla deflecting electrons generated fields19 show the external field to from the cathode surface into the insulator be a factor of two more effective in raising surface caused breakdown on the rising volt­ breakdown strength which may be a result of age waveform as expected. With the magnetic having the magnetic field present before field reversed to cause electrons to deflect saturation of the electron avalanche. away from the insulator surface, no breakdown was observed during the full 36 ns wide pulse. Breakdown did occur as soon as the voltage OPTICAL CALIBRATOR pulse reversed polarity"which made the outer metal surface a cathode and electrons were directed into the insulator surface. Typical A compact optical calibrator for fast elec­ disk voltage and integrated B-dot waveforms tronic streak cameras has been constructed are shown in Figs. 22a through 22d for break­ using a pulsed nitrogen laser as the input down with and without an externally applied light source and a fiber optic bundle array Bg field. A plot of mean electric field at to develop five optically delayed outputs. the insulator surface achieved for various values of Be is given in Fig. 23. The ratio A coaxial EG^G nitrogen laser is driven with of magnetic field needed to inhibit a surface a Kappa Scientific 30-kV pulser to generate flashover at a given mean electric field is a 4S0 psec risetime, 3.3-ns FWHM light pulse given by the relationship with an overall system jitter of less than 1 ns. A disk of Pilot-B scintillator is used to convert the 337 nm laser radiation to 408 nm emission for transmission through a Galileo fiber optic bundle has S outputs each delayed b<-*- in 5 ns increments as well as an optical to reference output which is internally coupled External magnetic fields have been employed to a photodiode to provide a 1.5 volt elec­ to improve shaMov angle insulator flashover trical to signal. This assembly has provided field strength •. lore than a factor of two adequate light output for calibration of the which is the limit observable in the present 20 ns sweep of an STL image converter camera. experiment. Comparison of the results of The coaxial laser head used in this calibrator this experiment and. an experiment using self can also provide a 200 psec wide p~.se when

•'VS

(a) (b)

Fig. 22. The voltage and integrated B-dot waveforms shown in 22a and 22b were taken with no external applied В field. Figures 22c and 22d were taken with an applied field of 0.46 Tesla with all other condi­ tions held constant. Voltage at 118 kV/div, integrated B-dot at 1 volt/div and 20 ns/div time sweep for all traces.

37 been used on the Ripple ccelerator to measure voltages up to 2.5 MV, a rates of change of voltage up to 8 x 10l* v/tec, in electric fields of up to 500 kV/cm.

The monitors are now installed on the MITE accelerator for evaluation for EBFA.

I - E»RIY IREAKOOWN J -HIE ПМЕ HEMDOWN J - Ml IBEAKDOWN N\_ I1 i-J

Fig. 23. Peak mean electric fields at the insulator surface vs. externally Fig. 24. Schematic drawing of nylon dielectrii

applied B9 for +14° insulators. 6 voltage monitor for water dielec­ • indicates »jarly breahdoim, tric transmission lines. with 5 ns of voltage peak, и indicates breakdown on the falling portion of the applied voltage SUPERFAST Z-PINCH waveform, ana C3 indicates no breakdown during the full 36 ns The development of magnetic flashover inhibi­ pulse. tion (MFI) suggests that the restrike problem in plasma devices may be overcome. The 0.3-ft Ripple accelerator was used to pinch a 0.5 can also provide a 200 psec wide pulse when torr D2 gas-filled pinch with Imax =2.0 MA. filled with neon which will be investigated Streak, photographs and current and voltage ' for calibration of an Tma-Con image conver­ measurements were obtained on pinches with

ter camera using a shorter delay inc.rer.ent initial radius r0 = 2 cm and length 1=2 cm. fiber optic bundle. Khen the current pulse was not crowbarred, the insulator restruck upon current restrike. At no time did restrike occur during the VOLTAGE MONITORS current rise v.hen MFI should he effective. The pinch impedance L was approximately 0.15 A new D type monitor has bci-n developed for fi and was reasonably well matched to the more reliable voltage measurements in water accelerator. dielectric pulse forming lines. Tae monitor uses a plastic dielectric across the linss The Z-pinch configuration is.shown schemati­ and avoids the fol1 owing problems _formerly cally in fig. ?5. Current (B) monitors and associated with water dielectric D monitor.'-' voltage f'Dl monitor? are in the acrylic insti­ (a) bubbles oi; the pick-off el&ctroJe, (bj ll tor f'ci-ij to the pinch. The voltage V and oxide coating on the pick-off, (c) streamer the rate of dianre of current I are.shown in initiation and electrical breakdown tetwrpn Ki". ?6 ar.-l *he integrals 01 V and I are shown the pick-off electrode and the ground plane, in i'l;'. .'". The resistance R from eiectron- (d) nonlinear behavior of the shunt res isior, neu'ru "ollisor.:-, electron-ion collisions, and (3) reduced frequency response by the and t •?•' "'ence was calculated to be < 0.01 Zl. stray capacity re grouid. The meriti-r h-i-- Since '.'. !>-. i.rjch lt""=s thtn the line impedance

38 ANODE CDR-iENE M? '.'OLTAGE IVAIFH DIELECTRIC \ '.'fi'.ITCBS \ PO'.VERFEED k-J / /^ -V- / / \ n ? 4 ь h D ? A 6 ь '4?5i PIKTHINO PLASMA ACRVLIC :\SULAros U'H

Fig. 25. Schematic of Z-pinch experiment. Fig. 27. The current I and the integral of the voltage LI are shown in a and b, respectively.

Fig. 26. The derivative of the current with Fig. 28. The inductance L = LI/I with a plasma respect to time t and the voltage load and a short circuit are shown V are shown in a and b, respec­ in a and b, respectively. tively.

Z0 or the ratio of V/I across the plasma, R within 10 percent. However, the initial in­ is assumed to be zero in the analysis. The ductance in the Z-pinch shot is low by 30 to inductance vs. tirce is obtained in Fig. 28a 50 percent from shot to shot, which is not from well understood. The pinch radius is computed from the change Vdt _ LI of the inductance uL as L = s /idt - r /exp An aluminum cylinder is inserted into the o pinch volume and the inductance that is obtained on that short circuit shot is shown The radius r and velocity U = f of the current in Fig. 28b. After the signal gets above sheath are shown in Figs. 29a and 29b, respec­ the noise- level, the short circuit inductance tively. The radius apparently collapses to

oi 2 liH agrees with the calculated value to ~0.3 r0, and the peak velocity of the sheath

39 о.» г 5 • a

-0.6

•1.0 ii—. . , о г г » s 9 г TIME I10"BMCI IIMEIlO'Vl

Fig. 29. The current sheath radius R and Fig. 30. The power Wp and energy W deliver­ sheath velocity U inferred from ed to the plasma are shown in a anc the inductance vs. time curve are b, respectively. shown in a and b, respectively.

EBFA DATA ACQUISITION FACILITY is ~ 60 cm/usec when one averages through the oscillations. The power W delivered to the Studies to determine computer system require­ plasma is given by " ments for the EBFA Data Acquisition and Control/Monitor systems indicate that a singl large minicomputer should be able to handle W-ll.12. both jobs. More detailed st.idies are current P 2 ly being made on control system requirements to verify this. Preliminary specifications К and the energy W = / W dt are shown have been written to define the CPU, periph­ p "' p о p erals, and operating system requirements for in Figs. 30a and 30b, respectively. The 3 kJ the data acquisition computer system. A of plasma energy corresponds to 7 keV for preliminary DOE 1830 implementation plan has each electron and ion in the initial gas. also been written. The computer RFQ will be The pinch radius stopped at 0.30 r when the 0 released as soon as the control monitor current was crowbarred at I = 600 kA. The specifications are ready. final ion number density was, therefore, ~S x 10l7/qn3. The ion line number density was "Sx 10l'/cm and' the temperature obtain­ We are currently in the preliminary stages of ed from the Bennett relation is 1 keV at defining the EBFA data acquisition hardware, equilibrium. The piston velocity obtained i.e., oscilloscopes and waveform digitizers. from the inductance measurement is consistent Some alternatives to Tektronix R7912 tran­ with a shock velocity of > 80 cn/usec and a sient digitizers are being examined. One temperature behind the reflected shock of possible alternate digitizer is the updated >1.3 keV. The energy containment times from version of the R7912, the 7912AD. This ion loss are 66 ns and 25 ns for T^ = 1 keV device is superior to its predecessor in the and 7 keV, respectively. The calculated following respects: the main frame and neutron yield for these extremes of Tj are plug-ins are fully programmable, and it 3 x 106 and 3 x 109 for Tj = 1 keV and 7 communicates with the computer via a micro­ keV, respectively. An attempt to measure processor-controlled IEEE 488 standard in­ terface. The disadvantages are: (1) it the neutron yield through 30 cm of H20 and 100 cm of air produced no significant counts costs $6 К more than its predecessor, and on a Ag-detector. This null result gives an (2) the programmable vertical plug-in avail­ approximate upper bound on the T^ of 2.5 keV. able only has a bandwidth of 200 MHz. Additional work is in progress to increase Tektronix is also developing a lower speed the crowbarred current and to improve the device called a 7612 transient digitizer. diagnostics on the pinched plasma. The suit­ This is fully programmable dual channel de­ ability of this plasma as a plasma simulator vice that can sample each channel as fast will then be examined. as every 5 ns. Both Sandia and Lawrence

40 Livermore Laboratory are developing Charge HYDRA, PROTO I, PROTO II - DATA ACQUISITION Coupled Device (CCD) transient digitizers. FACILITY The CCD's are used as analog shift registers to store a waveform in real time so it can The number of R7912 transient digitizers in be digitized later at a slower rate. Both the data acquisition facility for Hydra, developmental projects have sampling rates Proto I and Proto II has been increased in the S to 10 ns range. There are proposals from 15 to 29. The additional 7912 channels for developing 1-ns sample rate CCD digiti­ required extensive modifications to the appli­ zers. The current problem areas in CCD digi­ cations software. The changes have been made tizers are linearity, transfer efficiency, and the new software system is operational. and CCD channel matching. We are presently The number of addressable output data records pursuing the possibility of utilizing the on disc was increased from 80 to 120 for all newly developing CCD technology to obtain a three accelerator-. multichannel, high-speed, low-amplitude resolution transient digitizer which could The capabilities of the interactive data re­ be used to record switch simultaneity infor­ duction program F.sWTEK have been greatly mation from EBFA. We are currently following enhanced. We hav. included the ability to the progress of all types of new transient write programs in the simple FAWTEK language digitizers described above. The decision and then store them on disc. Users can then on the initial EBFA configuration will not run these programs whenever desired. This be made until spring of 1978. eliminates the job of typing the same sequences of commands after every shot. We have examined several cable shielding options for EBFA and have tentatively We have also added new commands to perform a decided to construct a shielded cable race­ number of common diode analysis functions. way between the bottom of the EBFA tank and For instance, a single command will compute the main radial I-beam supports by welding and plot corrected diode voltage, integrated plates to the bottoms of the beams to form B, impedance, power and energy. Other commands an RF tight enclosure. The cebles would do signal averaging, baseline adjusts, and then be routed around the tsr! in the 8 inch multiple overlay plots. Finally, we have space above the I-beams from the main junc­ included the ability to address signals on tion box in the pit under the accelerator disc by their output header names in addition to the desired energy point into the tank. to their record nvnbers. Based on MITE experiments we have decided to feed all diagnostic cables for the Marx The shot archive t.ipe software has been modi­ banks, pulse forming lines and transmission fied to reduce mistakes. Checks have been lines through bulkhead feedthroughs in the included to: bottom of the tank. This should result in much quieter signals and preliminary design 1. Prevent same shot from being taped for the main junction box in the pit has also twice. been completed. This box will contain patch panels for jumping cables from all EBFA diag­ 2. Insure taped shot numbers are sequen­ nostics to either the screen room or instru­ tial. mentation trailers parked outside the building. 3. Prevent reading a shot off tape onto The EBFA diagnostic screen room will be design­ disc over a shot that has not been taped. ed by Sandia Plant Engineering. They have completed a preliminary design including 4. Prevent firing a new shot before last specifications and drawings. The data acqui­ shot has been tapec. sition portion of the screen room will be 15' x 65', One end of the screen room will be isolated from the data acquisition system IMAGE PROCESSING for use by the laser holography pellet diag­ nostics. The need for another shielded area We are currently in the midst of a project to for the control room is currently being develop a general image processing capability. evaluated. The design should be finalized A comprehensive package of image processing and an RFQ let early in 1978. Construction subroutines, LADIES, has been obtained from will begin as soon as the building is ready Los Alamos. This system is now operational for occupancy. on the CDC 7600 after a rather involved

41 process. The only facility at Sandia for dis­ playing computer generated images is the Comtal device on the high-performance interactive graphic system. This system is interfaced to one of the CDC 6600's via a PDP-9 minicomputer. We have successfully displayed images and their Fourier transforms on the Comtal. The Comtal has two nice internal features for enhancing images. One is a pseudo-color mapping of black-white intensities into different colors on the display screen. The second is a function memory which allows interactive nonluiear inten­ sity mappings. We are currently using this system to enhance holograms of cylindrical implosions on Hydra.

42 REFERENCES:

1. Т. H. Martin, J. P. VanDevender, D. L. Johnson, D. H. McDaniel, and M. Aker, Proc. of Int'l. Topical Conf. on E-Beam Res, and Tech., SAND76-S122, Sandia laboratories, Albuquerque, New Mexico, p. 450 (Feb. 1976). 2. D. L. Johnson, Proc. Int'l. Pulsed Power Conf., Lubbock, Texas, IE2-1 (1976). 3. D. L. Johnson, Record of 11th Symposium on Electron, Ion and Laser Beam Tech., Boulder, Colorado, p. 445 (1971). 4. T. H. Martin, D. L. Johnson, and D. H. McDaniel, 2nd Int'l. Topical Conf. on High Power Electron and Ion Beam Res. and Tech., Ithaca, New York (1977). 5. S. Shope, J. W. Poukey, K. D. Bergeron, D. H. McDaniel, A. J. Toepfer, and J. P. VanDevender, submitted to J. Appl. Phys. 6. I. D. Smith, P. D'A Champney, and J. M. Creedon, Proc. of Int'l. Pulsed Power Conf., IIC-8 (1976). 7. M. S. DiCapua, P. D'A Champney, D. Pellinen, I. D. Smith, D. H. McDaniel, 2nd Int'l. Topical Conf. on High Power and Electron and Ion Beam Res. and Tech., Ithaca, lfew York (1577). 8. K. D. Bergeron, J. Appl. Phys., 48_, No. 7, 3065 (1977). 9. J. W. Poukey and K. D. Bergeron, submitted to Appl. Phys. Lett, 10. J. Mi Creedon, J. Appl. Phys., 48_, 1070 (1977). 11. K. D. Bergeron and D. H. McDaniel, Appl. Phys. Lett., 29, 534 (1976); and K. D. Bergeron, J. Appl. Phys., 48, 3073 (1977). 12. С W. Mendel, Jr., and S. A. Goldstein, J. Appl. Phys., 48, 1004 (1977). 13. R. A. Anderson, SAND75-0667, Sandia Laboratories, Albuquerque, New Mexico (Jan. 1976), 14. D. H. McDaniel, E. L. Neau, K. D. Bergeron, Bull. APS, 21, 1146 (1976). 15. I. D. Smith and P. D'A Champney, "Conceptual Design Studies of Pulse Generators for the Sandia E-Beam Fusion Accelerator," PIFR-819, Physics International Co., San Leandro, California (1975). 16. J. P. VanDevender, Proc. Int'l. Pulsed Power Conf., Texas Tech. University, Lubbock, Texas, Paper IIIE-3 (19?6). 17. K. D. Bergeron and D. H. McDaniel, Appl. Phys. Lett., 29_, 534 (1976). 18. K. D. Bergeron, J. Appl. Phys., 48, 3073 (1977). 19. J. P. VanDevender, previous section of progress report. THEORETICAL RESEARCH

OVERVIEW The goals of the particle beam fusion Theoretical studies of magnetic program include the development of pulsed insulation of vacuum transmission lines power drivers for fusion reactors and the were continued and compared with application of these drivers in fusion experiment. A new time-deponJent target experiments. The theoretical electromagnetic particle-in-cell code program has continued to provide target was written to study the evolution of design information for near-term magnetic insulation in coaxial trans­ experiments, including the recently mission lines that are longer than the reported neutron-producing magnetic electromagnetic pulse length. A good targets, as well as designs for Proto II transport efficiency was predicted by and future drivers, such as EBEA I. The the code, in agreement with experimental traditional 1-D Lagrangian hydrocode data for both 0.5 MV and 3.3 MV, design calculations for magnetic targets were complemented by 2-D M-ffi code studies Our understanding of electron and ion of the initial fuel preheat phase using beam production and focusing has been the BEMAG code developed at Sandia. greatly assisted by the use of large- Results from these computations verified scale direct simulation computer codes. the preheating of a small fraction of A newly modified code was completed to the fuel mass and the establishment of provide a complete variable zoning the magnetic field required for thermal capability. This flexibility was com­ insulation. bined with a new method for treating particle emission from tapered The baseline design for EBFA I presently cylindrically symmetric surfaces to employs magnetic insulation of the vacuum provide a tool for studying tapered, lines and the propagation of multiple hollow cathodes, including "parapotential" pinched beams in neutralizing plasma cathodes, a Soviet design for the channels. The beams are to be combined Angara I accelerator, and a cathode at a central target chamber, possibly proposed for an upgrade of the Los Alamos using enhanced electron deposition in a PHERMEX machine. target. Theoretical work was done to study the production and characteristics of current-carrying plasma channels in air, initiated by an exploding wire. Particle trajectory calculations were carried out to determine the efficiency of recombination of multiple beams near a target chamber. Our understanding of enhanced electron deposition in thin foils was advanced by a combined program of theoretical and experimental work on Proto I and Hydra. Hydrodynamic and diode code results were consistent with the measured deposition levels of ~ 10 TW/gm on Proto I.

Fusion reactors with standoff based on multiple beam propagation in background gases are faced with questions of target- produced shock-loading at the first wall. Hydrocode calculations of this problem have provided input data for reactor systems studies. FUSION TARGET PHYSICS

Single-Shell Fusion Pellets for EBFA with Electron or Ion Ablative Drivers lfe are investigating ablatively-driven electron and ion-beam fusion targets for the EBFA accelerator, which has been designed* for multiple diodes and beam combination. Nominal parameters used for the multi- module EBFA in the calculations are 2 MeV, 60 ns pulse length, 0.4 MJ deposited. Л 6.67 TW average power is assumed since it is estimated conser­ vatively that the peak power deposited will be about 10 TW, with a pulse rise Fig, 1. Pie diagrams showing optimized time and fall of 20 ns. shell configurations for 2 MeV electrons at EBFA power levels. A. Electron-Beam Targets Figure 1 shows the optimal shell The carbon/gold shell thickness has been thicknesses for different outer-shell only roughly optimized, by assuming that compositions with 2 MeV electron beams. the total shell pr is about the same A low-Z ablator (iron or carbon) is fraction of the csda range as for 1 MeV preferable because of the decreased electrons5 (0.93) and that the gold/ bremsstrahlung heating of the pusher and carbon mass ratio is 1/3-3/4. The the lower pellet cost. Electron-beam carbon/gold target requires a fuel energy deposition was determined using diameter of 1 cm or greater for efficient functions obtained from least squares transfer of beam energy to the pusher.5 fits to TIGER' planar deposition profiles for gold over the voltage range 0.2-3 MeV, Figure 2 indicates the expected sharp instead of performing separate least drop in neutron yield as target diameter squares fits for various materials at is increased or the symmetry requirement 2 MeV beam voltage. Using the deposition becomes less stringent. The compression profile of iron/gold or carbon/gold at ratio Ro/Rf, where R^ is the initial outer 1 MeV, these gold deposition functions shell radius and Rf is the fuel radius have been rescaled at 2 MeV to account at maximum compression, gives a measure for the broader profile and lower of the irradiation symmetry required** >' bremsstrahlung characteristic of energy (^ 21 for VRf = 50, ^ 4°a for R„/Rf = deposition in a low-Z/high-Z composite 30). For targets with diameters material compared to a high-Z material. exceeding about 0.8 cm, an irradiation The accuracy of this procedure has been symmetry requirement of 2-4? implies a DT checked by comparing the least squares fuel mass exceeding 25 ug. A longer solution for a 0.725 MeV carbon/gold machine pulse length would be preferable deposition profile with the profile as for these large-diameter electron-beam obtained from the rescaled gold functions. targets; implosion times are 100-350 ns The scaled solution has the same general for target diameters of 0.5-1.0 cm. shape but the profile is broader and the bremsstrahlung tail is Si higher at the For the same compression ratio Ro/Rf, csda range than in the least squares target diameter, and total energy solution. Target behavior has previously deposited, Fig, 2 shows that iron-ablator been found to depend on the level of the targets have a yield two orders of bremsstrahlung tail, and to be independent magnitude higher than gold-shell targets. of the precise shape of the major portion In contrast, breakeven calculations at of the deposition profile.'' 1 MeV and small target diameters have indicated" that iron-ablator targets are

46 better than gold-shell targets of the same diameter only for higher compression ratios. 0.014 cm

Fig. 3. Pie diagram showing optimised shell configuration for 2 MeV protons at EBFA power levels. J Figure 4 indicates the variation in neutron yield with target diameter for fixed compression ratio Ro/Rf and the Fig. 2. Neutron yield versus target variation in yield with DT fuel mass. diameter for iron- and gold- For target diameters exceeding 1 cm, ablator targets at EBFA power there appears to be little improvement levels. Curves are for constant in yield unless one goes to a larger

compression ratio RQ/Rf. compression ratio. An irradiation symmetry of 3-5» requires a fuel mass О 11 exceeding 50 ug for target diameters in Magnetic thermoinsulation ' has not excess of 0.8 cm. The gain (energy been included in these calculations; produced/energy deposited) is 0.001-0.006 such calculations are planned and it is for these targets with 6.67 TW deposited; 50-801 of the thermonuclear yield occurs expected that the yield will increase by the free-fall time. The aspect ratios significantly. for these targets (10-40) may be a problem B. Ion-Beam Targets unless an energy spread on the driving source is employedll to reduce sensititivitv y Figure 3 shows the optimal shell to fluid instabilities. thickness for an Li D-shell target with 2 MeV ion beams. A low-Z material must A machine pulse length of 60 ns is a be used for the absorption region, in closer match to the implosion time for spite of the reduced tamping effect of these targets as opposed to the electron- the outer part of the shell, to reduce beam targets. The 1 cm diameter targets radiation in the form of thermal x-raysJ have an implosion time of 60 ns; the and to improve tolerance to Rayle.ifih- 0.6 cm diameter targets have an implosion Taylor instabilities."'7 A range-energy time of 30 ns. relation in lithium for protons with energies less than or equal to 2 MeV,,. Calculations with magnetic thermoinsula- given approximately by10 Ep = 40.7X1'- , tion8,'H are planned. where Ep is the proton energy in MeV and X is the range in g/cm', has been used in the calculations.

47 Multiple-Shell Fusion Pellets for EBFA

We have begun an investigation of double - shell ablative targets for EBFA. The purpose of the study is (1) to determine neutron yields possible with double shells for electron-beams and ion-beams and (2) to determine some guidelines for optimization of multiple-shell targets.

Our major concentration so far has been on an electron-beam target, though a number of the guidelines we obtain should he applicable to ion-beams as well, in spite of differences in the energy deposition. The electron-beam target being studied is shown schematically in Fig, 5. Nominal EBFA parameters are the same as for the single-shell designs described elsewhere in this report. Ke have not used a liigh-melt-energy material such as tungsten for the inner shell in our initial study, in spite of the fact that the detrimental effects of bremsstrahlung preheat are reduced,12 Fig. 4. (a) Neutron yield versus target and we have not considered the possibility diameter for EBFA ion-beam of a magnetic-thermoinsulation target.13 target of Fig. 3. Curves are Both concepts are inappropriate for a for constant compression ratio parameter study since computer costs ,/Rf (b) Neutron yield versus are increased considerably. 5? fuel mass. Curves are for constant target diameter. 0.5(M2cm г-" Fe С Scaling to Breakeven 1.705g

We assume that machine scaling 0.4242 cm l— Au O.Wg to higher poKers is such that the beam 0.4000 cm V- pulse length would be the same as for EBFA and that the pinch radius would be the same or larger than expected for EBFA. BUFFER \ GAS A breakeven graphite-carbon-ablator target (cf. Fig. 1} for 2 MeV electrons has been modelled on CLYDE. The fuel diameter is &K 1 cm, fuel mass is 600 jjg, implosion time is 67 ns, beam power is 2000 TK, 100 MJ of energy is deposited, and the compression ratio implies 3$ uniformity of beam deposition and shell thickness. A gain of about ten would require 5000 TW of power. The major factor contributing to the high power requirement is target size 0T rather than beam voltage. It seems obvious that one must look elsewhere--to magnetic thermoinsulation8,11 or other advanced target concepts--for achievable breakeven designs. Fig. 5. Schematic diagram of double- shell electron-beam target under study for EBFA parameters. All the CLYDE calculations assume a DT we have restricted our study to targets fuel mass of 10 jig. The buffer mass Mb, with a simple gold shell filled with inner shell mass Nh_, and buffer to inner gaseous or levitated-cryogenic DT in shell thickness ratio ARu/AR- are varied order to reduce the number of parameters.

to maximize neutron yield, subject to < The two questions we have addressed in the following constraints: (1) ARb/AR; "v. the study are: 8. One-dimensional (planar) analysis1'' has indicated that a buffer to inner (1) Does voltage shaping increase shell thickness ratio of 8 or less the beam impedance needed for breakeven requires 1% uniformity of surface finish and gain? to limit the growth of instabilities, (2) RDTMRJ < 20 for shell stability.15'16 (2) Does voltage shaping increase (3) The ratio of the radius of the outer­ bum efficiency (the fuel bum-up most shell to the final fuel radius fraction) and target pr? RQ/RJ ,< 50 so that symmetry requirements are not too restrictive. We have chosen to express our guidelines Beginning o' tm] «I in terms of the mass ratios Mo/Mi and Beam pulse Ilium pi Mi/Mb and the thickness ratio ARb/ARj since these parameters immediately characterize the fabricated design. However, a more judicious choice of parameters could provide better insight into why target performance is better in some cases. For example, the ratio of the minimum thickness of the buffer to the inner shell thickness1'' may be a more relevant parameter than ARb/ARi. ГЪг an acceler.-iting shell to extract the most internal energy from the buffer, the inner shell should be thin compared to the minimum gas thickness.!?

Our best preliminary design to date lias without voltage shaping Mo/Mi = 10, Mi/^ ='9, and AVARi = ?• Neutron yield is 1,5 x 109, Rgr/ARi = with voltage shaping 8.1, Ro/Rf ^ 55.

Voltage Shaping with Electron Beam Fusion Targets Fig. 6. Schematic diagram showing variation in power deposition As an ablatively-driven implosion per mass with radial depth in proceeds, the shell pr increases and an e-beam target, with and energy is deposited further out in the without voltage shaping. ablator and is therefore used less efficiently late in time. If one increases the electron-beam voltage An increase in impedance is desirable during the pulse,13 energy penetrates since, for a single low-impedance, large- deeper into the shell late in the pulse aspect ratio diode, a substantial (Fig. 6), giving better coupling into fraction of the total cm-rent is in ions the target and faster inward acceleration and, even if ion flow is suppressed, of the shell. We have continued our pinch stability, gap closure, and radial analysis19 of voltage shaping with compression impose a lower limit of electron beams (the method is also 0.1-1 ohms on the impedance,1 Multiple applicable to ions) as a means of diode geometries22 are therefore necessary improving target performance. The to keep the impedance Z

49 combined beams (Zj/number of diodes) voltage shaping, the optimal shell small. We find that the impedance thickness is the same as with an unshaped increase of a factor of 2 to 4 obtained pulse; for constant-impedance voltage by voltage shaping is of the same order shaping the optimal shell thickness for as that obtained with simple design a given initial voltage V^ is greater improvements (iron ablator,4 velocity than for an unshaped design with constant multiplication,9 reduced deposition beam voltage Vj. The choice of which lengths and reduced bremsstrahlung at type of voltage shaping to use depends high currents23). The use of voltage on whether a lower breakeven current and shaping with one of these schemes or with the same or a lower power are desirable. more complicated target concepts which Constant-impedance voltage shaping allow even higher beam impedances produces the greater increase in imped­ (magnetic thermoinsulation,!3 snail ance, but increases the peak power and central ignitor24) will further reduce current. Variable-impedance voltage the number of diodes required in a shaping produces a smaller increase in multiple-diode geometry. impedance but reduces the power.

Figure 7 shows a typical voltage pulse Both of the advanced target concepts shape for an electron-beam-driven target. mentioned above (magnetic thermoinsulation The effectiveness of voltage shaping is and central ignitor) have the disadvan­ quite sensitive to the time shape of the tages thai, burn efficiency and target pr beam pulse with respect to the dynamics are reduced.24 Figure 8 indicates that of the metal shell. If the voltage rise voltage shaping can increase burn is more rapid than shown in the figure, efficiency and target pr at breakeven excessive shell preheat occurs, reducing levels for a simple gold-shell target peak shell density and degrading target filled with gaseous or cryogenic DT. performance; if the voltage rises more The targets--with and without voltage slowly than shown, energy doesn't shaping--have higher target pr and penetrate deeply enough and the advantage increased burn efficiency for greater of voltage shaping is lost. energy deposited (corresponding to higher initial voltages). At higher voltage the shell is thicker, ablation pressures are higher, and the shock wave starts further out, preventing the shock from entering the fuel and shock heating it until later in the implosion. The performance of cryogenic-ЕГ targets is improved by voltage shaping. Proper pulse shaping delays the arrival time of the inner edge of the fuel at the target center by generating a weak initial shock at low voltage and low power.

Similar improvement in burn efficiency and target pr can be expected with the advanced targets. Such a study of magnetic-thermoinsulsii->n targets is in progress. 0 35 Fraction of Beam Pulse Length

Fig. 7. Typical voltage pulse shape for an electron-beam-fusion target.

There is some possibility of independently varying voltage and current, so we have investigated two types of voltage shaping (cf. Table I). For variable-impedance TABLE I

Current and Impedance Requirements with Voltage Shaping

Trpe of Breakeven Current Breakeven Impedance Voltage Shaping Compared to "Unshaped" Compared to Unshaped

Constant-impedance Same - 3—li higher (Power a Voltage )

2. Variable-impedance 2/3 1.5 higher (Power is constant, linear, or trapezoidal function of time)

ConsUnHinpertaiHe Voil*].1 l

VdNiB'e Inpt-lidnL" VilllJUl'

Dl irvO<]fr.iL

jnMijpi'O. D! LIV.T":I

. _ llnsi'jpeii. Dl :

a_..j_ _i i_ ]A to

I Г Г I I ConslanlUiipt'ddnti! VoltJ

Variable Impedance Val1aqf "ihapinq - Dl <.гумг.пк

—. — Unshaped. Dl сгуярпц __ • Unshaped. Dl gas l_ I I I I L_ 6 8 10 12 U lb tncrqy Deposited iMJ)

Fig. 8. Burn efficiency and maximum target pr versus the deposited energy necessary to produce breakeven with and without voltage shaping. A larger amount of energy deposited corresponds to a larger voltage and greater shell thickness. Proto II Magnetic Targets The BEMAG 2-D Eulerian MUD code, described in an earlier report," was Calculations have been performed to used to compute the fuel behavior during project the performance of targets the preheat phase for the magnetic targets containing preheated fuel and magnetic used in the recent experiments. The fields, assuming the power absorbed in results for a typical gas discharge case the shell is consistent with that which are shown in Fig. 9. The figure plots might be expected from Proto II (.25-4 TW density contours at four different times absorbed). The present target preserved in the preheat phase. Contours for

the shell ^>lb of the targets successfully 0.25 o0 and 1.0 p0, where p0 is the employed in the RE11YD experiments initial fuel density, are plotted at each (.3 g/cm^), but was sized to provide an time. Trie flat horizontal electrode at implosion time consistent with the "- 50 ns the top of the target collects electrons current pulse mode of Proto II. Л target emitted from the cathode in the prepulse inside diameter of 7 mm was chosen. phase. This in turn drives a discharge current thi'"iigh the fuel region to the For magnetic targets, the fuel density is anode, represented by the cross-hatched the single most important parameter region at the base of the target. Since governing the convergence ratio of the the outer shell containing the fuel is implosion._ A DT fuel density of constructed of an insulating material, *> 1C|S g/cm-> for the 7 mm target and a the current flows down the center elec­ modest assumed preheat of 15 eV were trode supporting the collector and then found to produce substantial neutron around the oJtside of the fuel. As the yields (109-10l3) with moderate conver­ field diffuses in, the highest current gence ratios (10-13), for a variety of density (and ohmic heating rate) is levels of absorbed power. The results present near the electrode. This is of a series of target calculations is seen in the first portion of the figure found in Table II. Thermal conductivity at t = 0.2 psec. By t = 0.4 usee, the was assumed reduced by a factor of 36, ohmic heat.ng near the electrode has to the ion conduction level for DT. resulted in a flow of hot fuel towards the center of the sphere. This in turn leads to a pile-up of cold, dense fuel at the outer surface where it contacts Magnetic Target Preheat Calculations the insulating container. At later times the volume occupied by the hot, low density fuel increases. By The principle of magnetic thermal t = 0.8 usee, the central region contains insulation has been successfully employed only about № of the initial fuel which in electron beam fusion targets. (A does, however, have a temperature of discussion of recent experiments is found 15-20 eV. This fractional fuel mass elsewhere in this report.) Reduced heating is an essential feature of the thermal conduction and fuel preheat lead preheat phase. to much lower driver power levels than would otherwise be needed. The technique has promise not only for near term targets In addition to heating the fuel, the but also for high-gain advanced targets.il prepulse current also must establish For these reasons the design of such the magnetic field required for insula­ targets are of great interest. There are tion. This can only happen, however, two basic phases in the operation of a if the diffusion time of the field into magnetic target. The first of these is the fuel is shorter than the prepulse the preheat phase, in which a discharge time. That this occurs is shown clearly current is passed through the central in Fig. 10, which plots contours of region to heat the fuel and to establish total enclosed current at t = 0.8 ysec. the magnetic field required for thermal Most of the current flows in the central insulation. The second phase is the fuel region and the later target implosion of the target by the main beam implosion by the main pulse is fast pulse. This latter phase is usually enough to keep the field frozen into studied using 1-D Lagrangian target the fuel plasma. design codes.

52 TABLE II

Convergence Ratio T Specific Power Total Power yield max (TW/g) (TW) (no. of neutrons) Ml (g/cm2) -It 5 .27 It.7 X 109 10.3 7U0 3.3 x 10 ч

i -It 10 .53 2.8 x 1010 11.2 980 lt.0 x 10

20 1.07 2.0 x 1011 12.1 1360 It.6 x 10"U 12 40 2.13 1.7 x 10 12.8 1930 5.0 x 10"1*

13 -it 80 It.26 l.lt x 10 13.5 2960 5.2 x 10

FRACTIONAL FUEL MASS PREHEATING

T-D, 2 «ec T"0.4|iS«

Fig. 9. Density contours (0.25 p0

and 1,0 p0) at four different times in the preheat phase.

T-o. 6 psec Т-0.8(1МС

MAGNETIC FIELD PENETRATES ON PREPULSE TIME SCALE

С

2 1/3 Fig. 10. Iso-current lines at t = 0.8 usee.

\\\\\\\\

53 Hydrodynamics of Power-Deposition Driven

Expansions" 1/Z 2 „ - [Y(Y l)e] t Expansion x Analytical and numerical studies have o 3 + a Wave Position been performed to model the expansion of 1 г material under the influence of continu­ ous, in-depth deposition of power, such p = p (Case 1) Density as power deposition by high energy parti­ о о cle beams (see Fig. 11). The governing M hydrodynamic equations have been studied p = (Case 2) Density and found to reduce to a self-similar set o Л1Г for the two planar cases: (1) ablation from the surface of a thick wall, and (2) the expansion of a finite mass foil of negligible thickness. For each case, an ideal gas equation of state is employed and negligible initial internal energy is assumed. Self-similarity is obtained for a time dependent, spatially uniform power deposition function of the form P = e(l + a) ta. High energy • particle flux —^ (eg. e , ions,. . .) The equations of continuity, momentum, energy, and state employed are:

Continuity |£+P^+v||=o

H+ v2v+ l_3p_ = Momentum 3t Эх р Эх Case 1. Ablating Wall Case 2. Exploding Foil

Energy |£ + v |e + E |v = = E(1 + a) ta at эх р эх Fig, 11. Energy deposition from particle

E.O.S. E = CY - 1) e beam into a thick wall (Case 1) or thin foil (Case 2). It has been found convenient to eliminate the pressure, p, in favor of density, p, and internal energy, e, and to normalize these parameters and the velocity, v, to .Moving with J sound speed, C the internal energy, e , speed of sound, 0 c0, and initial density, p0, for case 1 or mass per unit area divided by magnitude of the expansion wave position XQ, for case 2 (see Fig. 12). The independent position variable x is also normalized to XQ, which is obtained by integrating the speed of sound c0 with respect to time. The normalization variables are

(1 • a) e0 = et Energy Fig. 12. Expansion wave from an 1 + о ablating thick wall.

1/2 2 c0 = [Y(Y - l)e] t Speed of Sound The normalized, self-similar variables are e + (Y - 13 е-ё-. *- V£M^ ) for which plots are shoiiii in Pig. 13. E = — ,

R-2-. °o Tlie self-similar equations are Energy

dE ^ „ dE + dV 6E - 5 T Vv ж CY - 13 E £- - df 3? d? Momentum

6 ,, , dV , dV 1 dE I E dR = , 2 V " k d£ d? Y d? " Y R dC Continuity (V - 5) j2. + R g^ = 0 Case 1 (wall) I-'ig. 13. Solution for exploding foil expansion.

gr [R(V - 53] = 0 Case 2 (foil) In terras of dimensional variables, the solution for Case 2, the exploding foil, with boundary conditions for Case 1 V(-l) = 0 *-(rh)h EC-ID = l J\ * a) R(-13 = 1 1 + and for Case 2 Ш' V(0) = 0 and m / (1 + q)(3 * a)

R(53 d£ = 1 (Y " 13

r 2 fl + o)(3 + a) x exp where C3 + a 8(Y- 13 Et ) 2(1 • a) 3 + a An asymptotic solution of the Case 1 equations, for large values of 5, is The solution for Case 2 is

V= 5

55 increasing leading edge velocity as a function of increasingly fine zoning of the leading edge. In real flows, when the collision time, x, exceeds the time scale t in the far blowoff region, the continuum formulation of hydrodynamics breaks down and a kinetic theory approach is appropriate.

(Y - 1) Another significant feature of these solutions is tiie nearly uniform tempera­ ture implied by E •* const for large £;. and This suggests that even at high tempera­ ;Vs2 + 4L + s2] tures, conduction energy transport will R ^ Rr exp be small, because of small temperature 4t J > gradients. Finally, it is noteworthy that for t < TJ/2 the Case 1 solution where applies to finite thickness foils, where т\/2 is the time for the expansion wave L = Y(Y - 1) + BY to reach the center of the foil, from each surface. For t » TI/2, the Case 2 The full solution for Case 1 is shown in solution applies. Fig. 14, which is obtained numerically.

REACTOR AND STANDOFF RESEARCH

Plasma Channels for REB Propagation

The propagation of multiple pinched electron beams*i26 to a target chamber has many attractive features, including a standoff capability for a reactor and reduced pulsed power requirements compared with ultra-high-current single or double diode configurations. A summary of recent work in this area is described elsewhere in this report. A key element of the propagation scheme is the formation of a long uniform plasma channel to neutralize the space charge and current of the injected beam. This Fig. 14. Solution for ablating wall plasma channel also carries an externally expansion. supplied discharge current to confine the beam electrons to the channel. An interesting feature of the power driven expansions studied here is the Present experiments have used an exploding unbounded domain of the solution, which tungsten wire to initiate a capacitor results from the assumption of uniform bank driven air discharge. It is felt power deposition per unit mass and the that the exact mechanism for initiation continum formulation. This compares of the discharge is unimportant, so that with the bounded domain of the adiabatic a laser preionization could be used for expansion solution for which the driving a reactor based on beam propagation. energy is the non-zero initial energy of We have constructed a 1-D MID code to the material. Numerically, using study the evolution of the discharge. Lagrangian hydrodynamic codes, the These code comput? '.ions have been used unbounded solution for power driven to provide a comparison with density and expansions is manifested in an ever- temperature measurements of the channel,

56 as well as providing a design tool for In this study, an injected electron Proto II and EBFA scale experiments. distribution was sampled by varying the initial phase space coordinates, calcu­ The code, entitled CHART-B, combines a lating the 3-D trajectories, and recording 1-D Lagrangian hydrocode (with radiation the distance of closest approach to the diffusion) with a magnetic field solver. center of the configuration (r = 0, z = 0). The hydrocode is described in the Note that the assumption of current aforementioned summary contained elsewhere neutralization implies that the electron in this report. The most recent modifi­ trajectories are noninteracting and cation of the code is the incorporation therefore simple superposition occurs in of a circuit equation to describe the the beam overlap region. Scattering and electrical coupling of the channel and electric fields are assumed negligible the capacitor bank. The peak channel and symmetry of target irradiation is currents computed compare with the not evaluated. measured values to about 10-205. In addition, the treatment of the electrical The variation in the distance of closest conductivity has been improved by approach to the center of the configura­ incorporating theoretical data published tion (Rmin) attained for different by Viegas and Peng27 for the conductivity injection conditions is shown in Figure of air at low temperatures 16. The results of Figure 16, when (2000°K < T <^ 20000°K). weighted with a realistic, non-hollow, injected beam distribution should actually produce better beam concentration than apparent in the figure. In Figure 16a, REB Combination 1 MeV electrons were injected at the

channel radius (rc) with a velocity nearly The general problem of REB overlap is to parallel to the channel axis. The determine a plasma and field geometry electrons execute betatron oscillations which achieves efficient beam superposition approximately in the injection plane of at a target and is consistent with target the channel. Inside the channel overlap requirements. As a first step, we have region there is cylindrical symmetry, so analyzed a single, simple case and have that when the injection plane is aligned neglected specific target requirements. with the x-y plane (a = 0), all of the betatron oscillation can go into angular Of the many possible channel transport momentum (рф) in the overlap region. and overlap schemes, the one considered However, for a = 90°, none of the consists of 40 uniform current-carrying betatron motion appears in рф when the electron passes into the overlap region. channels (radii rc) arranged circumfer- entially around the target with return Figure 16b shows the result of varying current channels forming the axle of a the amplitude of the betatron oscillation wagonwheel configuration.28 Figure 15 for an intermediate injection plane angle shows one concept of how such a wire (a = 45°). In Figure 16c, the axial configuration would fit in the front end variation of the electron distribution of a multi-beam accelerator. For a given is obtained by moving the injection point (r = rc, a = 45°) along the channel for plasma channel radius, rc, the converging channels begin to overlap at a radial a betatron cycle. In Figure 16d, electrons distance RQ from the center of the are injected at the channel edge with an configuration. The plasma current along increasing non-axial velocity component. a beam channel is assumed to be constant in as far as the channel overlap radius From these results, it appears that most (RQ), where it linearly decays over a of the injected electrons from a realistic specified distance as the individual beam can get inside a radius roughly one- currents merge into a disk current. A quarter of the channel overlap radius at channel current of 20 kA was chosen for the hub of the configuration, regardless 1 MeV electrons to provide good contain­ of the length of the channels. An estimate ment of the beam, while remaining below of the beam concentration (current density the Alfveri-Lawson critical current of gain G) in these calculations is obtained about 50 kA. The two axial return current by comparing the net current density channels are assumed to have expanded to crossing the cylindrical region of radius twice the radius of the beam channels. Rmin and height 2rc with the current

S7 density in a single channel:

G = Jinet' Ji Jc = 10 гc /'R mi. n~ 5. This result depends sensitively on the actual plasma current configuration near the center of the system. We should note that the channel studies reported here have not Been optimized, and higher gains may be obtained by reducing the overlap radius using different channel configurations and/or smaller channels. I

Fig. 15. Concept for multi-beam front end for EBFA.

Fig. 16. Summary of point-of- closest-approach results from 40 beam calculation.

58 Electron Deposition in Thin Targets for studying magnetically enhanced deposition in targets of arbitrary Recent interest in electron deposition thickness. The diode code can predict has centered on enhancement of colli- deposition (A) in the limit of negligible sional deposition resulting from particle beam target interaction or (B) with a dynamics in strong self beam and/or diode simplified scattering and energy loss macroscopic fields. The increase in model. This model contains self- deposited power per unit mass is simply consistent E and В fields and is useful the result of an increased path length for computing deposition in very thin in a given amount of material which can foils in realistic diode geometries. be caused by electrostatic reflexing2J or by magnetic beam stagnation^-30 or Theoretical predictions of deposition diffusion.31 for the Hydra experiment are shown in Fig. 17. Plotted is specific power In general, electron deposition can be deposited in the focal volume vs. Ip = described as current enclosed within pinch radius rp. The focal volume is the region in the

foil with r < rp and rp is 1/2 of the FKHM of the current density profile. PM = e fIS) 8c SIS) d£ , CD The voltage is 1 MeV, rp = .IS cm, and the foil is a 6 pm thick, planar Au л> foil. The upper two curves are Monte where P., = deposited power/mass (w/gm), Carlo code predictions with non-self- e = 1.6nx 10-19 col f(g) = electron 3 consistent E and В fields for either density/energy (cnr Volt -1), 6c = isotropic or normal incidence to the electron speed (cm/sec), and S(S) = foil. A gaussian J and п profile, stopping power CVolt/gm/cm2), The beam ь truncated at r = rp, is assumed with density n. is given as E = 106 v/cm in front of the foil and AAX E = 0 behind the foil. It is also assumed that В penetrates the foil. nb = f <£) dS . (2) Note the increase in Рм as Ip increases for Ip > 1д = 47.4 kA, reflecting For a monoen°rgetic beam P„ = deposition enhancement. Also shown are analytic estimates obtained from Рм = e nb 8c S(<£0). In the energy range of 6 e % Sc S for charge neutralized Haraner- interest 1-10 MeV, 6 = 1 and S s 10 3 Volts/gm/cm2 and depends weakly on Rostoker ^ equilibrium beam and for a material. Therefore to increase Рщ the Bennett equilibrium beam. For these estimates one obtains n from the equi­ beam density must increase. If we D reference Рм to a cold, normally incident librium beam conditions with the beam with current density J (amps/cmz), remaining parameters fixed by the beam then an enhancement factor x can be voltage. It is seen then that enhanced defined as x = Щ e 8cJ_1 = 8/8N, where beam density is implicit to high current бм = <%>/c and <%> = average normal beam equilibria and that deposition velocity. One can further relate the estimates are somewhat reduced from the normal drift velocity due to curvature non-self-consistent Monte Carlo calcu­ and grad Б drifts, assuming В а г or lations. For these estimates it is В о r"l, such that x * I/rA> where I is assumed that the beam equilibria is not the enclosed current and 1д = 17000 Sy. significantly modified by the interaction This enhancement results from magnetic with the foil. The variation in the stagnation for large v/y beams since the upper four curves is thought to qualita­ beam density increases as the beam drift tively represent the theoretical velocity becomes small. uncertainty due to uncertainty in the source beam. The lower line represents the reference deposition level of a cold, The conputational models used for studying paraxial beam. enhanced deposition include both (I) Monte Carlo models and (2) diode codes. Versions of the Monte Carlo models are Examples of solutions using the Monte (A) collisions only, (B) collisions with Carlo model with self-consistent non-self-consistent E and В fields, and magnetic forces are shown in Fig. IS. (C) collisions with self-consistent В A Hamroer-Rostoker bean with charge fields.4 The Monte Carlo model is useful neutralization is injected from the

59 left boundary and propagated into a Examples of diode code calculations for foil as shown. Plotted are a few the Hydra parameters are shown in Fig. representative particle trajectories for 19, Figure 19a shows a diode configu­ a beam with energy 1 MeV, radius = .IS cm ration with a planar hollow cathode. The and total current = 94.8 kA (v/y = 2), anode consists of an absorbing boundary The case on the left is for gold foil of at 720 kV which approximates a carbon areal density 1.93 x 10"3 gm/cm2 expanded anode in the experiment. A typical to .01 cm. The dot density plotted electron trajectory is shown, The diode within the foil is proportional to the code predicts a pinch with FWHM3 1.5-2 mm local P[,;. For this very thin case the which approximates the experiment although beam is not significantly perturbed by the current density was ^ 2-4 x higher the foil and uniform deposition of than in the deposition experiment. "o 4 TW/gm is calculated within the beam Figure 19b shows a similar diode except radius, which is consistent with that that a thin 6 urn Au foil, backed by given by the unperturbed beam density vacuum, is located near the axis. This and particle energy. The example on the configuration is similar to the thin right is similar except that a foil with foil deposition experiment. Note that areal density of 1.93 x 10'2 gm/cm2 is this model indicates an increase in beam assumed. For this case, scattering is density on axis for the thin foil case. more pronounced and there is some The depositon level inferred from the reduction in Pj^ near the beam axis which peak beam density is ^ 6 TW/gm which is results. The deposition averaged over qualitatively in agreement with the the focal volume is ъ 3.2 TW/gm which Nfonte Carlo calculation for pinch radius represents only a 20% reduction of the of .15 cm and pinch current of 100 kA. unperturbed beam case. Since these parameters approximate the conditions of Comparisons between calculations and the Hydra experiment, we would not expect experiments were also made, A planar a substantial reductions in F» due to anode foil experiment was performed with modifications in В due to scattering in a tapered (10 ) hollow cathode o.d. the foil. 7.6 cm and i.d. 3.56 cm was used with a thick carbon anode (.635 cm) with a hole of 2.54 cm dia. in the center. Various foils were then mounted over the anode hole in the plane of the anode. There was vacuum on both sides of the foil. Surface temperature of the rear surface HAMMERROSTOKER of the foil was measured using x-ray diodes, optical diodes, and spectro­ scopy. The beam was also diagnosed using time-integrated x-ray pinhole photography and a collimated PIN detector.

Fig. 17. Theoretical predictions for Hydra experiments.

ok^-r—i 1 1 1 1 0 50 100 150 200 250 CURRENT IN PINCH FWHM (К AI 3.0 1 3.0 • • ' .... • ' • • ... • • ' ' 2.3 •= 2.8 2.6 » 2.6 • ? 4 2.4 э t 2.2 2.0 2.0 i.S i.S 1.6 а 1.6 ~~-^—^ 1.4 ^w^3 1.4 1.? 1.2 у^Дл/ 1.0 1.0 о. я (1.4 т\л N • 0.6 0.6 0.4 0.4 0 ? 0.2

•1. 0 п. С 0,< 1? 1,я ?Л 2. J ••A 4.(1 0,4 П. 8 1.? 1.6 2.0 2.4 2.S г ,10-' »,о-'

|«п Л 1 1011 1(1/,|" 4п ГОН

Fig. 18. Beam-foil interaction with v/y = 2 using Monte Carlo model with self-consistent В fields. Beam with Hammer- Rostoker equilibrium injected from left.

У/

331 A /

К ~э 155 (Г / / 2 с: _3 V 0.00 L 0.00 0.43 086 0.00 086 z Z SOLID ABSORBING ANODE VACUUM BACKED bum FOIL NEAR AXIS

V(KV) 720 720

le (KA) 290 300 lj (KA) 135 125 1, (KA) 425 425

-3) 05 I 1015 12 I 101S nbma- x

Fig. 19. Diode code solutions for Hydra diode.

61 Figure 20 shows results of this experi­ with №1= .16-.2 cm. For these ment and is a plot of temperature vs. conditions ке would expect PJ,J * 10 TK/gm time for shot #5706 for which a 6 pi from previously discussed theoretical Au foil was used. Calculated solutions models. (---) as well as experimental data using XRD measurements ( ) and optical diode measurements (—•—) are shown. "I 1 1 The calculated solutions are obtained /4 5TW/GM using the CHARTD code34 with planar geometry which assumes the foil is optically thick. The deposited power/ HYDRA mass is constant in time and space at the »h SHOT 5706 level indicated. The hydrodynamic 6 /-m Au FOIl calculations indicate a temperature profile which is peaked at the center of the foil and which decreases to some plateau near the outer portion of the

blow-off. Both the optical and soft OPTICAL x-ray depth are such that temperatures OIODE in the plateau region are measured and both diagnostics should measure essen­ tially the same temperature. The XRD data corresponds to spatially averaged black body temperature associated with the hot focal region of the beam. The associated error bars correspond to uncertainties in sensitivity of the detector and the source area. The optical diode data results from an absolutely calibrated photo diode. Spectroscopy was also employed to insure that strong line radiation was not emitted in the range observed by the photo diode. Both measurements are consistent and indicate F>1 ^ 1 TW/gm. The data here Fig. 20, Temperature vs. time for are typical of many shots for which temperatures •$ 5 eV were measured. Hydra experiment.

If we take the most optimistic values Results of surface temperature measure­ for current density and pinch radius as ments and hydrodynamic calculations are that given by x-ray pinhole data for a shown in Fig. 21. Plotted is tempera­ thick carbon anode, then the peak J is ture vs. time for (A) a 6 ym Al foil only 600 kA/ciri2 and the pinch current and (B) a 6 p Au foil. For the Ip = 45 kA. This implies deposition calculations, the CHARTD code was used levels of about 1 TW/gm (see Fig. 17) with planar geometry and the tempera­ which indicates little enhancement and ture obtained at a depth of 10 "4 gm/cm^ which is consistent with experimental from the front surface corresponding to measurements. the maximum depth observed by the soft x-ray temperature measurements. The Another series of experiments was per­ deposition was assumed uniform in space formed on the Proto I accelerator. The and constant in time with t = 0 diode physics are different than that corresponding to pinch formation time. of Hydra since a much smaller cathode Radiation losses were assumed negligible. fy 2.5 cm diameter) is used in a plasma Note that calculations predict that for filled diode. A much tighter pinch is a given deposition level a higher produced with higher J and with a large temperature is produced in the gold foil fraction of the total current enclosed. than for the aluminum foil which is a X-ray pinhole photography indicates result of greater foil inertia, and less that a single beam incident on a thick expansion cooling, for the gold. carbon anode produces peak J г 5 MA/cm'-

62 100 ; i 1 r . I- - 1— i : : - - s* *L-—"" *>s' ^^*^U> -

• /* ^»--"-"—"~*^' 16

ю '•. // il^-—•""—""*^ - ; /ф^—" * i - /j&& ^8 : ^ P = 1 TW/gm - • /?т&£*^~\^~- -"J—"^ 1 . 1/ S 2

рм= 1TW/| i • • 20 40 60 20 40 60. 1 TIME (usee) TIME (ns«c) bum Au FOIL 6A/m Al FOIL

Fig. 21. Temperature vs. time for Proto I experiment.

Also shown are surface temperature Preliminary Studies of Blast Loading at measurements obtained from x-ray diodes the First Wall of an Electron Beam for the respective foils using a single Reactor Chamber side of Proto I. Time = 0 corresponds to the pinch formation time. Again Estimates of the blast wave pressure at uncertainties in the emission area lead the first wall of a gas-filled electron- to uncertainties in the measured tempera­ beam reactor chamber have been determined ture. The Al foil data, however, is from the CHARTD34 code, using the Lock­ pessimistic in that the entire apertured heed tables35 for the radiative area is assumed to determine the lower properties of air, and analytic point temperature bound. For the Au foil, source solutions for a spherical blast x-ray pinhole photography and PIN wave. Such a model has previously been detectors were used to estimate the used to_study atmospheric weapon emission area. From this data we con­ effects56*37 (i.e., fireball growth and clude PM * 4-6 TW/gm for a 6 um Au foil shock propagation from nuclear and Рм г s-16 Ttf/gm for the 6 um Al foil. explosions), which is analogous to the problem studied here. The pressure Diode code calculations for the above pulse at the first wall was obtained as conditions indicate that scattering in a function of the energy from the pellet the gold foil tends to reduce the implosion, the ambient ^ir pressure in deposition over that of a 6 um Al foil the chamber, and the chamber radius. where scattering is negligible. We feel The energy from the pellet implosion this and differences in ion diode current is the energy deposited in the pellet due to the ion mass may explain the plus the energy in the PT fuel and in differences in deposition levels between x-rays after thermonuclear bum: neutrons Al and Au. will not contribute to the srowth of the blast wave.

*S Calculations were done on CHARTD for two We conclude that for these pellets, the hypothetical pellet implosions, with 1 reactor chamber radius will be determined atmosphere chamber fill pressure, "sing,, by the neutron flux because the reflected the energy partition of M. J. Clauser's38 shock overpressure from the pellet debris 2.4 urn diameter gold/DT electron-beam decays rapidly with distance and is pellet. Characteristics of the energy in «: 100 atmospheres at 2 meters for these two pellet implosions are given in pellet gains of interest (-6 50). The Table III. based on CLYDE calculations peak value of the reflected shock over­ for Au/DTS8 and Fe/Au/DT4 ablative pressure with 1 atmo chamber fill

pellets. At the onset of a CHARTD calcu­ pressure P0 is about 144 atmo at 1 m, lation, energy is deposited on the pellet 12 atmo at 2 in, and 1.8 atmo at 4 m for to simulate the energy deposition; at Pellet #1; for Pellet #2, it is 137 atmo the implosion time 1/5-1/4 of the thermo­ at 1 m, 22.5 atmo at 2 m, 2.6 atmo at nuclear energy produced is deposited in 4 m. Figure 23 shows shock overpressure the fuel to simulate the results of the UP and time elapsed since energy burn. deposition in the pellet began versus reactoT chamber radius for Pellet *2. Reflected overpressure ДР is obtained Figure 22 shows the growth of the fireball Г and the blast wave for Pellet #1. Rapid from ДР using the Rankine-Hugoniot radiative growth of the fireball occurs equation, on a time scale of 10 ns due to beam deposition. The fireball sits at about CY+1) ДР 10 cm until debris expansion from the ДР = 2 ДР + + pellet implosion reaches the fireball at 2y P„ CY-D ЛР about 1 usee and the fireball begins to "shock up" and grow by compressing and doing work on the surrounding air.

TABLE III

Energy of Fictionalized Pellets

Energy Thermonuclear Energy Energy Pellet Deposited Energy Released in Ions in # (MJ) (MJ) and X-Rays Meutrons (MJ) i£ (MJ) (*)

1 It l+O 8 20 32 80 2 3 60 15 25 <*5 75

64 Fig. 22. Growth of fireball and blast wave with time since the pellet implosion for Pellet #1 (cf. Table III).

0" I 1 ! 1

3 - \ -5 0- \ / - i

Э - \ / - 3

o- - 2

D - 4 / ч у -

REACIOS CHAMBER RfttMUS (netsrs'

Fig. 23. Shock overpressure and time elapsed since the pellet implosion versus reactor chamber radius for Pellet #2 (cf. Table III).

65 At increasing distance from a finite but TABLE IV sudden source of energy, the resulting blast wave appears more and more like Shock Overpressure as a Function of that of a point source. Equations for Reactor Radius R and Energy E the point source spherical blast wave 36,39-40 are P = 1 atmo о

Shock overpressurerAP: Y = 1.4

= Х for ДР/Р >. 10 (1) ДР/Р,о Y*T [ГС 0

0 4 1 m 2 m h m 5 m 2Y J -2 (2] ДР/Р„ [0.14 X" + 0.11S X Y*T

1 5 MJ 9 2 O.k 0.3 + 0.244 X - - 0.016] for 10 MJ 15 3 0.6 0.5 0.1 1 ДР/Р 1 10 0 20 MJ 30 5 1 0.7 1*0 MJ 9 2 1 100 MJ 5/2 19 3 2 (3] t = RQ/C0 Х for ДР/Р0 >_ 10 2/5 S/ (4) t = 0.543 RQ/Co [(1 + 4.61 X J TABLE V

-1] for 0.1 £ ДР/Ро < 10 Shuck Overpressure as a Function of Reactor Radius R and Fill Pressure P where X = R/R с о о г E il/3 E = 20 MJ C5J Rc = 211 о Y = 1.4

В = 5.33 for Y = 1.4

E = energy, in MJ, deposited in pellet 1 m 2 m к m 5 m 0 0 ^v plus energy in ions and x-rays

P = ambient air pressure in atmospheres 0.1 1+ 0.6 0.3 0.2 1» 0.7 0.1» С = sound speed 0.5 5 0.8 0.5 The dashed lines on Fig. 23 indicate 1 30 5 1 0.7 the point source solution obtained by 2 30 6 1 matching the CHARTD solution to the 1 point source equations at a pressure of 5 37 7 2 1 about 15 atmospheres, using у = 1.4, 3 2 the value for room temperature air. 10 kz 9 » Such an adiabatic Y of 1.4 gives a reasonable fit to the CHARTD results. For Pellet #2, RQ from CHARTD calcula­ tions is 524.6 cm; from equation (5) using у = 1.4, it is 523.6 cm. The impulse of the blast wave can be a significant parameter in predicting From the point source solutions we can damage to the chamber wall. For Pellet obtain estimates of how the shock over­ #2, the pressure pulse duration is pressure varies as a function of E_, estimated to be 0.1-1 millisecond for reactor chamber radius Rr, and fill gas pressure loading at a fixed point. This pressure in the chamber P0. These estimate is made by using the pressure results are tabulated in Tables IV and V, pulse width (FWHM) at a given time and dividing by the shock velocity. Work is in progress to extend the CHARTD the effects of space charge on the shock calculation to study shock fields. Three others were based on interaction with a 1 cm steel wall at the parapotential model of Ref. 43, and 4 m. The purpose of such a calculation a fourth was based on the steady state is to determine what the dynamic model of Ref. 44. The parapotential stresses on the wall are as a result of model predicts different values of the

the ion and x-ray fluences and to deter­ line impedance Zs depending on what

mine whether radiative transport to the voltage, Vm, is specified at the edge wall can be recovered bv wall heating. of the electron sheath. Defining у = 1 + eV/roc2,

z 1 V o" CY0- UCYHntY,,,* CY^-1) ] POWER НШ THEORY AND DIODE PHYSICS fo»"1. <» Magnetic Insulation of Vacuum Transmission Lines

The use of magnetic insulation by means where Z is the free space line imped­ of the self-magnetic field of the line ance. current in vacuum transmission lines continues to be a vital technique for These predictions are plotted in power flow development. There are two Figures 24a and 24b. As can be seen, important cases which should be the "saturated" theories (2 and 3) distinguished. There is first the describe the data fairly well for "short pulse" case when the pulse dura­ voltages less than .9 MV, but at higher tion or its risetime is short compared voltages, the "boundary current" theory to the transit time from the input of the (#4), proposed by McDaniel,42 is the line to the load, and we are interested most successful, although the minimum in the propagation of the pulse before it parapotential current theory (#5) is reaches the load. Secondly, there is the roughly correct. Also shown in "short line" case in which the time Figures 24a and 24b (circles) are the scale for voltage changes is long compared results of computer simulations using to the propagation time, so that we can a PIC code in coaxial geometry which treat the load and line as being held at was developed specifically for this a fixed voltage. It is important to make problem. As can be seen, the agree­ this distinction, because in the crucial ment between the simulations and curve transition zone between the region in #4 and the data is quite good. which electrons can cross the gap and the region in which they are trapped The success of the nonsaturated near the cathode the orbits of the theories leads us to suspect that a electrons could be quite different; in much smaller portion of the cm rent the short pulse problem they experience is carried in space charge close to dB/dt and dD/dt effects which are the anode than the saturated theories unimportant in the long pulse problem. would suggest. The computer simula­ tions also support this conclusion. This is considered an advantage from the point of view of transport efficiency because there is less Magnetic Insulation of Vacuum Trans- likelihood in roostconfiguration s of Mission Lines - The Short Line ProbTem large losses occurring in the transition region between the line and A comparative analysis has been made of the load. a number of theoretical predictions for the self-limiting current and of experi­ ments of Shope and Toepfer on Hydra,*! and of VanDevender on Hermes.42 Five different theoretical models were studied and compared with the data. They are tabulated in Table VI, One of them was the single particle model which neglects

67 TABLE VI

Theoretical Predictions for Self-Limiting Current

I CURVE MODEL SELF-LIMITING IMPEDANCE

1/2 Single Particle Z_ = Z. ГлГ± (Non Self-Consistent)

2 1 Saturated parapotential Eq. (1) with vm = Y0

3 Saturated ., Zss = zPP (F/G)(See Reference 45 for Steady State s s values of F/G) (Self-Consistent)

It Boundary Current Eq. (1) with Y = Z /Z га о

5 Minimum ParapotentiaI Eq. (1) with ^i- =0

68 (a)

(b)

Fig. 24. Self-limiting impedance of coaxial line. Solid curves are theoretical predictions identified in Table VI. Circles are computer simulations, (a] Data is from Hermes, operating Kith a 139 fi (geometrical im>edance) coax; Cb) data is from Hydra Kith a nominal 11 fi impedance. Triangles are for a curved coax.

69 Magnetic Insulation of Vacuum Trans - the potentials via conservation laws. Mission Lines - The Short Pulse Problem Although this yielded some interesting results, the model soon became so complex We now consider the problem when the line that we turned to our final and most length L is longer than the pulse length general approach, a 2-D, time dependent, [L > ст, where т is pulse time). A particle-in-cell code which allows us schematic of the problem is shown in to study the details of the electron Fig. 2S. A voltage pulse V(t) enters a behavior. The electron emission is coax at z = 0. The inner boundary handled using a Gauss' law technique (r = a) is assumed capable of emitting valid for time-dependent fields.4'.48 space charge limited electrons. The All results given below in this section problem is to calculate V(z,t) for z > 0. were obtained with this particle Cylindrical symmetry is assumed. simulation code.

To illustrate the results, we consider the case L = 5 m, a = 6.35 cm, b = 10.58 cm, and a pulse which rises

linearly in 4 ns to V0 = 5 MV, remains constant at 5 MV for 7 ns, then falls FIELDS VOLTAGE linearly to zero in 4 ns. The solution PULSE E at t = 15 ns is shown in Fig. 26. V(z) at this time is plotted with the beam envelope curve (E) to show that the ELECTRONS leakage current (40 kA here) flows only in a narrow region at the pulse front. п м м t м м The average electron energy reaching к r = b is l.S MeV (not 5 MeV). It is also important to note that the electrons in the "cutoff" region behind the front do not turn near the anode; they are confined to a sheath which is relatively close to the cathode. This may account for the absence of leakage behind the front region and has beneficial impli­

cL cations for transport efficiency through regions of nonuniform electrode structure (e.g., the diode-transmission line interface region). Another feature of this solution is that the front of the Fig. 25. Schematic of coax line problem. pulse sharpens as the pulse propagates; A = anode, К = cathode. at t = 0 the 10-90 risetime is 3.2 ns, while at t = 15 ns, it is 1.5 ns. In addition, it is found that the front Our first approach to this problem was propagates at a velocity less than c, to use the equivalent circuit code while the pulse rear propagates at с SCEPTRE.46 Чу using nonlinear circuit This front erosion, as well as the elements, one can model many of the front sharpening, were also observed in qualitative features which have been experiments49.50 and in the equivalent observed in experiments, such as pulse circuit analysis.46 front sharpening and erosion. In our second approach, a fully tine-dependent electromagnetic field solver was written We have also studied the case where e and tested (by comparing with analytic electron emission is delayed until a solutions for simple problems), and a threshold Er is reached at the cathode. nonlinear conductivity (including For the case shown in Fig. 27, this magnetic cutoff) was included. Results threshold was .7 MV/cm. The resulting were compared with those obtained using behavior is similar to the above except SCEPTRE, and excellent agreement was that the high voltage part of the pulse found. We thus proceeded to include a is eroded more rapidly at first, and a cool fluid model, in which charge and precursor pulse moves at с ahead of the current densities are determined from main pulse, as shown in the figure.

70 This behavior is, again, qualitatively similar to the experimental and circuit analysis results.

V -

/ / • / / / / 1 [ s / -—v1 <* 1 71 e - i\ \

Fig. 26. Simulation results at t = 15 ns for a 5 MV pulse propagating

down a 5 m line with Z0 - 31 £2. E is the envelope of particle trajectories of which three are shown.

Fig. 27. Voltage V vs. z, showing effect of delaying emission until E (K) > E .

71 Results for a number of runs are that which has been observed in short summarized in Table VII. Several coax (steady^state) experiments and interesting features are evident. First, simulations.51 it appears that If and I]j scale quite

closely with Z0 at fixed V0, whereas The particle code was also used to there seems to be a different dependence simulate a coaxial line with the same of II. the conduction current which geometric impedance as the triplate actually leaks to the anode, on line line in the present MITE experiments. parameters. There may also be a weak The results of this work are discussed dependence of the front velocity, elsewhere in this report, Vf = cSf, on the dimensions of the line (compare runs 4, 5, and 6), The propor­ tion of leakage current total current is seen to decrease dramatically with Double Sheaths in Plasmas increasing voltage. This is in accord with the approach of Bf to 1 as voltage Under certain circumstances a plasma increases because energy transport may respond to an applied electric efficiency becomes very good as Vf field by forming a narrow, non-neutral approaches c, so losses must decrease. region in the body of the plasma across The transport efficiency (as defined in which essentially the entire potential Eq. 9 of Ref. 46) for the 11 nsec pulse drop occurs. These structures are after 3 meters ranged from 66$ at 1 MV referred to as double layers or double to 965 at 10 MV. sheaths. They are of current interest for several reasons; they may play a role It is interesting to compare these in electron emission processes from results with the parapotential theory of sharp points and edges,52 in plasma- filled diodes, and in the FOSIL fast- Ref. 45 for IT, defined by Iy = V0/Zs opening switch concept.42 with Zs given in Eq. (1). The flow pattern of Fig. 26 suggests that Vm = V0 ("saturated parapotential theory") would Using analytic models, several features not be appropriate, so we need a of double sheaths have been investigated. prescription for the choice of Vm. Since One of the most important questions is we have IJJ, we can use another result under what circumstances double sheaths from the parapotential theory, Ym = IT/^B» will form. We have obtained a simple to determine if these results are consis­ expression for the critical density which tent with it. As seen in Table VII, the marks the transition from double sheath agreement between Ip calculated in this behavior to a turbulent discharge (in way and lj is remarkably good, especially which the field is essentially constant in view of the fact that the electron across the plasma). We have also orbits are known to be quite different obtained an expression for the maximum from those assumed in the parapotential plasma density for which the plasma is theory. It should also be noted that the unstable to the growth of density per­ simulation results are within 104 of the turbations, a process which is thought minimum parapotential current, Imin> to lead to the nucleation of a double discussed in Ref. 45. sheath. What is interesting is that both expressions can be put in the same ". Fig. 28, two of the quantities from form: in cgs units, Table VII are plotted against voltage and compared with the function C(Yo " l)/(Yo + I)1'2- If space charge is neglected, one obtains this expression, both for Z/Z0, and as presented in Ref. 50, for Bf. One would expect these to Here, л is a measure of the effective be correct at low voltages, but it is collision frequency referred to the ion plasma frequency, v ff = n upi; E is very interesting that the agreement for e 0 Bf is also quite good at high voltages. the applied field; and T is an effective Also shown are two experimental points temperature whose form depends on whether for Bf, which are in good agreement with we are discussing the nucleation the simulations. On the other hand, condition or the sheath maintenance condition. In many cases, T = Te, Z/Z0 appears to reach a plateau below the solid line in a manner similar to the electron temperature. TABLE VII

geometrical impedance, 60 Jtn b/a; Ix is the total current flowing in the line while Z is V/ly, II is the conduction leakage current which actually intercepts the anode; Ig is the "boundary current" which flows within r < a; Ip is the parapotential current calculated as described in the text and should be compared with Ix; 3f is the front velocity Cmeasured at 2/3 V) relative to с

1 V0(MV) IT(M) z/z Ip(kA) e Run z0(«) ' 0 hJh V ! f 1 1. 31. 59. .57 .U2 .51» 59- ^6 2 2.1* 8. ><50. .67 .38 .51 It 1*0. .75 3 3. 31. ll+O. .70 .29 -.U6 lUO. .79 1* 5. it. 1670. .75 .35 .1*3 161*0. .82 5 5. 31. 220. .73 .20 .1*2 215. • 87 6 5. 72. 93. .74 .lit .1*1 91. .87 7 6. 31. P.60. .75 .l»t .38 260. • 90 8 10. 31. 1*10. .80 .10 .37 1*00. .95

Fig. 28. Reduced front velocity, Sf = Vf/c (open triangles) and reduced

running impedance, Z/Z0 (open circles) vs. voltage from the simulations. The solid line is an expression for both quantities from a single particle analysis. Also shown are two experimental measurements of Bf (solid triangles), one at 460 kV*y and one at 3.4 Ml'su.

73 In Figure 29, we have compared Eq. (1) plasmas as having isothermal distribu­ with some results for the transition den­ tions has been used to determine thermal sity taken from the experimental work of effects on the beam current densities. Lutsenko, et al.^3 the value of n chosen This in turn affects the balance of was .86, which is the value predicted by forces among the species. It is found Buneman-" and verified in reference 55. that there are no net forces on the The agreement is quite good, considering sheath even when there is a pressure how simple the model is for such a difference between the plasmas on complex system. Better agreement is opposite sides of the sheath. This obtained by increasing n to 1.2, as may explain why double sheaths have shown by the dashed line. This value been observed to be remarkably statio­ of n is well within the uncertainties, nary in some experiments. since the experiments on anomalous resistivity in this parameter domain are not all in good agreement. However, even with n = .86 we get the dashed line with Electron Emission from Cathode Plasmas T = 8 eV rather than 4 eV. This is also in Kelativistic Diodes plausible because the 4 eV value quoted by Lutsenko, et al. is the initial value, The mechanism of high current electron and a good deal of electron heating can emission from cathodes of relativistic be expected in the plasma. electron beam (REB) diodes is a process which has only recently begun to К understood in a detailed manner. Much attention has been paid in the recent literature to the initiatory phase of the transition to space charge limited flow. This is in part because for many applications, the goal is to prevent vacuum breakdown. Several experiments have observed the process of explosion of microscopic metal particles at the cathode surfaces. It is assumed generally that when enough such points have exploded in an REB diode, the plasma flares coalesce to form a rela­ tively uniform expanding plasma front, which has been photographed holograph- ically. If the plasma is sufficiently dense, one expects and observes the space charge limited current.

However, there is still a missing element in this analysis, which is to explain how current continuity is main­ tained across the metal/plasma inter­ face at the cathode. It is proposed Fig. 29. Critical density for transition that it is not necessary to assume from double sheath to turbulent electrons cross the metal/plasma discharge. Data is from Ref. boundary. Instead, current continuity 53. Theoretical curves are can be maintained by a flux of positive from Eq. 1 with T = 4 eV and ions from a high density plasma region a nitrogen plasma. across a positive sheath into the metal lattice. Another aspect of double sheaths which has been studied is the balance of It turns out that there are two electrostatic forces among the various requirements on the plasma: it must be species (electrons and ions in the beams sufficiently thick. The density and in the plasmas]. A model which requirement is found to be treats the beams as cold fluids and the n = 1.48 x 109 (V3A/T J1/2 d"2 where A is the ion atomic number per was itself an interesting problem, charge, T„ is in eV and all other units because the eigenfunctions of the

are SI. For Te » 3 eV, d = .005 m, cylindrical wave equation are Bessel V = 1 MV, A = 1, we find n,- = 3.5 x 1022 functions rather than trigonometric m-3, which corresponds to 9.3 kA/cm2. functions, as in the planar case. The thickness requirement is obtained by The orthogonality of these eigenfunctions setting the number of ions in the thick­ could not be used and the inverse ness т equal to the number of electrons Laplace transform had to be performed delivered to the anode. For the same explicitly in an integral which contained parameters it is found that т = 2 mm. the Bessel functions. Careful attention Both of these conditions are expected to had to be paid to the singularities in be easily met. Several other consistency the integral (at the zeros of the Bessel checks have also been made.59 There function) of which there were an are a number of conclusions one can infinite number. drew if this hypothesis is correct. For one thing, the uniformity of electron Studies using this code are still rather emission is determined primarily by the preliminary, but several interesting extent of the cathode over which a plasma results have emerged. (1) the effect with density greater than nc has formed. of the emission (if the entire cathode This depends on the dynamics of lateral face turns on) is to cause V(r) to plasma motion. In addition, it answers decrease as r •* 0, rather than the the question of how dense a preformed increase found in the vacuum case. plasma need be to act as a space charge (2) Runs with an emission threshold limited emitter. (i.e., no electron emission until a critical field is reached at the cathode surface) look interesting (at least for the case studied, where V rises linearly Theory of Radial Power Flow in 4 ns to 2 MV in a 2 m diameter system with 1 cm gap) in that the region near Calculations of the radially inward r = 0 turns on rather suddenly, so that propagation of a voltage pulse are of currents in the MA range are produced interest for several reasons. First, in times of less than 1 ns. (It should such studies are relevant to present be mentioned that the currents in this work on Proto II to produce a low first study were not well focused, and impedance diode for power concentration. a critical field was required which may Second, we wish to know more about be somewhat higher than physical.) magnetic insulation in a radial disk (3) Adding a Bg from an external source vacuum transmission line. And, most (e.g., wire on axis) did not help, since generally, the calculations may teach it insulates most near r = 0 without us something about how to concentrate shutting of emission at large r. Also, power in the form of electromagnetic the pinching was not helped appreciably, waves. as also observed with the steady state diode code.60 (We are assuming that Thus, a new version of the time dependent, these times of < 10 ns are too short for 2-D, cylindrically syirmetric, electro­ аррт т'able ion production.) What seems magnetic particle code was written to to •'.- aeeded to make this scheme of study the probla* of a voltage pulse power concentration work well is to propagating radially between two plane insulate with an externally applied Bj. parallel plates, with one plate emitting field (e.g., from a cusp), although we space charge limited electrons. There have not yet studied this possibility are several restrictions on the present with the code. form of the code; it cannot treat tapered geometries (i.e., a spacing between the plates which varies in r), and (assuming the pulse enters the system at r = R at t = 0) it is only valid for a time 2R/c. The code was checked against analytic vacuum solutions, which show that the pulse amplitude increases sharply as the axis (r = 0) is approached. The obtaining of these analytic solutions DIODE CODE DEVELOPMENT AND APPLICATION equipotential lines as shown in Fig. 31a would result. If instead a triangular area weighting is used in this zone, the correct equipotentials, as shown in Variable Zoning Diode Code Fig. 31b, result. An example of a slanted mesh simulation is given in a A new version of the diode code.6 0 has following section. recently been written which increases the code's accuracy and flexibility in treating different diode geometries. The new +2.

The effect of thin foil anodes in diodes A few Proto I simulations were also done has received much attention lately. The for a single sided case at 1,5 MV with diode code has been applied in support the same cathode as described above. of enhanced deposition and pinching foil The foil in these simulations was mounted experiments on several accelerators. The over a hole in the anode of radius foil is simulated as a constant potential 4.76 mm and depth 4.96 mm. Three surface in the diode. A simple small different foils were considered and the angle scatter and stopping power routine, results are summarized in Table IX. written by M. M. Widner (5241), has been This series of simulations investigates added to the code to treat the effects the results of going from low Z to high of scattering and deposition in these Z foils and the effect of changing the thin foils. ion species. The results demonstrate that lowering the foil Z increases the 1. Proto I electron density (bv a factor of two for this case) at the foil as expected. In an attempt to understand the Changing the ion species slightly lowers recent Proto I experiments with thin the density, mainly by decreasing the foils, a series of diode code runs were total current. made. The first diode configuration considered was that of a hollow cathode 2, Hydra of inner radius 0.635 cm and outer radius 1.27 cm. The diode voltage was Simulations of foils in Hydra 1.5 MV and the total diode current was diodes were done to investigate the 160-200 kA per side. Four different foil effects of scattering in the foil and cases were considered and the runs are also the effect of varying the mediuir simmarized in Table VIII. The infinitely properties in the hole in the anode "ver thin foil case is a foil with no scattering which the foil was mounted. Three or deposition. The solid anode case is simulations were done with the same a perfectly absorbing boundary (no back geometry of a foil mounted over a .95 scattering). It should be noted that cm radius hole in the anode. The cathode the 50 \m thick foil case is probably inner radius was 1.59 cm, the outer stretching the applicability of the radius was 3.81 cm and the diode voltage scattering and deposition models whidi was 720 kV. The three foils considered are valid only for thin foils. were perfectly absorbing, 6 um gold with no scattering, and 6 pm gold with The first thing to note from this table scattering. The electron part.cle map is that the electron density near the foil for the third case is shown in Fig. 32,. center was higher for the thinner foils The resulting maximum electrc i densities and highest for the zero thickness case. at the foils were .5 x lOlS^.m-S, The peak of the radial distribution of 1.3 x 1015 an"3, and 1.2 X "0*5 cm-3, absorbed power moved farther away from respectively. Scattering is seen to the axis with decreasing foil thickness. have little ei\Tect on the maximum electron density since magnetic field The current density je, defined here as current/area absorbed by the foil, and effects dominate for this case.

77 TABLE VIII

Sunmary of 2-Sided Proto I Foil Simulation Results

Current Maximum Density Foil Type Electron Power Absorbed I. in r < Л mm Density at in r < .8 mm h 1 p [Щ (WO (MA/cm ) Foil (cm"3) (W) Infinitely thin 150 60 - 12 x 1015 0 6 ми Au 115 h5 < .1 2 x 1015 2 x 109 50 Wn Au 150 30 1* 6 x 101? 8 x 1010

lU 11 Solid Anode 13 1.3 x 10 175 35 7 x 10

T.ABLE IX

Summary of One-Sided Proto I Foil Simulations

I I. Maximum Electron e 1 Foil Type Ion Species (Ш) (kA) Density at Foil (cm J)

6 ^m Au H+ 170 35 3.U x 1015 6 wn Al H+ 18C 35 6.2 x 1015 6 wn Al Al+ 170 5 k.2 x 1015

Fig. 32. Electron particle map of 6 urn gold foil in a Hydra ш diode.

78 A second Hydra diode configuration con­ In run #3, the total power is distribu­ sisting of a hollow cathode of inner ted as: target 501, ions 305, anode radius 1.90 cm, outer radius 6.35 an, 151, foil S%. Since 1^ is so large, A-K gap of .89 cm, and diode voltage there does not seem to be much point in of 1 MV was simulated to investigate the increasing R/d. Even if one is willing effects of various fills in a 1.60 cm to live with 30% in ions, one should be radius hole in the anode over which an able to obtain a better pinch using infinitely thin foil was mounted. The plasma injection. Our conclusion from results of these simulations show that this work is that this type of diode does a plasma filled (E = 0) hole behind the not seem very- promising, and this con­ foil produces a maximum electron density clusion is consistent with the prelimi­ at the foil greater than twice that nary results obtained by D. J. Johnson which results if the region is perfectly (5244) on Hydra. It is important to absorbing (solid anode). Further, if point out, however, that the poor the hole is not plasma filled but pinches obtained with these thin foils allowed to have full E and В fields and do not mean that the absorbed power/mass form its own potential well, the electron is low, and in fact the calculated density is enhanced by a factor of 4 over density near the foils was generally the solid anode case. The pinching in large. As shown by runs 2 and 3, the these three cases varied only slightly. problem is to get the electron energy The conclusion is that the foil always into the foil during its 2 or 3 passes. enhances the electron density over a solid anode case. 4. Foil Diodes with Bz

3. Thin Foil NRL Diodes The diode code was used to study

the effect of an applied uniform Bz on A series of runs was made for a double sided, KAAK diode. Each anode a small, R/d = 6, Hydra type diode to A consists of a foil F. Figure 34 shows examine the idea of using a thin planar a sample calculation (only the left half foil placed parallel to the anode and of the symme ric system is shown). Ions connected to it by a stalk on axis. were neglected, and the region between (This idea was proposed by Shyke foils was vacuum. Some electron orbits Goldstein, NRL.) A sample case is shown (or parts thereof) are shown; note the in Fig. 33, where the dots are simulation scattering in the 10"3 cm Au foil. The electrons, and K, F, A, and S stand for average electron makes 14 passes through cathode, foil, anode, and stalk, the foil and deposits 70 keV per pass. respectively. Electrons absorbed in the The Bz effectively prevents much pinching. foil pass through S to the anode, One can tailor the energy/pass and the contributing to Bg in F-A. Table X region of the foil where the deposition shows results from five of the most takes place by varying foil thickness interesting runs. The most important and Bz. This result is interesting column is the electron power deposited mainly because of the large number of in the foil in r < 3 mm. The presence passes per electron. of a "target" in run #3 means that an electron absorber of radius 3 mm was 5. Conclusion on Foil Effects located at the foil; this run gave the best results of the foil cases, having Some general conclusions can the most power deposited and the least be drawn from tlw • ove foil simulations. leakage to the anode. This case was It is seen that thin foils consistently done with and without a charge neu­ increase the maximum electron density at tralizing plasma in F-A, with no sub­ the foil position over that which would stantial difference found. Run #4 had obtain if the foil were replaced by an no foil, the F-A region being anode absorbing anode. The thinner the foil, plasma; j (pinch) was 2.4 MA/cm2. Run ; the greater this enhancement The low «5 had no fo l or anode plasma; j (pinch) absorbed powers observed in the simula­ was 1.5 1 Adding a B = 1 kC to NA/cm . f tions does not imply that the absorbed these cases destroyed the pinch. power/mass is low but rather that the electrons do not deposit much energy into the foil during their few passes.

79 Finally, an applied Bz field can increase Proto II Diode Studies the number of passes an electron makes in the foil, thus increasing the A low voltage simulation of a Proto II deposited power/mass ratio. The applied diode provided information on expected

B?, of course, prevents much electron current densities in the first pinching pinching. experiments on the machine. The diode considered consisted of a hollow cathode with an applied voltage of 500 kV.

F К Шт..

;---'*^-;'...

Fig. 33. Electron particle map from a .95 MV thin foil (F) diode. K-F = 4 mm, F-A = 2 mm, outer (inner) cathode (K) radius =3.8 cm (1.6 cm).

TABLE X

Summary- of Thin Foil "Shyke" Diode Simulations

Power (OW) % e Lost Deposited to A, I (kA) I (kA) Run Foil Target e ± Passes/e r < 3 шш r < 3 mm 1 10 v.m No 2k0 95 2.5 .№' 31 2 1 № Ко 250 100 3.2 .002 50 3 1 vm Yes 2l»0 100 2.1» .17 35 4 0 No 240 80 .20 11 5 <я 250 100 1 .Ik kS on axis was low, it was expected that the results would improve with increasing voltage.

Higher voltage results have been recently obtained which suggest a very tight pinch is obtainable under certain diode configurations. A 63.5 cm outer radius, 15.2 cm inner radius, tapered cathode with a .254 cm cathode tip-to-anode gap was simulated with a 1.5 MV diode voltage. Various combinations of ion emission, wire on axis, taper angle and a special solid cathode were considered and are summanzed in Table XI. In this table ji and \\ represent the current density and percent of the total electron current hitting the anode in r < 1.9 cm. Case A represents a tapered cathode (1° taper) and Case В i-epresents a flat cathode (0° taper). In both of these cases, ions are emitted only over the range 0<_r^l5.2cm. Cases С and D are for a special cathode consisting of the tapered cathode of Case A but Fig. 34. Sample electron trajectories with the cathode extended from the tip from a diode code run with to the axis resulting in a solid cathode instead of a hollow one. In Case С ions 10 vm Au foil and applied B = 1 kG. К = cathode, F = are emitted as above. In Case D, the z ions are replaced by an I = 1 MA foil (anode). Only the left w half of the symmetric КАК wire current on axis. Perhaps the most system is shown. K-F = 2 mm, interesting cases shown in Table XI are F-A = 1 mm, V = 1.5 MV, and A and С Case A demonstrates that a outer cathode radius = 1.2 cm. large fraction of the electron current can be pinched from a large cathode radius to a relatively small area on the anode. In Case С almost all of the The dimensions in this simulation are electron current arrives at the anode Ro t = 6.35 cm, Ri = 2.47 cm, and d = U n in the 1.9 cm radius area with the 1.5 mm resulting in an R/d of 42. An sacrifice of the energy devoted to ion infinitely thin anode plasma is assumed current. Case A has a smaller I| than to allow the emission of ions over the e case С because the E, field neai the entire anode surface for r £ Rout- Th axis is lower (the cathode is hollow). cathode emits electrons from the face, Case В has a still lower 1^ because of top and bottom shanks, and corners. the lack of electron pinching. Figure 35 is an electron particle map for The steady state results for the above case С In summary, these results show simulation are an electron current of that a tapered cathode pinches better 475 kA, and an ion current of 640 kA. than a flat one, and that a line current This total current of 1.115 MA is slightly from a wire cannot replace the distribu­ higher than the predicted parapotential ted ion flow,60 It should be pointed current of Ip = 9S0 kA because of the out that these are steady state simula­ inclusion or shank and corner emission. tions ; time dependent effects can play The electron beam undergoes a moderate an important role in pinch development. pinch with 35% of the electron current However, these steady state runs should hitting the anode in r £ 3.5 mm. The correspond reasonably well to the resulting current density in this area behavior of the diode at the peak of is .43 MA/an2. One half of the total the voltage pulse. electron current arrives at the anode in r < 1.06 cm, and 855 in r <^ 2.47 cm. WhTle the low voltage current density

81 TABLE XI

Proto TI Large R/d Simulations

I I I. Jl w 1 (MA) (MA/on2) *1 (MA) (MA) A 0 3.3 O.Y 0.1 25 В 0 b.6 0.2 0.0 6 С 0 O.y 2.2 0.1 96 D 1.0 1.6 0 0.0 1

Tig. 35. Electron particle map from a Proto IT diode, case C. P^, =

64 cm, Rj = 15 cm, d0 = 1.1 cm, _di = 2.5 mm, V = 1.5 MV\

82 Los Alamos PHERMEX Diode Simulation Large Diode Simulation In response to a request from F. VanHaaften A large diode has been simulated to at Los Alamos, the newest version of the investigate electron beam pinching from particle code was used to simulate a hollow (Router = 32 cm> 'Чппег = 2& cm) proposed diode for the PHERMEX facility. large aspect ratio diodes (к/3 = 80). The diode consists of a 30° tapered The assumed voltage is 1 MV. The simu­ hollow cathode with 5 mm А-К gap, lation has importance in understanding R = = 1>27 c,n Proto II diode operation. The simulation inner -35 ram. and RoUter • The simulations were run with 1 MV and model assumes that an infinitely thin 2 MV' across the diode. The resulting plasma covers the anode from which ions current densities on axis were respec­ are emitted over the range 0 £ r <_ Router. tively 1.8 MA/cm2 and 3.5 MA/cm2. The Electrons are emitted over the entire 1 MV case is shown in Fig. 36. The cathode including both inner and outer results of these simulations indicate shanks, corners, and face. The simula­ that it is not unreasonable to assume tion is steady state in that it that the desired 1 to 2 mm spot size includes no time dependent effects such containing 50% of the total electron as time dependent emission, plasma current can be achieved, at least at formation, or 3B/3t effects. The results early times in the pulse. Plasma of this simulation are an electron motion at later times may change this current of 525 kA and an ion current of result, possibly for the better. 3.1 MA. This total current of 3.6 MA implies an impedance of 0.3 Q. Of the total electron current, about 90% hits the anode within a 1 cm radius of the axis, resulting in a pinch containing an average current density of 0.15 MA/cm2. This value of total current is in good agreement with the parapotential theory which predicts a current of l„ = 3.5 MA. The theoretical ratio of ion to electron current Ij/Ie = 3.9 i.= slightly lower than the simulation value of 5,6, but the Goldstein formula does not strictly apply to this case of a very hollow cathode. Dividing the parapotential current according to the

theoretical ratio of Ij/Ie yields an ion current of Ij = 2.8 MA and an electron current of 700 kA, in fair agreement with the simulation.

The pinch formation time is expected to be quite long, and it is not clear whether the large anode area "Router^ can really be covered with dense plasma, as this might require a considerable fraction of the beam energy.

Fig. 36. Electron particle map from a Hermes II Ion Diode Studies proposed Los Alamos PHERMEX Two different diode geometries were diode. simulated for the Hermes accelerator in an effort to determine if they could produce an anomalously high ion current, as suggested by some preliminary experimental results.54 The first simulation consisted of a 7,62 cm

•3 radius spherically tipped cathode in which the cathode has RQ = 3.87 cm, opposite a flat anode with a 28.6 era A-K Ri = 1.59 cm, and the diode voltage is gap. The applied voltage was 10 MV. The 950 kV. Three A-K gaps are considered. second geometry considered was a hemis­ The first is a tapered (parapotential] pherical cathode of radius 7.62 cm cathode with 10° taper angle. (See opposite a hemispherical anode of radius Fig. 38. The other two cathodes are 22.9 cm. The applied voltage in this both flat faced (0° taper angle) with case was 7 MV. different A-K gaps. The results of these three simulations are presented in Both of the above described simulations Table XII. predicted that only about 10% of the total diode current Kould be in ions. Other possible mechanisms for producing higher ion currents were examined. No plausible mechanism was found which pro­ duced much more than 105 in ions, except for a small possibility of electron reflexing in some of the experiments. The conclusion that we draw is that Hermes II in these configurations should not produce an anomalously high ion current.

Angara Cathode

.An early time simulation of the Angara cathode, used at the Kurchatov Institute, %. has been done using the newest version of the diode code. An electron particle map for this cathode is shown in Fig. 37. The cathode tip radius is ! mm and the outer radius is S mm. The tip to anode gap is 2 mm and the diode voltage is 500 kV. The steady state simulation results are an electron current of 30 kA and an ion current of 1 kA with 12» of the elec­ tron current arriving at the anode within iig. 37 electron particle map of a radius of 1 mm resulting in a current Angara cathode early time density over that area of 0.1 MA/cm^. behavior. The simulation validity is restricted to early times in the voltage pulse because plasma motion is neglected. There is The results indicate that the para­ very strong experimental evidence that potential cathode pinches as well as much greater total currents and current the flat cathode when the former runs densities are achieved with this diode at a much lower current level. It also and that the diode behavior is very pinches better than a flat cathode much dependent upon prepulse conditions. whose A-K gap has been adjusted to This suggests that plasma motion plays provide the same total current. The an important role in this diode. pinch is better for the parapotential cathode because the emission is pre­ dominantly from the inner part of the cathode face, as opposed to the flat Parapotential Cathode Simulation cathodes where the emission is much more uniform over the entire face. The slanted surface capability of the new diode code version has been applied to the investigation of the behavior of tapered "parapotential" cathodes. A particular Hydra diode is considered N?-V- 4 •

г К N*':-. •'

\3jC,

1 Vs-' vU^W' • >

• -- Z

Fig. 38. Parapotential cathode electron particle map

TABLE XII

Summary of Parapotential vs. Flat Cathodes

I. A-K Gap h Cathode (mm) (HA) (kA) MA/cm Parapotential 2.7 180 35 2.0 Flat 5.0 2^0 8o 2.0 Flat 6.5 160 1*0 1.5

85 PARTICLE BEAM SOURCE DEVELOPMENT AM) conducting" drift tube. It was quickly THEORY found that a purely transparent tube (e.g., quartz) drastically and unfavorably altered the IREB propagation characteristics from those that occur Collective Ion Acceleration in a metallic tube, A number of hybrid drift tubes were then constructed A collective acceleration program is in containing various combinations of glass progress which is designed to demonstrate (quartz, pyrex), conductive coatings, fcasibilit/ of the Ionization Front and metal (brass, copper, etc). After Accelerator (Ш).65'66 The three main substantial experimentation, a suitable goals of the experimental feasibility "transparent/conducting" drift tube was phase of this program arc summarized in created that does reproduce the IREB Table VIII. Hie status of this program transport properties of a metal tube, as .-: 4 ptember 1977 is also indicated yet also has windows along the sides in lius table. Note that the full I FA for use in the IFA.6" system (IRF.B, dye laser, fast shutter, frequency-doubled mby laser, and heated (ii) laser sweep. The dye laser Cs experimental clianber) is now opera­ beam is to be injected into a light tional for the first time. Following pipe array to produce a swept light 'lable XIII, we will now briefly summarize front along the IFA drift tube. To progress on the П'А experiments to date. demonstrate that such a swept light front can be made, and to examine the Coal 1 of this program was to demonstrate actual uniformity of the front, a that Cs is a feasible working gas. This series of streak camera experiments involved TRF.B stopping and transport were performed. In these experiments, experiments in air and in '.s (see the dye laser output (8521°A).was made previous semiannual report). The results visible to the streak camera (P-ll of these experiments clearly demonstrated photocathode) by the use of a frequency- the pressure threshold for IREB-induced doubling crystal. The streak pictures ionization of Cs and showed that for a obtained clearly demonstrate that the reduced Cs pressure of < ЗП microns, light pipe array does produce an IR1IB-induced ioi.ization processes should accurately-programmed light sweep. In not interfere with the I FA operation. 6 •' addition, use of the fast shutter The Cs density can therefore be up to assembly (optical risetiir.e % 1 nsec) lfllS спгЗ, which is 1ГР times larger than demonstrated that a very sharp swept the typical IRER density of 101' cm-3. light front car. be made.68 This means that the Cs has to be ionized only 0.12 to control the potential well (iii) scintillator/streak beam motion in the I FA. front diagnostic! To observe the beam lront motion in our controlled beam Goal II ir this program involves front experiments, it was necessary to e.xpi r.' i :•• s to demonstrate laser- develop a new time-resolved beam front c .. .'. IHEB beam front motion. To diagnostic. This consists of a ас:. е this goal, the five prerequisite scintillator lining the inside of the problems listed in Table XIII had to be drift tube which is excited by the IREB solved. The first four problems have electrons hitting the drift tube walls. now been solved, and the fifth is near The emitted light is then recorded by solution: a streak camera. This technique has now been perfected so that we can (i) transparent/conducting dri ft readily see, with subnanosecond tube studies"! All of our earlier 1КЕБ resolution, e.g., stopped IREB's (for transport studies in air and in Cs were injection into vacuum) and fully- performed in a drift tube with "perfectly - propagating IREB's (at optimum trans­ conducting boundary' conditions" (i.e., port pressures).68 a metal tube). For the IFA, it is of course necessary to have windows along (iv) high temperature modification. the drift tube for the swept dye laser Because the^xperimental chamber must light to enter. Therefore, it was be heated to ^ 250°C to achieve the necessary to develop a "transparent/ desired Cs operating pressure, a number of complications arose concerning This program is iointly sponsored by materials problems. These problems have DOE and AFOSR. been adequately solved and we are now ablt to operate at the required tempera­ tures rather routinely. Calculations of Ion Beam Neutralization (v) low litter synchronization of lasers and IREB. To achieve the This work was done in support of the required synchronization of the lasers pulsed linear ton accelerator and the IRI-:B for the IFA, it is necessary experiments of S. Humphries. Л crucial to minimize the jitter and delay times problem in such a device is the as much as possible. The commercially neutralization by electrons of the ion available trigger-to-Pockel pulse delay beam, since even a relatively small times of 300-400 nsec were reduced to degree of non-neutralization would cause 4- 50 nsec by developing and constructing beam spreading in the long drift regions. special 1'ockel cell drivers. Our present The calculations were made with a 1-D jitter measurements yield standard rolativistic electrostatic particle deviation values of % 6 nsec for the IREB code. A_uniform ion backgi'?und n± = (start of blumlein charging to diode 10^- cm"3 was introduced into an L = 1 pulse), < 4 nsec for the lasers (trigger cm gap with a .5 ns risetime. Space to laser pulse out), and % 1 nsec for the charge limited electrons were emitted fast shutter (trigger to shutter pulse self-consistently from the walls. The out). Ke have had some laser pretrigger- exact numbers chosen are not important, ing problems caused by Marx generator since potentials scale line п^Ь'. The noise. Ke are in the process of solving solution after .5 ns oscillates about these problems and ensuring as accurate an equilibrium in which the ions are a synchronization as possible. ovemeutralized near the walls, under- neutralized in the middle. The maximum The first system shots have been taken potential is 40 kV (for no neutraliza­ without the fast shutter, so the swept tion, it would be 225 kV), which is dye laser pulse has a rather long bad since the ions would seea risetime. Nonetheless, in some of our defocusing field of order 10S V/cm. first shots the synchronization was favorable ; < the streak pictures did As pointed out by Humphries,6° in a roughly im. -e that the beam front ; ?al device there will always be effects was controlled by the lasers.68 Ke .lich give the electrons a velocity await further tests, and use of the component parallel to the wall, which fast shutter, to finalize this conclusion, prevents them from returning to the wall, and allows more complete neutra­ In summary, the key results of the IFA lization. This was modeled in the 1-D experimental program to date are that code by simply reducing the velocity we have: (perpendicular to the wall) by a fixed fraction f each time an electron (1) demonstrated Cs is a feasible crosses the midplane. Both f = .9 and working gas. f = .7 fairly well. The .P case reduced the maximum potential to < 2 kV (2) demonstrated transparent/ in 1.5 ns, while the .7 case took only conducting drift tube. 1 ns. To within the accuracy of the code, good neutralization was achieved (3) demonstrated accurately- everywhere. These results seem to programmed laser sweep. confirm the idea*^ that cold electron neutralization can occur provided the (4) demonstrated diagnostic that electrons can gain a velocity parallel accurately shows time-resolved beam to the walls. front motion.

Ke are just row beginning full system shots, and the study of laser-controlled beam front motion.

87 TABLE XIII

IFA Feasibility Experiments

I. Demonstrate Cs is a feasible working gas

IKEB stopping and transport in air completed March, 1977 IREB stopping and transport in Cs

II. Demonstrate laser-controlled IKEB beam front motion

transparent/conducting drift tube j laser sweep . I scintillator/streak beam front diagnostic completed September, 1977 high temperature (250°C) modifications ) low jitter synchronization of lasers and IEEb j first system shots

III. Demonstrate collective ion acceleration

ion diagnostics REP' vliNCES

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89 Proc. of Sth Intl. Conf. on Plasma Phys. and Cont. Nucl. Pus., Tokyo, November 1974, Vol. II, p. 347; R. Burke and M. Sapir, 17th Annual Meeting of Div. of Plasma Phys., Amer. Phys. Soc. (St. Petersburg, 1975), post deadline paper. 19. M. Л. Sweeney and M. J. Clauser, in Electron-Beam Fusion Progress Report, SAND-77-1414, Sandia Laboratories, October-March 1977, p. 60. 2(1. B. D. Meixner, Los Alamos Scientific Laboratory, unpublished; II, Smith, Lawrence Livermore Laboratory, unpublished. 21. J. W. Poukey, Proc. of Intl. Top. Conf. on E-Beam Res. and Tech,. SAND-76-5122, Sandia Laboratories, February 1976, Vol. 1, p. 247. 22. L. I. Rudakov and M. V. Babykin, Proc. 7th Europ. Conf. on Contr. Fus, and Plasma Phys., Lausanne, 1975, Vol. 2, p. 172; P. A. Miller, R. I. Butler, M. Cowan, J. R. Freeman, J. W. Poukey, T, P. Wright, and fi. Yonas, Phys. Rev. Lett. 39, 92 (1977). 23. G. Yonas, J. W. Poukcy, K. R. Prestwich, J. R, Freeman, A. J. Toepfer, and M. J. Clauser, Nucl. Fus. 14, 731 (1974); D. Mosher, Phys. Rev. Lett. 35_, 1228 (1975). 24. J. H. Nuckolls, R. 0. Bangerter, J. D. Lindl, IV. C. Mead, and Y. L, Pan, Europ. Conf. on Laser Interaction with Matter, Oxford, England, September 1977. 25. L. Baker and .1. R. Freeman, Electron-Beam Fusion Progress Report, SAND-77-1414, Sandia Laboratories, Albuquerque, N. M., p. 74 (October 1977). 26. P. A. Miller, et. al., Phys. Rev. Lett. 39, 92 (1977). 27. J. R. Vlegas and Т. С Peng, J.A.R.S. 31_, 654 (1961), 28. T. P. Wright, "Transport Studies of Cusp and Multi-Channel REB Fusion Concepts," submitted for publication. 29. M. M. Kidner, J. W. Poukey, and J. A. Halbleib, Sr., Phys. Rev, Lett. 38, 548 (1977); and references contained therein. 30. M. J. Clauser, L. P. Mix, J. IV. Poukey, .1. P. Quintenz, and A, J. Toepfer, Phys. Rev. Lett. 38, 398 (1977). З:. D. Mosher and I. B. Bernstein, Phys. Rev. Lett. 38_, 1483 (1977). 32. J. A. Halbleib, Sr. and M. M. Widner (to be published in IEEE Trans. Nucl. Sci., December 1977). 33. D. A. Haraner and N. Rostoker, Phys. Fluids 13, 1831 (1970). 34. S. L. Thompson, Improvements in the CHARTTi Energy Flow--Hydrodynamic Code V: 1972/1973 Modifications, SLA-73-0477, Sandia Laboratories, Albuquerque, October 1973! 35. R. K. M. Landshoff and J. L. Magee, Thermal Radiation Phenomena, Vol. I (New York. Plenum Press, 1969). 36. G. I. Taylor, Proc. (London) Roy. Soc. А 2ГЯ, 159 (1950); Ш, 175 (1950). 37. W. E. Baker, P. S. Westine, and F. T. Dodge, Similarity Methods in Engineering Dynamics (New Jersey: Hayden Book Co., 1973), Chap. 4 and references therein. M. J. Clauser, Phys. Rev. Lett. 34_, 570 (1975).

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E. I. l.utsenko, N!. D. Sereda, and L. M. Kontsevoi, Sov, Phys. Tech. Phys. 40, 484 (1975); 42_, 1050 (1976); Sov. J. Plasma Phys. 2_, 39 (1976). ~ 0. Puneman, Phys. Rev. Lett. 1_, 8 (1958). S. M. llamberger and M. Friedman, Phys. Rev. Lett. 2]_, 6741 (1968). S. P. Bugaev, A. M. Iskol'dskii, C. A. Mesyats, D. I. Proskurovskii, Sov. Phvs. Tech. Phvs. 12, 1625 (1968); D. I. Proskurovskii, V. P. Rotshtein, A. F. Shubin, and E. B. YanTcelevich, ibid, 20 1342 (1976); G, N. Fursei and V. M. Zhukov, ibid, 21_, 176 (1976). F. M. Charbonnier, C. J. Bermette, and L. W. Swanson, J. Appl. Phys. 38, 627 (1967). L. P. Mix, J. G. Kelly, G. W. Kuswa, D. W. Swain, and J. N. Olson, J, Vac. Sci. Tech. 10, 931 (1973). K. D. Bergeron, A Mechanism for Electron Emission from Cathode Plasmas in Relativistic Diodes, RS 5241/180 (Internal Memorandum), Unclassified, May 1977. J. P. Quintenz and J. W. Poukey, J. Appl. Phys. 413, 2287 (1977). J. P. Quintenz, to be published.

91 62. R. Lee and Shyke A. Goldstein, private communication,

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68. С L. Olson, 2nd Int. Top. Conf. ILigh Power Electron and Ion Beam Research and Technology, Cornell University, October 3-5, 1977.

69. S. Humphries, Jr., Cornell Report EPS 225, June 1977. PARTICLE BEAM SOURCE DEVELOPMENT

OVERVIEW An additional area of beam research has been the development of a small pulsed X-ray Electron beam research directed toward provid­ source to yield high quality flash X-radio- ing improved in-diode pinched beam sources and graphy of pellets. Л source yielding - 100-um establishing the efficiency and feasibility resolution of objects has been demonstrated for -superposition of many beams progressed in and work continues to '""—ove the convenience three major areas. and reliability of tin. ..ource.

It was established that heating of the anode The effort to extend the capability of higher to ~ 1000°C is a useful means for reducing power conventional pulse power generators to hydrogen content of anode plasmas. The con­ accelerate ions (rather than electrons), and sequent reduction in proton current can allow assess the feasibility of this technology a reduction factor of 2 to 3 in the energy variation for target experiments and reactors lost to ions in large aspect ratio diodes and lias progressed. We have focused our efforts allows more flexibility in the design of future on the use of externally applied magnetic systems for in-diode pinch target experiments. fields to suppress electron flow and completed an exploratory set of experiments on Hermes-II. Focusing stability has been improved from large We identified a source of energy loss caused effective aspect ratio (radius/gap of emitting by electron flow and have nearly completed surface) diodes. The means used is to split design of a follow-on experiment on Proto i cathode surfaces into segments isolated one which should eliminate the electron loss and from another by their own self-inductance. A allow us to test other critical factors central cathode is adjusted to provide a bias affecting ion beam focusability. current which guides the current produced by the outer ring of cathodes into the central Progress toward development of a multistage region. Since the beam does not strike the accelerator for ions with pulse power tech­ anode as it sweeps inward toward the center, nology centered on devalopment of a new labora­ producing plasma on the anode surface as was tory facility and design and procurement uf the case for previous diodes, ion current loss hardware for a five-stage test apparatus for i? reduced by a large factor, and the formation the Pulslac concept. In addition, we obtained speed and stability of pinches were enhanced. encouraging theoretical results indicating TV- principles involved have been demonstrated that effective space charge neutralization on Hydra and Proto I, but the full benefit re­ will exist between acceleration stages of mains to be determined in lower impedance Pulslac. Other "heoretical results indicated operation on Proto II. An additional develop­ that the limits required for gap voltage ment made by the Pulse Power Croup shaved that, stability to maintain longitudinal beam focus using self-magnetic insulation, electromagnetic should te obtainable for realistic parameters energy- could be transmitted to within about of experimental hardware. 20 cm of the axis of Proto II with minima] loss. This development will very likely result in improved formation of pinches at Proto II IMPEDANCE CHARACTERISTICS OF HEATED REB power levels, either with conventional cathodes DIODES or the segmented cathode approach. Recently, considerable emphasis has been plac­ Substantial progress toward establishing the ed upon the use of low-impedance relativistic feasibility of combining beams guided along electron beam (REB) diodej to produce a ionized current-carrying channels has been source of electrons for inertial confinement made. Two beams have been transported, and fusion experiments.1 At diode power below overlayed on a target. Theoretical and experi­ 10" W and impedance above 1 ohm, these mental measurements on channel formation have diodes generate a tightly pinched electron resulted in specifications for the capacitor beam focus with peak current density of bank channel initiation system for a 12 beam approximately 1 MA/cm2. However, for appli­ combination experiment on Proto II, and hard­ cations requiring higher power and lower ware for this experiment is being procured. impedance a rapid collapse2 of the diode

« impedance occurs prior to the termination ^ the X-ray flux emitted through the 50-mm- of the electrical pulse. The investigation" diameter hole in the cathode. of plasma formation in 15 ohm diodes with pulsed holographic techniques has demonstrated that although copious supplies of anode and cathode plasma with densities 2 5 x 10^° electron/cm^ are produced, the impedance is specified by even lower density plasmas which are unobservaMe with holography. Therefore, although the impedance collapse is thought to be caused by the rapid motion of diode plasma which effectively closes the diode gap during the pulse, the relative importance of anode and cathode plasma is uncertain and the gap closure velocity is only inferred by modeling•' the diode impedance. A second mechanism which accentuates the impedance collapse is the in­ creasing efficiency with which ion current is generated 5 as the diode aspect ratio R/D increases. This work reports a successful attempt to reduce the impedance collapse due to these mechanisms by heating the anode to a temperature of approximately 1000°C which is adequate to remove all but the last one or two monolayers of absorbed hydrogen.6 The data obtained are consistent with recent observa­ tions ' of a reduction in the rate of imped­ ance collapse for anodes heated to 400°C. E-BE»M STOP' fOll «NODE

Measurements were made with the Hydra accelera­ tor8 which was operated with the central Fig. 1. A cross-section view of the pinched electrode in positive polarity. A hot cathode electron beam diode and anode heat­ (filament) electron beam was used to heat thin ing apparatus. foil anodes and the inner edge of the REB cathode for 30 minutes prior to and during data acquisition. The heating apparatus, As has been previously observed,4 it was anode foil stretcher, and REB diode used are noted in this experiment that the diode shown in Fig. 1. The anode and cathode current for a pinched REB with cold anode temperatures were estimated to be 1000° and could be expressed approxmately by 200°, respectively, based upon an equation I = Ie + Ij. In this expression the ion of the electrical powers delivered to the current is given by I^(A) = I x (еУ/ап^)5* radiation and conduction heat losses. The e R/D and the electron current by Ie(A) = use of thermocouples or an optical pyrometer 9 x 103(y2 - 1)^R/D. The relationship for for temperature measurements was determined ion current is based upon a simple argument to be impractical because of the high voltage that the space charge of the ion beam should and luminosity of the 2S00°C filament. equal that of the electron beam for electrons that pinch to the diode axis. The electron Diagnostics included the electrical character­ current is functionally similar to the istics of the REB diode, signals from biased critical electron current for self-pinch ion collectors,9 collimated and uncollimated I (A) = 8.5)! 10%2 - I)JSR/D. in these X-ray flux measurements,10 and X-ray pinhole relations у is the relativistic electron photographs. The ion collectors were config­ mass correction, с is the velocity of light, ured with 10~3 cm2 apertures and located at пц is the mass of an ion in the ion beam 17 to 40 cm from the anode. The collectors (taken to be the mass of a proton for a cold were mounted in pairs at radial positions of anode), and R/D is the diode aspect ratio. approximately 1.2 cm on opposite sides to the The diode gap is taken to be D = D, -- vt, diode center line to ascertain the ion beam where v is a fit parameter (v ~ 2.3 cm/usec symmetry. The collimated X-ray flux was for cold anodes in this experiment) which is obtained with a silicon PIN diode which observ­ assumed to relate to the motion of anode ed a 1-cm-diameter region at the center of the and cathode plasma which begins at current anode. The uncollimated PIN diode observed initiation. In this model the diode aspect

94 ratio is chosen to be the larger of the two duration time steps and by dispersing the ratios determined from the inner and outeT current at each step accordinR to the radii of the cathode. The data shown in simultaneous diode voltage. The resultant Fig. 2 are the total diode current, corrected discrete current was smoothed with a 6 db voltage, and impedance for Shot 6095 with low-pass filter between 30 and 60 MHz. The heated tantalum anode. Also shown are the very low-intensity ion collector signal calculated ion current and diode impedance. beginning at 70 ns in Fig. 3 is coincident These curves were obtained using the inner with the expected arrival time of protons. cathode radius aspect ratio and v = 1.7S cm/ When cold anodes were used this time coin­ usee. The value of щ was assumed to be cided with the onset of a lar e signal which that for C+2 on the basis of time-of-flight continued until approximately 50 ns, in analysis of the emitted ion beam. This has agreement with the calculated -irrent for reduced the calculated ion current by a proton flow. At 200 ns the signal increases factor of 2.5 and raised the calculated imped­ rapidly and agrees on a "first order" basis ance by approximately 20 percent with respect with the calculated current for '+2. The to the values obtained foT proton flow. discrepancy observed in Fig. 3 Ы tveen the calculated and measured ion currents is attributed to the presence of ions other than С in the ion flow. After 200 ns, the signal increases again suggesting a component of the current due to Ta+2. The di/.crepancy between the observed and calculated ion cur­ rent varied considerably from shot t> shot and was often different for the pair of collectors. However, the general treed was always consistent with the assumption of weakly ionized carbon and proton emission for the hot and cold anodes, respectively. The absolute magnitude of the ion collector signals could not be used to varify the cal­ culated reduction in ion current for the heated anode shots because of the possible variation in collector efficiency as a func­ tion of ion species. A few shots taken with 125-um-thick copper foils at room tempera­ ture and approximately 500°C gave the same general behavior with more of a tendency for С** and Cu+1 emission for hot anodes.

The X-ray measurements were of little assist­ ance in diagnosing the heated diode behavior. T{nnc) Although it was expected that the hollow electron beam collapse11 velocity would be decreased with a heated anode, because of the Fig. 2 . The continuous curves are the requirement of the beam to heat the anode to corrected diode voltage, total extreme temperatures to create high-Z ions, diode current, and impedance for the collimated PIN diode signals did not Shot 6095. The dotted curves are indicate such a decrease. The X-ray pinhole the calculated ion current and photographs did indicate a difference in the impedance assuming a C+2 ion beam. azimuthal electron beam symmetry however. See text for details of these FOT hot electrodes, eight well-defined radial calculations. spokes were noted which correlated with pie- shaped sections removed from the anode near the outer edge of the cathode. This suggests that the warmer inner edge of the The results shown in Fig. 3 are the ion cathode, which no longer retains an abundent collector signal for Shot 6095 obtained at supply of absorbed hydrogenous material11 17 cm from the anode, and calculated currents 1 2 +2 +2 (> 5 x lO ^ atoms/cm ) suitable for conver­ at the collector location for C and Ta sion to plasma during cathode whisker explo­ ion beams. The calculated currents were sions, plays a less important role in obtained numerically. This was done by electron emission for the heated diode. dividing 1} into approximately 0.4 ns

9S be generated at low impedance (.4.0.S ohms); 1 ' 1 ' ! i for parapotential electron flow12 this . necessitates employing diodes with large radius cathodes and small anode-cathode gaps. Pinch size, however, has been seen -i- г-- li(T +?) - to increase with both increasing cathode z >,H :-^ * i (c* > 3 s radius* and radius to gap ratio (R/d). A - - beam compression ratio (radius of cathode/ half-current radius of the pinch) of about — V r — 10 at R/d of 5 has represented the maximum г pinch "quality" for some time. At R/d greater than about 10, difficulty in turninj " v.; on large area annular cathodes uniformly over their emitting surface and possible instabilities in the radial flow of very i I i i i i . i i thin sheet beams result in the generation о к» гоо зон 400 500 of highly filamented beams which, because of рэог symmetry, generate magnetic fields which preclude the formation of reproducible Т (мм) well-centered pinches and often result in early impedance collapse due to off-axis gap closure by plasmas. In addition, loss of one-half or more of the total diode power Fig. 3. The continuous curve is an ion collec­ to either electrons striking the anode at tor signal from Shot 6095 obtained large radius or to the generation and flow 17 cm from the anode. The dotted of large ion currents-" frequently occurs. curves are calculated ion currents for this collector position assuming C+2 and Та*1 icn beams. In an attempt to alleviate these problems, we have investigated an alternate technique for generating high current electron beams In conclusion, it was observed that although at low impedances. By employing arrays of impedance collapse is not as rapid foT heated discrete cathode elements located on circles diodes, extremely hot electrodes do not pre­ of successively larger radii, and a single vent the collapse. It was not possible to axially mounted cathode of the proper design establish with certainty the origin of the to establish the boundary condition for plasma causing the impedance collapse because parapotential flow,12 we have been able to the electrodes were not independently heated. generate pinches from relatively large The reduced ion current striking the inner diameter (from 12 cm to 1.2 m), large R/d edge of the cathode for the heated diode would (11 to 47) cathodes with improved beam probably not result in less cathode plasma compression ratios and significantly reduced being formed if the quantity of absorbed gas ion currents. Here we report on preliminary were unchanged because of the higher stopping experiments done at relatively high imped­ power of the high-Z ion beam. It can there­ ance (e.g., 1 to 5 ohms), which characterize fore be inferred from the X-ray photographs the production of multiple electron beams, that heating the cathode affected the quan­ their radial propagation, and their combina­ tity of cathode plasma produced by whisker tion on the diode axis. These multiple explosions and at least partially assisted cathode arrays shortly will be employed in minimizing the impedance collapse. This in Proto II pinching experiments at 0.25 indicates that the impedance collapse per­ to 0.5 ohms. haps could be further reduced by use of cathodes heated to 1000°C. The use of biased arrays of segmented cathode has several advantages over a single, large, very hollow annular cathode. Since the ELECTRON BEAM PINCHING FROM DISCRETE LARGE latter operates in an "ion induced pinch" model6 (i.e., ions from anode plasma locally DIAMETER CATHODES reduce the diode E field, thereby enhancing radial direction of the E X В drift), a Over the last few years, attempts to produce significant amount of energy is lost to ion high power density pinches from multimegamp generation and in certain cases the pinch electron beams have met with a number of sweep-in rate is partially controlled by problems. For MeV energies, these beams must

96 the anode composition and rate of ion produc­ by plasmas produced an overbiased condition tion. 17 The former cathode configuration, leading to magnetic cutoff of the outer however, operates in the "bias current pinch" cathode emission and possibly drawing the mode,18 for which early in the beam pulse, pinching electrons off the anode surface radial flow is enhanced by increasing the since the bias cathode was now very near magnetic field with an axial current. anode potential. Third, none of the bias Although anode plasma may still be necessary cathodes used actually produced a pinching for tight pinch formation,19 it will be shown beam itself. The outer electrons therefore later than ion flow can be localized to a would not follow parapotential flow once small area in the axial region and power loss they penetrated the center current sheet, and to ions is significantly less than that pre­ may have struck the anode across the inner viously predicted." cathode. And fourth, the outer cathode, a single-tapered annulus (10 cm O.D., b an I.D., Discrete cathode arrays also exhibit better 50 taper) operating at R/d of 11, generated large scale turn-on uniformity than a large a highly filamented beam with poor azimuthal annulus, partly because of field enhancement symmetry (observable in X-ray pinhole pictures) effects at the ends of the emitting surfaces and highly variable direction in which the and partly due to inductive isolation of each beams were launched. Here we show that the cathode within a circle. For typical geo­ use of biased discrete cathode arrays can metries and current rise rates used in these greatly reduce many of these initial problems. experiments, a discrete cathode which had not yet turned on would have from .100 kV to 200 kV more voltage drop to the anode than would a Diagnostics and Apparatus cathode which had turned on. The formation of nonreproducible beam filamentation and For this series of experiments, the Proto I large angular moment-nn beams by annular and Hydra accelerators were undermatched to cathodes of R/d > 1. an thus be alleviated progressively approach low impedance opera­ by using this technique of "forcing" the turn tion. Proto I (nominally 7 ohms/side) were on of each cathode segment. Although multiple operated single sided from 3 to S ohms with rather than a single beam exists in the diode, typical beam parameters of 1.0 MV, 300 kA, the number of beams now is controlled by the 24 ns; Hydra (nominally 2 ohms) was operated number of cathodes (N) and the magnetic field both normal polarity for X-ray diagnostics possesses N fold symmetry. As will be shown, and reversed polarity for ion diagnostics proper shaping of the emitting surface of with typical beam parameters for both cases the discrete cathodes for generation of a of 900 kV, 450 kA, 100 ns. purely radially directed beam can reduce the angular momentum of the beams. Various tip geometries used for discrete outer cathodes are shovn in Fig. 4. Except In previous work with a biased hollow annular for the cylindrical tip (B), all others were cathode,*3 the bias current was supplied by made with straight or slightl/ curved single- resistively or indictively isolated axial edged emitting surfaces in an attempt to loads (rods or solid cathodes). Although generate beams with a radially inward trajec­ increases in pinch sweep-in velocity by tory and as little azimuthal velocity as factors of 3 to 6 were attained, major possible. In this way we hoped to be able improvement in pinching was not realized, to use the multicathode approach and to in­ for several possible reasons. First, to corporate the recently described paravector- establish the boundary condition for para- potential diode.18 Small cylindrical tapered potential flow of electrons from a single cathodes were used for providing axial bias outer cathode, the axial load must supply current. The following dimensions were used: the current IMas > CVY)Itotal, but for either 2.5 cm O.D., 1.3 cm I.D., 15° taper, or 3.8 cm O.D., 2.5 cm I.D., 10 taper, run the work reported in Ref. 13, only 10 to at inner edge R/dg of 1.4 and 2.5, respec­ 20 percent of the requisite !•..}_„ was tively. Beams from these cathodes have been achieved. Second, the temporal response of studied previously^ and were found to pinch bias current should satisfy the previous rapidly (within 15 ns) and tightly (2 to 4 condition throughout the power pulse. But mm FVHM). Depending on the current they are for the small rods used, their inductance required to supply (they run at relatively resulted in slow current rise early in the flat impedance typically for 25 to 60 ns) the pulse; the outer electrons therefore were duration of most of the experiments of in­ being underbiased during this time and for terest here. Fig. 5a shows a diode setup the larger solid cathodes, rapid gap shorting

97 Fig. 4. Emitting surface geometries for outer cathode tips:

A. Hemispherical edge, concave or convex. B. Cylindrical, 15° taper. С Concave curved edge. D. Straight edge.

for Proto I employing a small diameter bias cathode on axis and a circle of six outer cathodes on a 13-cm-diameter circle; the

latter are operated at an R/dQ of 9 to 15, depending upon the desired impedance. Fig. Sb shows the cathode insert plate for a Hydra experiment where eight outer cathodes

on a 20-cm-diameter circle (run at R/d0 of 20 to 28) surround a larger diameter axial cathode. Also shown just outside the outer discrete cathodes is a single hollow annulus (23 cm diameter) used to generate beams for comparison with the discrete cathode case. The electrical characteristics of each circle of cathode elements was monitored by azi- muthally spaced arrays of dl/dt loops at various radii and with one or more voltage monitors at the edge of the diode. Besides monitoring the current feed symmetry, the current loops were used to determine the current bias relationship between the cathode arrays and the time-dependent characteristics of each circle. Beam formation and propaga­ Fig. 5a. Diode setup for Proto I experiments. tion were diagnosed by recording the brems- Cathode dish with discrete cathode strahlung from anodes of either graphite or arrays is above, anode plate with aluminum of at least one residual range central witness plate is below. thickness. X-ray pinhole photography at Note six-sided damage pattern and various magnifications provided time-integrated axial hole blown through witness information on individual beam structure and plate.

98 I'll — — 7 4} ~ :;. „ :j "v. z - ^ N t

0 4 12 1С i- 0

.IV ,Л Ы Ь Fig. 5b. Cathode insert plate for Hydra experiments. their combination on axis. Three silicon PIN diodes provided time-resolved data on the dose ''.» >№*, 'V. to the anode; two diodes were used with 0.48- cm-diameter aperture lead collimators to observe radiation from the axis and from just / /IrtT lift]* \ inside one of the outer cathodes, while the third diode measured total radiation. Arrays of biased charge collectors' were used to -^--щ,~ estimate the localized current density of ions from the anode plasma, to examine the . --ДГ ion front profile, and to determine the effect of the current biasing condition on ion production. Either 3 or 4 detectors were mounted at various radial and azimuthal ... • .,.. positions and observed ions through various - — holes and slots in the cathode mounting plate (visible in Fig. 5b). A negative bias of 50 -. 1 to 100 volts and flight path of 7.S cm from -- _... ._ - : \m anode to detector were used.

Most results on pinch quality reported here are qualitative; techniques for more quanti­ У *c X tative measurement of pinch size, current 1 density, fraction of total beam energy _/- ...r^ . -,.._ within the pinch region, and sweep-in velo­ city presently are being developed......

Experimental Results Fig. 6. Bias condition, i.e., current ratio The bias conditions for three Hydra shots are plots of I /I , and electron shown in Fig. 6. In these plots, when the total bias falls ust under у plotted vs. time for three cases: current ratio ^tal^bias J underbiased annulus (top), overbias- the у plot and has about the same temporal ed annulus (center), discrete cathode shape, the outer electrons are properly array (bottom). biased during the entire machine pid.se. The

» three cases to be compared are an underbiased cases, the flatter ion profile is a result hollow annulus (hop), an overbiased annulus of electron loss and energy deposition at (center), and an array with 8 discrete outer large radius, as can be seen in Fig. 9. cathodes (bottom). The time-dependent imped­ Here collimated PIN diode signals from X-rays ances of each cathode for these cases are emitted within a 0.18 cm2 are on axis and plotted in Fig. 7, and the ion signals just inside an outer cathode are plotted detected at r = 0, 5, and 10 cm (the latter for underbiased and properly biased cathode being just inside the emitting edge of the arrays. The increase in the axial PIN signal outer annulus or discrete cathode) are shown and decrease in the radial signal under pro­ in Fig. 8. per bias is apparent especially at late times. These results correspond to a reduction by a For the underbiased condition, the annulus factor of 5 in the anode dose at large radius. impedance falls during most of the power pulse at a rage consistent with a gap closure velo­ Several time-integrated X-ray pinhole photo­ city -3 of 4 to 4.5 cm/ysec. Although the graphs of discrete cathode array pinches ion flow is peaked on axis, the signal across are shown in Fig. 10. In 10a and b, the from the annulus is still one-fifth the axial electron loss pattern across the outer value. The fast closure and relatively flat cathodes and the expansion as the beams sweep ion profile suggest that the diode is operat­ inward can be seen to differ fnr different ing similarly to one employing a single un­ tip geometries and bias conditions. Where biased annulus.22 In the early overbiased the individual beams overlap, electrons appear case, when the inner impedance begins to to be lost at localized regions on the anode, rapidly fall as a consequence of axial gap as evidenced by a symmetric pattern of circu­ closure, the annulus becomes magnetically cut­ lar and "petal" shaped X-ray emission sites off; experimentally its impedance appears to in the pinhole pictures and small regions of rapidly increase, but the annulus never spall between the cathodes on the witness actually stops emitting electrons as will be plate. This beam loss may be due to partial explained shortly. The ion signals show cancellation of the magnetic field, with the ion (and consequently the electron) electrons consequently crossing equipotential current density to be very peaked on axis. lines and striking the anode, or it may be a For the near-optimum biased cathode array, result of electrostatic repulsion when the the outer cathodes emit with nearly constant beams combine and the charge density increases, impedance for almost 60 ns and exhibit little since this occurs in a region devoid of ions or no closure (0 to 1.5 cm/ysec). The axial for charge neutralization. It therefore cathode impedance reaches a plateau then appears to be desirable to keep the beams increases, probably due to space charge re­ separate as they sweep radially inward. pulsion between the axial cathode electrons Under higher magnification (10c), the beams and those from the outer cathodes, i.e., can be seen to propagate into the region of this hump in the impedance curve indicates the bias cathode pinch and eventually coalesce success in drawing the outer electrons in on axis. toward the axis. The ion signals still show an axially peaked ion profile. Compared to the underbiased case, the signals at 10 cm Conclusions radius are comparable in amplitude; however, since the ion signal is non-zero only across Our understanding of the operation of biased from the discrete cathode tips, the total ion discrete cathode arrays can be summarized current at large radius is actually reduced using a simple diode model shown in Fig. 11. by the ratio of cathode emitting areas, in With little or no bias current, electrons from this case by a factor of 4, while the axial an outer cathode will follow a trajectory value has increased by a factor of 7.5. similar to No. 1, i.e., weakly pinch, deposit most of their energy at large radii, generate The data in Fig. 8 suggests that ion flow a large ion current, and sweep in at a few occurs in two different regimes in the mm/ns radial velocity. This happens because, diode. On axis, where the inner and outer just as in an ion induced pinch, the electrons electron beams combine, the ion current need the increased magnetic field from ion density is on the order of the space-charge flow to achieve parapotential flow. Under limited value, but across from outer cathodes proper bias condition, which has been deter­

signals only a few percent of the axial value mined experimentally to be Ibias ~ (l/Y)Itotal are detected under proper bias conditions. throughout the pulse, outer electrons will flow Thus, a large area of source-limited ion radially inward at a significantly higher velo­ flow exists at large radius. For underbiased city than for the ion induced case and combine

100 Л —— ~J — "X 10Ш V — r- ti'.l i ^ . '--

— \ .... «Л « / V И\ L ;'U \u ± f1 ' JE^^v 4 /V 14/ AT • i к 1 i Ml AT -10 CM 4.. L N it, г О

д^ ^ HYDF > SI В

T1 I > torn зг : , V tJ- •—' / X J Л / h IOM N kAk/^^^^Vflift утч"г- ,;;* ^ >>*\7/v

oo 1i. e 2 * 3D 4.0 5i ч \o~7 II! ,„.... .i» n a HYDS ' 61 4

/ T / - J OH ft -- .1 \ r \ — lion \ \ \ U V s * f 1(4 \\ ,U| ^ r' s с ч *e^ t 1 о a i.e 4P Ь 0

Fig. 7. Time-dependent impedances of the Fig. 8. Signals from biased ion collectors outer cathode(s), inner cathode, at three radial positions for the and total diode for the three cases of Fig. 6. Signals, not cases of Fig. 6. corrected for 7.5-cm flight path, are mainly from H+ and C^.

101 Fig. 9. Signals from collimated PIN diodes looking on ax,is and just inside outer discrete cathode for two bias cases. Area under curves is pro­ portional to dose delivered to anode at that position.

Fig. 10a. X-ray pinhole photograph of pinch Fig. 10b. X-ray pinhole photograph of pinch using 8 cathodes "A" of Fig. 4. u^g g cathodes "C".

102 Fig. 10c. X-ray pinhole photograph of Proto I pinch using 6 cathodes "D", at higher magnification. Diffuse circular ring in center is 24 mm diameter.

with the other outer beams and electrons from the axial cathode on the diode axis, where the magnetic field rapidly drops (path No. 2). If the outer electron flow is initially over- biased or becomes so during the pulse because the axial A-K gap closes too rapidly from plasma motion, electrons will follow path No. 3, strike the axial cathode, and even­ tually be emitted toward the anode but at the much smaller R/d of the center cathode, i.e., at higheT impedance and inductance. Kith greater overbias, outer electrons will be magnetically cutoff so fast [path No. 4) that they return to the cathode plate outside the inner BDOT position and contribute to the power flow into the axial cathode. Since the outer cathode current is obtained by measuring the difference between inner and outer BDOT signals, path 4 gives zero outer current Fig. 11. Cross section of diode showing (though outer cathode emission is not zero) possible electron trajectories and explains why the impedance appears to rapidly rise at late time in Fig. 7. from outer cathode(s).

For highest power deposition on axis and low- impeda::c.2, low-inductance operation, diode conditions for the approaching Protc II experiments will be optimized for beam focusing from a path No. 2 type mechanism.

103 REB PROPAGATION AND COMBINATION IN PLASMA concentric grooves with sharp edges machined CHANNELS into it. The grooves in the cathode face caused rapid turn-on and uniform emission to The ignition of thermonuclear pellets by rela- occur. For pinch diagnostics, thin, flat tivistic electron beams (REB's) will require carbon anodes were employed. They allowed the development of sources capable of deliv­ highly resolved X-ray pinhole photography to ering tens or hundreds of terrawatts of be performed and the anode blow off generall; electrons at voltages near one megavolt and did only minor damage to the cathode face. at very high current densities. While the use Anode-cathode gaps of 2 to 4 mm were - all) of self-pinched electron diodes is a promising used, with impedances in the range on L СО 6 approach to this problem, one encounters ohms resulting. significant difficulties in going beyond the one terrawatt level in a single diode. These X-ray diagnostics of the pinched beam were difficulties include the following: [1) power performed with an X-ray pinhole camera (time- loss approaching 50 percent at the one mega- integrated) and spatially apertured PIN diode volt, one meganrp level can occur in a diode Measurements of the latter type (described hy the generation of a counter-streaming ion elsewhere)2'* indicated that, once the pinch beam if suppression measures are not employed; was formed, all (> 90 perr.pnt) of the X-rays (21 radial compression ratio limits of only produced came from within a 10-mm-diameter 10:1 are commonly seen for efficient, intense region on axis. From this information and pinch generation; (3) anode-cathode gaps tend densitometer traces of pinhole pictures we to short circuit with effective gap closure infer that the pinch was 3-mm HVHM •:tl i a pea velocities of a few cm/us; and (4) pinch current density of 3 MA/cm3, when ;V diode stability becomes poor for diode aspect ratios power was 0.4 TW. For work descriLed below, above 20:1. Taken together, these restrictions two separate anodes and cathodes were driven imply a practical limit of near one terrawatt by a single Hydra line, so that only about at a few olims for the generation of efficient, 0.2S TW were available per cathode. Under intense pinches by a single REB diode. While these conditions, the available pinch inten­ these restrictions can be overcome somewhat, sity was reduced substantially. a substantial breakthrough is required to obtain useful pinches well above the one- terrawatt level. Consequently, in the REB Injection Into Channels pursuit of fusion by electrons one is led to approaches involving the generation of Once a satisfactor)' pinch has been obtained, many independent pinched beams and their the REB must be successfully injected into a subsequent combination.23,24 jn tj,is channel. The channeJ is a current-carrying section, we present results of preliminary plasma which provides for beam current and studies of .;uch a system. In the arrange­ charge neutralization. Ideally, the channel ment under consideration, dozens of separate properties are chosen so that once a beam pinched beams are to be transported via electron enters the channel, it should drift plasma channels from diodes to a target freely down the channel under the influence legion. At the target, suitable beam over­ only of the channel fields. The beam electror lap must be achieved ind suitable incidence should experience no collective forces. Con­ on the target must -.•-. The distinct sequently, a simple abrupt transition from elements of the >oi>'i.'-: ? be discussed are the diode to tlie channel plasma and fields (1) pinch formats v. in r: J.iode, (2) REB should provide satisfactory injection. A injection into t» .-iinsiv. channel, (3) channel particle simulation of this process lias been formation and b?;, • | ••.-•-; Ration, and 00 beam performed and reported elsewhere.24 Note overlap at the tar^ .. that for beams injected with large transverse energy, some electrons will experience excur­ sions beyond their initial radius of injection As a result, the beam current density in 'he Pinch Formation channel may be lower than at the anode r-r le. In the course of other work, a simple anode- cathode geometry was developed for the Hydra In experiments, the channel termination at the accelerator^^ which has characteristics anode plane can produce problems. The channel: suitable for the present study. The cathode used in this work were formed by wire-initiatet was a 76 mm O.D., 33 mm I.D., hollow brass discharges, in the ambient air. The 2.54 x 10"- cylindrical cathode with a 10° taper. The cm diameter tungsten wires were attached eithe: front face of the cathode had a series of directly to thin foil anodes or to wire bridle:

104 directly behind the anode. The thin anodes served as electrical grounds for the diode m _ (p + q) Э_ (ru) fields, as return current paths for wire and dt г Эг channel currents, as mechanical wire supports, and as pressure windows. In all cases tried, substantial radial beam blowup was experienced at the injection point, so that the trans­ ported beam was of much loweT arrrent density than available at the anode plane. In addi­ tion, some beam loss occurred at the injection dll _ Э_ Г И)9 3_ 3u (rll) point. In the best cases, 80 percent injec­ dt " Эг I 4nrc Эг Эг tion efficiency was ob:uined. Л radial blowup factor of two or three was common because the channel size was several times larger than where the pincli diameter.

d 3_ 3 RUB Transport in Plasma Qianncls dt 3t 37 The channels used to transport the RGB's from the diodes to the target vicinity are current- Hie parameters, p, u, p, T, and Ц are the carrying, highly conducting, plasma discharges. air density, radial velocity, pressure, They satisfy the need for a small diameter temperature, and specific energy, while q column which is charge and current neutralizing is the usual artificial viscosity. 'Die magne­ and has a "rigid" imbedded magnetic field. Hie tic field II is assumed to be axiro.ith.al and channels could, in principle, be formed in the current density axial. Л simple Ohm's many ways. In this work, they were initiated law and Maxwell's equations were used to by air by fine tungsten wiTes which were obtain l-.q. H). driven by a 20 kV, 15 pF bank. The tungsten wires play mainly a fonnative role in the The set of equations was closed by the addi­ discharge process, with the wire material tion of a tabular equation of state (which remaining on axis while the cylindrical dis­ included ionization effects) relating the charge column expands in air about the wire. specific energy to the pressure and Lcmpcru- Holograms of the channel are shoivn in Fig. ture. Magnetic field boundary conditions 12 for an earlv time and near peak current were provided by specifying the total current (50 кЛ). carrjed by the channel as a function of time, using values measured experimentally. It was A 1-t) magnetohydrodynamic OHD) model has assumed that the wire served only to initiate been used to provide a means for comparison the air discharge and its presence was not with the experimental results and estimates included in the calculations. The initial con­ of channel parameters required for beam ditions required included the temperature and propagation models. The model is quite radius of the discharge at the start of the similar to one described by Aleksandrov, computations. These were not known from experi­ 27 et al.26 and Pavlovskii, et al. The ment, and values of T0 - 0.6 eV and r0 ~ 0.06 conservation equations governing mass, cm were used. A solution to the set of equa­ momentum, energy, and magnetic field in­ tions was obtained by modifying the existing tensity in cylindrical coordinates (practical CHART-D 1-D L'agrangian hydrocode.28 These cgs units) are: modifications included adding the Lorentz force term, ohmic heating and a magnetic field solver. dp _ £ 2_ (ru) r Эг cu The solution to the equations also required ar specification of the transport properties, CJ, the electrical conductivity, and <, the du radiative diffusivity. The conductivity л - э_ (p + q) - 0.1 jH (2) a Эт included contributions from both electron ion2^ and electron-neutral collisions. The

105 nuimminuuiiuiuv

! Ч ЧГ. шшшшш}А Fig. 12a. Hologram of channel at 1 ps.

Fig. 12b. Hologram of channel at 6 ps.

104 commonly available tabulaT Rosseland mean Simple estimates may be made of REB-induced free paths AR (required to determine the effects on channel dynamics. Using conducti­ radiative diffusivity) are not suitable for vity, mass distribution, and energy deposition these calculations because they neglect information such as calculated above, one may absorption of the more energetic photons. estimate REB energy loss, degree of current Aleksandrov, et al.26 and Pavlovskii, et al." neutralization, and beam-induced channel expan­ used an approximate AR = 6 x 10 "s T1>33/p sion. Such estimates show that adverse effects CT in eV). should not be prohibitive with air channels near 1 m long at REB currents near 1 MA at The present computations studied both the densities of several MA/cm». For the case of Pavlovskii approximation and a simplified very much more intense REB's strong beam- multigroup radiation approach where an effec­ channel coupling could occur, and variation of gas species and channel formation teclini- tive AR diffusion approximation was used for the short-wavelength photons (T > 7 eV) and que would need to be considered. a loss term E = 4ogTfypA„ was used for longer wavelength photons (T < у eV). Die parameter A« is the Planck mean free path and ag is the REB Overlap Stefan-Boltzmann constant. In general, the code predicts the existence of a high tempera­ The general problem of REB overlap is to ture (few eV), low-mass-density channel core, determine a plasma and field geometry which surrounded by a cold, high-density tube of gas. achieves large beam superposition at a tar­ This is seen in the comparison of integrated get and is consistent with target require­ refractivities in Fig, 13, where the Pavlovskii ments. As a first step, we have analyzed AR was used for these calculations. The com­ a single, simple case and have neglected parison of both the channel boundary expansion specific target requirements. velocity and refractivity is good. The cal­ culated current density distributions are Of the many possible transport and overlap nearly uniform in the hot core. schemes, the one considered consists of 40

uniform current-carrying channels (radii rc) arranged circumferentially around the target with return current channels forming the axle of a wagonwheel configuration.30 Fig. 14 shows one concept of how such a wire configu­ ration would fit in the front end of a multi- beam accelerator. For a given plasma channel

radius, rc, the converging channels begin to overlap at a radial distance R~ from the center of the configuration. The plasma current along a beam channel is assumed to be constant in as far as the channel overlap

radius (RQ), where it linearly decays over a specified distance as the individual currents merge into a disk current. A channel current of 20 kA was chosen for 1 MeV electrons to provide good containment of the beam, while remaining below the Alfven-Lawson critical current of about 50 kA. The two axial return current channels are assumed to have expanded to twice the radius of the beam channels. In this study, an injected electron distribution was sampled by varying the initial phase space coordinates, calculating the 3-D trajectories, and recording the distance of closest approach to the center of the configu­ ration (r = 0, z = 0). Note that the assump­ 3 6 9 mm tion of current neutralization implies that the electron trajectories are noninteracting and therefore simple superposition occurs in Fig. 13. Radial profiles of fringe shift the beam overlap region. Scattering and count from holograms in Fig. 12 electric fields are assumed negligible and and corresponding theoretical symmetry of target irradiation is not evaluated. predictions.

107 From these results, it appears that most of the injected electrons from a realistic beam can get inside a Tadius roughly one-quarter of the channel overlap radius at the hub of the configuration, regardless of the length of the channels. An estimate of the beam concentration (current density gain G) in these calculations is obtained by comparing the net current density crossing the cylin­

drical region of radius J^jn and height 2rc with the current density in a single channel

G - jnet/jc - 10 rc/Rmin „ *

This result depends sensitively on the actual plasma current configuration near the center of the system. We should note that the channel studies reported here have not been optimized, and higher gains may be obtained by reducing the overlap radius using different channel configurations and/or smaller channels. In fact, recent preliminary results predict that gains of - 10 can be attained, and the ultimate gain possible has not been determined but could be even greater. Fig. 14. Concept for multibeam front end for Thus far, experiments have addressed several EBFA. parts of the multiple beam conioination problem. The efficient generation and transport of a single beam has been reported previously.^p The variation in the distance of closest More recently, a dual diode front-end was approach to the center of the configuration constructed for the Hydra accelerator. It was (R^yj) attained for different injection con­ used to generate two independent beams driven ditions is shown in Fig. 15. The results of by a single Hydra line. The interaction of Fig. IS, when weighed with a realistic, non- the two beams at a combination region was then hollow, injected beam distribution should studied. actually produce better beam concentration than apparent in the figure. In Fig. 15a, 1 MeV electrons were injected at the channel The total energy available to the two anodes was 50 kJ. Total power and net impedance radius (rc) with a velocity nearly parallel to the channel axis. The electrons execute behavior is shown in Pig. 16a. The two diodes betatron oscillations approximately in the were generally balanced in energy to within injection plane of the channel. Inside the 10 percent. A top view of the experimental channel overlap region there is cylindrical arrangement is indicated in Fig. 16b. The symmetry, so that when the injection plane flat carbon target was pulsed positive by is aligned with the x-y plane (a = 0) all the bank and 'the two wires were driven in of the betatron oscillation can go into parallel (60 kA total peak current). X-ray angular momentum (p*) in the overlap region. pinhole photographs were taken with the However, for a = 90 , none of the betatron camera in the position shown, so that images motion appears in Рф when the electron passes of the anodes, wire support structures, wires, into the overlap region. Figure 15b shows and target were all recorded. Two types of the result of varying the amplitude of the shots were taken: first, the wires were dis­ betatron oscillation for an intermediate placed vertically from each other at the injection plane angle (a = 45°). In Fig. 15c, target; second, the wires were attached to the axial variation of the electron distribu­ a common point at the target. X-ray images tion is obtained by moving the injection point such as in Fig. 17 were obtained. The anode assembly image is relatively bright because (r = rc, a = 45°) along the channel for a betatron cycle. In Fig. 15d, electrons are of the presence of high-Z material and because injected at the channel edge with an increas­ the injection efficiency was роотег than that reported previously.30 ch ome shots, the ing nonaxial velocity component. S

106 mm-lion pl>ne IB ch»nn»l ovtr milancr of vHociljr • nt» •ii.l H punt one liTmor radio» .

4-0 r'Smm ch»nn*\ axil tn0* a v;t»Vj.vJ

"\

Fig. 15. Summary of point-of-closest-approach results from 40 beam calculation.

0.5 TW

JODNS 200 N5

Fig. 16a. Total Jiode power and net impedance for dual diode experiment.

109 X-»»V С»М!Я»

Fig. 16b. Schematic top view of dual diode experiments showing pinhole camera position used for Fig. 17. Wires were staggered vertically as target for some shots.

Fig. 17. X-ray pinhole images showing anode structures, wires, and target. Spacing between anodes = 11.5 cm; wire lengths = 31 cm; spacing between spots at target in top image = 6 cm.

J 110 I high density tube of air at the channel edge In addition, a unique requirement of the FXR was visible in the X-ray photograph. system arises from the necessity to discrimi­ nate against the background REB bremsstrahlung From this experiment, we conclude that no pulse. We have been successful in developing gross problems arise at the intersection of an FXR system that will for the first time two beams and channels at a target. Some resolve sufficient details to begin the current density enhancement was obtained experimental examination of instabilities. at the overlap region, but no calculations It consists of a 600 kV, ~ 100 um diameter have yet been performed for this configuration and 3 ns FXR source and a time resolvable which are useful for quantitative comparison. duo-microchannel plate X-ray camera that can be pulse actuated to discriminate against the background REB X-ray pulse.

Initial studies of the basic elements of the A 3 ns, 600 kV commercially available FXR multiple REB combination scheme have been source has been modified to produce a X-ray carried out. Results of theoretical analyses spot size of ~»100 ym. With this small source, and experiments concerning beam pinching, magnification flash X-radiography of small injection, transport, and combination via objects several mm in size can be achieved. plasma channels are encouraging. Ultimate This is demonstrated in Fig. 18. The objects limits to power density achievable by the in this radiograph consist of 0.02", 0.01", technique have not yet been determined. and 0.005" tungsten wires spaced 0.02", 0.01", and 0.005" apart, and four gold spheres, two with 3 mm diameter and 0.002" wall thickness A HIGH RESOLUTION TIME RESOLVABLE FLASH (note the small dent on one of these), and X-RADIOGRAPHY SYSTEM two 4 mm diameter with 0.008" wall thickness. The object to source distance was 10 cm and Hydrodynamic instabilities have been predict­ the object to film distance was 30 cm. This ed for Relastivistic Electron Beam (REB) arrangement gave an image magnification of inertial confinement fusion targets during three. implosion. Hue to the high density target materials used, for example gold or iron, Figure 19 shows a schematic diagram of a duo- observations of the target implosion dynamics microchannel plate X-ray camera. The input and instabilities are only possible with channel plate performs as an X-ray converter flash X-radiography (FXR) techniques. The and a preamplifier with a gain of ~ 300. It FXR system resolution and penetrating power serves to convert the incident photon image required for observing the different instabi­ to an electron image which is accelerated by lities are listed in the following table. the gridless electrode to the second channel

TABLE I CHARACTERISTIC LENGTHS AMD AERIAL DENSITIES

CHARACTERISTIf PENETRATING INSTABILITIES LENGTH (cm) POWER (E/cm2)

Gross Instability resulting 0.1 < 0.25 from target and/or irradia­ tion assymmetry

Ablator-pusher 0.02 -0.25 - 1.5 Rayleigh-Taylor

Pusher-fuel 0.301 -1.0 - 4.0 Rayleigh-Taylor

III " Щ framing intervals can be achieved. In addi­ tion to projection X-radiography, this versii can be used in conjunction with a pinhole arrangement to provide time resolvable X-ray !,,t!| pinhole photography. Mill Coupling the FXR source with the duo-channel i ' i i • plate camera, the system can be used to recoi the target behavior after the REB pulse. Synchronizing the gating pulses with the FXR pulses after the REB X-ray pulse eliminates the background bremsstrahlung from the image. Using a projection arrangement to provide a magnification factor of three, the system re­ solution is unaffected by the camera resoluti and only determined by the projection geometr The system spatial resolution is ~ 100 urn and temporal resolution is potentially 1 ns. The penetrating power of this system as deter mined by a Au stepwedge is ~ 1.5 gm/cm2. Thi Fig. 18. X-radiograph made with the 100 vim system provides us a useful tool to study mos source. of the possible instabilities with the excep­ tion of those occurring in the pusher-fuel interface.

HIGH POWER MAGNETICALLY INSULATED RADIAL DIOD

^BIMiy. I [Г'В-IK It has long been recognized by workers in ICF that the creation of intense focused ion beams would best satisfy pellet ignition requirements, but the requisite sources were discerned to be improbable. Recently, as a result of the development of potentially suitable ion beam ignition source technology, calculations predicated upon ion beams have

been published. 32-34 we report here progress toward extending previous ion diode results Fig. 19. Duo-microcharmel plate X-rav camera. to create an intense ion beam which will be useful in pellet fusion studies. The final goal is to utilize pulse-power technology plate. During transit the ""„-ctron image to create a beam which will be useful in quality is preserved by a 4-k gauss magnetic pellet fusion studies. The final goal is to field. The gyro-diameter (0.2 mm) of the utilize pulse-power technology to create a electrons in this field defines the intrinsic beam which can deposit ~ 10" W/cm^ symmetri­ resolution of this camera. After further ampli­ cally upon a pellet while maintaining good fication (~ 300) by the second channel plates impedance match with the power source for the image is rendered visible by a phosphor the duration of the pulse. screen and recorded onto film via contact printing. Since the channel plates can be operated in the pulsed mode, by pulsing on Principles of Operation the bias voltage for the channel plates, the camera can operate with a shutter speed as The nominal 10 MeV 140 kA Hermes35 accel­ fast as 10 ns. erator was employed as the pulse-power source for the experiment described; the diode is A second version of this camera with multiple shown in Fig. 20. Magnetic insulation to framing capability has also been constructed suppress radial electron flow was provi3ed and is now undergoing tests. In this version, by pulsed coils with a period of 10 milli­ the input stage consists of five separately . seconds to allow nearly unimpeded penetration gatable channel plates. In this way, five of the field lines through the stainless steel consecutive 10-ns X-radiographs with variable

112 polycarbonate plastic was inserted in the cathode shank to serve as a prepulse suppressor giving a capacitive division ratio of > 4:1 to the 800-kV prepulse. PERFORATED" METAL CATHODE4 Diagnostics and Results 20 cm Three В loops were positioned 120° apart on the input and output faces to estimate WITUCSSPLATE*- current flowing in the diode. Azimuthal variations of the В signals were insignifi­ cant. A pinhole X-ray camera was used to STAINLESS STEtlS, image the ends of the diode to determine the position of the electron leakage and thermoluminescent detectors (TLD's) and PIN CURRENT LOOPS (3) diodes were used to compare the X-ray flux levels from different shots at several loca­ tions. c*s. 22 shows an example of electron leaka^ - .surements at the drift tube end of the Jioi • obtained with the pinhole Fig. .'(). Radial ion diode apparatus. camera for ri = Bcrjt.* It is apparent from the white patches on the drift tube wall in the figure that a substantial current of housing. The calculated field plot is shown electrons was lost in radial flow along the in Fig. 21; measurements confirmed that eddy field lines in the forward direction. The currents did not alter the field configura­ faint elliptical pattern surrouding the tion. The magnetic field lines were nearly cathode indicates a small electron loss to perpendicular to the electric field in the the anode where it joins the drift tube wall. highly stressed portions of the anode and The pinhole camera viewing the input region cathode to reduce electron leakage current, of the diode indicated that < 10 percent of and the path length along lines connecting the leakage current appeared in this region the anode and cathode was made as long as and this was confirmed by TLD's. This is possible for the same reason. The gap spac­ expected because the radial electric field ing and the area of the ion-emitting source and azimuthal magnetic field due to current were chosen by assuming Child-Langmuir limited in the cathode shank results in an axial flow of protons with an applied voltage of drift force on electrons so that radial 7.S MeV. Other factors considered were leakage would be in the forward direction. possible plasma closure of the gap during the 800 ns prepulse and the magnetic field limit of about 20 kG which could be produced with The targets were either polyethylene or easily constructed coils and available energy aluminum sheets, or conic sections which storage. The ion source was a piece of nylon served both as witness plates and as targets plastic window screen placed in contact with for neutron production. The purpose of the the stainless steel anode surface. During conic sections was to determine the azimuthal the voltage rise capacitive energy is stored uniformity of the ion flux; no azimuthal within the plastic and eventually the surface structure larger than the cathode screen breakdown strength is exceeded causing the mesh size was in evidence. Damage patterns stored energy to dissipate in a surface on plates showed clear outlines of the per­ plasma. We have not yet attempted to analyze forations in the cathode screen CFig. 23), this process in detail or to optimize the indicating that space charge neutralization plasma layer formation. For several shots, of the ion stream was rapid and nearly com­ a layer of deuterated polyethylene powder plete and that ion beam divergence was at suspended in a volatile liquid was coated most a few degrees. Electron transport over the screen. We assumed that hydrocarbon along the В field lines within the cathode contamination in the diode would cause at least half of the ion flux to be protons, is the field for which an electron rather than deuterons,„consistent with pre­ crit viously reported data.j(' A 10-cm length of gyrogradius equals the gap spacing.

113 5 -

I x 1Q5 CATHODE SURFACE THIS FIELD LINE RETURNS TO ANODE FLANGE AT 18" RADIUS

1 2 3 4 5 G DISTANCE IN INCHES ALONG AXIS

Fig. 21. Plot of magnetic lines of force in gap.

I

Fig- 22. Pinhole photograph of electron leakage current in forward direction.

IM and field emission from the regions where d(d,n)He3 reaction. In the absence of CD2, these lines intersect the cathode is the it was possible to study proton induced means postulated for neutralization. It reactions. The neutron detectOTS included was also noted that the intense region of two silver activation counters38 and two ion emission was restricted to areas covered scintillation time-of-flight (STF) detectors39 by the plastic mesh. placed 8.8 and 17.9 meters from the diode. The STF's had a lineaT response and peak cur­ Based upon current measurements and neutron rent ratings of 0.6 and 1.2 A, and were shield­ output, we estimate that approximately 20 ed with 20.3 cm of lead. percent of the current was carried by protons. The balance of the current consisted of An example of STF data obtained from the detec­ electrons which were diffusely deposited on tor located 8.8 m from the diode is shown in the drift chamber wall. The peak diode volt­ Fig. 25. (A CD2 source and a CH2 target were age, corrected for inductance, was ~ 4 MV, employed.) The first peak is caused by and the total diode current was 160 kA. A bremsstrahlung from leakage electrons. The leading edge of the second peak is attributed plot of diode voltage and current for a typi­ 13 14 cal diode is shown in Fig. 24. Since the primarily to fast neutrons from the C (d,n)N reaction. The change in slope at 475 ns after inductive correction to the voltage was > SO the X-ray signal may be caused by the peak pro­ percent, errors in calculating the impedance duction of neutrons from the Cl2(d,n)Nl-> reac­ are considerable. Nevertheless, we note that tion. Of course, complications caused by the impedance appears to drop considerably other reactions which can occur for the mate­ toward the end of the pulse. The impedance rials in the diode in the 4 to 6 MeV ion would be expected to stay fairly constant energy range available, and the 100 ns neutron over the 100 ns power pulse because the gap production interval make precise measurements is > 2 cm and plasma closure at 3 cm/psec difficult to carry out. would change the effective gap by less than 10 percent, teasurements by others37 have For a CD -coated ion source and a CH2 target, shown that the presence of magnetic fields 2 > B tends t0 hol| the neutron flux recorded on the silver crit i back plasma expansion detectors was 5 x 109 to 7 x 1010. Using the and hold the impedance constant. It is value 4.2 MV for corrected diode voltage and likely that either plasma generated during total neutron yield cross-sections,40 we cal­ the prepulse or increased electron leakage, culate the diode current to be 20 kA of ions. perhaps from field emitted electrons on the We assumed in this calculation that about 50 cathode end are responsible for the fall in percent of the ions accelerated were protons impedance. from diode contamination, in agreement with previous data.3f> Since the reaction cross- Because of the extreme potentials developed sections have a large functional dependence by Hermes and the high diode feed inductance on voltage, the ion current inferences may (-1600 nh) it was impractical to incorporate be in error by as much as a factor of two. leads to an external electrical recording system in the cathode. Therefore, calori- metry was •--tempted by using a liquid crystal This experiment differs from a previously thermomet . attached to a box behind an reported radial ion diode experimental for aperture. This device was to be photographed which 60 percent generation efficiency was a few seconds afteT the shot. Unfortunately, measured. In the previous work, the exclu­ hot vapor from the diode appeared to damage sion of field lines from the cathode would the liquid crystal surface and may also have have caused a bulge in the field pattern contributed to the deposited energy in the in the vicinity of the end of the cathode. box. We expect that the new ion diede out­ The flow of electrons would have been inhibit­ lined below will be more accessible to calori- ed because E and В would be approximately metry. Additional ion production data was orthogonal, a condition less well preserved obtained by measuring the neutron output and in this experiment. In addition, it is energy from several combinations of ion source possible that the glass insulator in the and target during the course of the experiment. previous experiment could have reflected A deuterated polyethylene coating was employed some of the electrons. If a single current on the ion source screen together with a CH2 feed such as employed in our experiment were taTget to produce the C12(d,n)N13 and used in the future, we would suggest using a Cl3(d,n)№W reactions. For a few shots, a В field modified to be mirror-like at the open thin CD2 coating was used over the CH2 target end for the purpose of reducing leakage cur­ in an attempt to superpose some yield from the rent.

IIS .-'•pri'l '•!''•' •i {'(• (ч•)'i• i• >•:• t• Ь: 71 el ! Q! J*SS''-;

Fig. 23. Aluminum witness plate showing damage pattern. The structure is due to the pattern of the perforations in the cathode mesh.

50 100 TIME Insecl

Fig. 24. Ion diode voltage and current (inductance corrected}.

116 radial feed interposed. This allows the mag­ netic coils to be readily connected to a capacitor bank and keeps the target at ground potential providing easy diagnostic access. The azimuthal magnetic fields caused by the diode current itself will peTturb the focal spot size. This effect can be partially compensated by shaping the gap cathode, so that the original velocity of the ions is altered in a manner which causes•the ions to converge for a particular diode current.

AXIS OF SYMMETRY MAGNETIC COILS m RADIAL PULSE LINES OFPROTO I OR II

Fig. 25. A typical oscilloscope trace on the 8.8-m TOF detector showing the X-ray ANODE ^ DIELECTRIC MESH bremsstrahlung signal and the ion- (GOOD CONDUCTOR) ION SOURCE beam target neutrons. The horizon­ CATHODE MAGNETIC FIELD LINES tal sweep rate was 200 ns/div and the vertical sensitivity was 4 volts/ div. Fig. 26. Next generation two-sided feed, quasispherical, ion diode experi­ ment.

Similarities in observations of beam quality, integrity of the acceleration gap and ion Certain questions will be addressed to assess source after many shots, space-charge neutra­ the utility of ion diodes for fusion research. lization, and insulation against electron We will, for instance, explore the dependence flow across the annular region of the gap of beam quality on cathode mesh size and ion give us confidence in the utility of extend­ source design. We will find out whether ing this technique for fusion research. electrons moving freely along field lines will allow those lines to effectively smooth out. potential variations so that the field Direction of Future Research lines can be viewed as equipotentials. To operate at small gap spacing and low imped­ The defects of end leakage of electrons and ance, we will have to confirm and extend magnetic deflection of the protons to an previous results showing that В fields can off-axis locus can be circumvented, by using delay plasma closure of the gap so that a the design conceptually sketched in Fig. 26. good impedance match can exist through the In this design, the magnetic coils are placed pulse. The diode self-magnetic fields may within the cathode structure, which is magne­ further aid in reducing electron leakage as tically transparent. The anode is a good high powers are attained. The spherical conductor which expels the field so that focus experiment nasi being planned for the magnetic lines of force are tangential to 1 to 2 TW Proto I machine is expected to pro­ the anode surface. The total magnetic flux vide answers to many of these queftions dur­ traversed by a particle moving from the anode ing the next year. A cross-sectional view to the center is zero, so that ions undergo is shown in Fig. 27. no net deflection. Since the lines of force close around the cathode structure-and drifts are inward from both ends, electron leakage should be small, por the sketch shown, the Sandia Proto I and II accelerators rated at 2 and 8 terrawatts are ideally configured with two grounded radial feeds with a positive

117 this heavy ion current represents a 104 extrapolation over present experience with light ion beams. If multistage linear accelerators could be developed with increase current carrying capacity, a vast parameter space of intermediate mass ions would be accessible which could have great practical advantages over the two extreme cases con­ sidered. The energy of the intermediate mass beam would be chosen to fulfill two requirements: (1) the beam current should be easily handled using conservative pulsed voltage technology, (2} the acceleration of the beam must reduce the beam divergence enough to achieve the required standoff dis­ tance from the final lens. A 1.S TW module using 50 kA of 0+ at 300 MeV with 3-m stand­ off from the target would be well suited to reactor studies and is theoretically possible.4< >

Fig. 27. Proto I ion diode in cross section. Compared to light ion diodes, multistage intense accelerators would have the follow­ ing advantages for fusion applications: (a) the decreased divergence angle from accel HIGH INTENSITY PULSED LINEAR ION ACCELERATORS eration of the initial injector distribution would allow standoff from the target, (b) A relatively small (1 staff and 1 technical physical electrodes are not necessary; assistant) experimental program has been therefore, there is the potential for high underway since spring 1977 to develop linear duty cycle operation, and (c) the require­ ion accelerators with vastly increased pulsed ments on power flow in the conversion of output capabilities compared to conventional electrical energy to beam kinetic energj devices. The experiments are aimed at a are reduced since the conversion process demonstration of the principles of space is spread out in space and time. The advan­ charge neutralized ion beam transport tages over conventional accelerators using described in Ref. *2. In this report, the heavy ions are: (a) the ability to operate relevance of this work to the inertial con­ at lower beam energy makes the accelerator finement program will first be discussed, system more compact, less complex and much followed by a brief review of the physical less expensive, (b) the possibility of principles of the device. The work done to using strong electrostatic beam focusing date is listed. To conclude, experiments eliminates the vast magnetic field energy planned fOT the next six months are outlined. that would be needed to contain heavy ion beams, (c) the.system is modular, so that tests can be made on devices costing a The advantages of ions for initiating fusion small fraction of the cost of a demonstra­ 13 bums of fuel pellets are well-known.' Two tion plant, [d) the requirements on the possibilities for the production of the re­ injector and low velocity acceleration sec­ quired ion energy fluxes have been proposed, tion can theoretically be met, (e) the 44 light ion diodes and heaw ion beams from accelerator has a basically uniform design conventional accelerators.^ Light ion throughout its length and no complex switch­ diodes, an outgrowth of recent progress in ing and transport elements are needed for the ability to produce proton beams using current multiplication, and (f) magnetically the technology developed for relativistic insulated diodes driven by pulse lines have electron beams, can be designed for high a demonstrated high efficiency. power levels in the voltage range appropriate for protons (1 to 10 MeV). Consideration of heavy ions (U+) is a consequence of severe The theoretical basis for multistage magne­ current transport limitations in conventional tically insulated ion accelerators are accelerators; to achieve the required power described in detail in Refs. 42 and 47. at low current levels CI kA) very high beam Referring to Fig. 28 the accelerator con­ energies (100 GeV) must be used. Even so, sists of a number of annular drift tubes

118 separated by narrow acceleration gaps per­ presented in Ref. 47. It was suspected that meated by radial magnetic fields. Inside nonrelativistic beams accelerated by loaded the drift spaces, which form the bulk of pulse lines would be subject to instabilities the accelerator length, the beam can be resulting in filamentation of the longitudi­ neutralized by cold electrons since the nal phase space, degradation of the current magnetic fields prevent the backflow of profile and, hence, difficulties in matching electrons, allowing the interior of the and controlling the beam in downstream sec­ tubes to be at uniform potential. Sources tions. The instability comes about because of cold electrons line the tube walls and, voltage variations in upstream gaps (which consistent with experiment observations, are inevitable if the beam is to draw signi­ these electrons can rapidly flow into the ficant energy from the pulse line) produce beam causing an almost complete cancellation extrusions of the longitudinal phase space. of space charge fields. In the acceleration These can velocity bunch so that current gap, the beam will be unneutralized, but the spikes are produced in downstream gaps. magnetic insulation allows strong fields to These produce larger voltage variations, be impressed on the gap (MV/cm) eliminating and hence the process bootstraps. In order longitudinal space chaige effects. In to investigate the severity of this problem, addition, if the magnetic fields are shaped a computer simulation code (PILAC) was as is shown in Fig. 2S. the electrostatic developed to trace particle orbits through equipotentials will be similarly shaped multigap systems driven by realistic pulse since the magnetic field lines determine line models. The code allows arbitrary the location of virtual electrodes. Thus, initial phase distributions, addition of transverse electric fields can be impressed shunt circuit elements, and adjustment of which are large enough to overcome transverse the pulse charge, characteristic impedance, space charge effects. An additional use of and timing of the various acceleration the insulating magnetic fields is that they stages. Testing was done at low beam loading can allow controlled, voltage dependent with a positive ramp in the applied voltage electron flow which can decouple the gap (controlled by shunt circuit elements) and, voltage from the instantaneous ion current as expected, the beam behavior was similar and prevent klystron instabilities, as to that of an RF driven linear accelerator. described below. At higher loading, bunching instabilities set in very quickly, as shown in Fig. 29. It appeared that for reasonable efficiencies (10 percent or greater), broadening of the velocity spread of the injected beam could not prevent the growth of bunches, although the onset was somewhat delayed.

A possible cure for klystron instabilities is the use of controlled electron flow in downstream gaps to decouple the gap voltage from ion current variations. Magnetically insulated gaps, if run near the critical cut­ off magnetic field, exhibit many of the properties of zener diodes. Electrons will flow until the gap voltage is depressed below that needed to allow electri-л crossing. If the magnetic fieid is adjusted to main­ tain the gap voltage at V(pulse line)/2, then the voltage will be clamped near this Fig. 28. Magnetically insulated acceleration value as long as the maximum ion current gap. System is symmetric about is less than V(pulse line)/2Z. •; as ions centerline. (Calculated field courtesy of J. Freeman.) flow in, they slightly depress the voltage which has a large effect on the electron current. and hence the electrons are replaced The initial theoretical work for the magne­ rath ions. When the Y-I electron curves for tically insulated multistage ion accelerator the annular gaps of the accelerator are was performed at Cornell University (Ref. 42). obtained experimentally, they will be incorv" Additional theoretical work has been per­ porated into PILAC to investigate the utility formed at Sandia Laboratories and was of this method.

119 has evolved and is currently under construc­ tion. A sketch of the initial stages of Pulselac is given in Fig. 30. A list of design and testing work follows:

1. An unused Marx generator assembly (Including oil tank, oil, switches and capacitors) was located and rebuilt to yield 25 kJ at 450 kV. A fast, high reliability switch was installed, and the Marx was succes; fully tested at maximum voltage into a high resistance load.

2. Cable for the cable Blumlein lines of Pulselac were tested to 400 kV. In order to fire the Blumlein with low jitter, trigatron switches were designed, A low-inductance, multi-output trigger Marx generator was designed, built, and successfully tested Fig. 29. Typical P1LAC run. Gaps powered by to 100 kV. 50 ohms pulse lines charged to 1 MV wit)-, a 30 nH series inductance and 3. Multielement wall flashboards to supply a 100 ohm shunt resistor. Initial the cold electrons for beam neutralization phase space distribution is rec­ were designed. A twelve output capacitor tangular. bank for flash initiation was built and tested

(a] Emerging phase distirbution 4. Vacuum chambers, vacuum insulators, (vz - z) and gap voltage drift tube assemblies, and magnet coils for (dotted) and current (solid) the accelerator were designed. Delivery is profiles for Gap 1. expected in January 1978.

(b) Emerging phase distribution 5. The main magnetic field banks were (note change of scale) and designed and are being assembled. The total voltage and current profiles stored energy is 30 kJ. for Gap 2. 6. Two complete vacuum systems with con­ trollers were built. Other theoretical work included studies of transverse focusing of ion beams using 7. A fast pulsed gas valve was built and imperfect lenses with negligible space charge successfully tested. forces. The computer program developed will be useful to provide a baseline against which At the same time, a new laboratory was set up to compare experimental results on beam focus­ specifically for multistage ion accelerator ing. Members of the theoretical group have experiments. contributed to an understanding of the accelerator concept. Magnetic field solu­ To the present,, the bulk of the time has been tions for the experiment under construction devoted to design, construction and testing were provided by J. Freeman (see Fig. 28). of subsystems. Experiments will be initiated The process of beam neutralization by cold in early 1978. The Pulselac is designed for electrons has been studied by J. Poukey with 300 to 400 kV/stage. The design is modular very encouraging Tesults. These studies will so any number of coupled stages can be inves­ continue with more complex models for the tigated up to a maximum of 5. The injector electron dynamics. should be able to supply 10 kA of protons. The maximum degree of tunability has been The major thrust of work in the last six incorporated into the device. Gap spacing months has been in the design of an initial and effective magnetic file curvature as well five-stage accelerator, Pulselac, to investi­ as the pulse length and characteristic imped­ gate the critical concepts of high current ance of the cable Blumlein lines can be varied. beam transport. A complete machine design In addition to the primary goal of obtaining

120 SLOT RHFI.HCTOMHTRY FOR DIAGNOSING DIODE PUISELAC Modules 1 and 2 PLASMAS

Slot rcflectometry is a diagnostic method for measuimg electron density and collision rate by looking at the conductivity of a p'asma which is terminating a waveguide. The diag­ nostic is best suited to measuring plasmas near a metal surface. It lias been used in the past to measure anode plasmas on a Ncrcus diode, prcpulsc plasmas on the Proto I accelerator and gun plasmas stagnating against metal plates. While the last mca- siiicment was originally used to character­ ize the guns used in plasma-Pilled diode work, there is another region where slot reflectometry will lie of value. 'Hie ion diode work uses plasma covered surfaces as ion sources. /\t present, there are metal- dielectric structures which flash to make plasmas. In addition, the use of gun Pig. 50. Initial stages of Pulselac experi­ plasmas stagnating against the anode offers ment. potential as a uniform source of ions in self-pinch diodes (for diodes employing external magnetic fields the flow of plasma the maximum ion current at the highest bright­ across the field must he considered). Guns ness, the following areas will be studied: presently used yield 1()1S cm-3 at the surface

- where the plasma stagnates and 1Г)13 сщ-л away 1. Effectiveness of cold electron neutraliza­ from the surface. tion. Measurements of time-dependent trans­ verse phase distributions as beam propagates through magnetic field. The theory of slot reflectometry has been done rigorously and the calculation of 2. Measurements of phase distributions electron density and collision rate is from injector. readily accomplished either graphically or by small programmable calculator. The 3. Experiments with gas and pulsed plasma experimental apparatus has been re lined to ion sources for injector. yield the highest possihlc .-^curacy for fast time scales (rise times of a few nsl. 4. Determination of focal lengths of The measurement range CO-5 x lCllS to 10l$ electrostatic lenses. for present apparatus) is unavailable with our other diagnostics. 5. Experiments in pulse shaping using shunt circuit elements in pulse lines.

6. Confirmation of velocity bunching instabilities predicted by PILAC.

7. Attempts to cure velocity bunching by electron clamping.

8. Experiments with plasma-filled magnetic lenses to bring output beams to high intensity focus.

121 REFERENCES:

1. G. Yonas, J. W. Poukey, K. R. Prestwich, J. R. Freeman, A. J. Toepfer, and M. J. Clauser, Nuclear Fusion, 14, 731 (1974).

2. M. Di Capua, J. Creedon, and R. Huff, J. Appl. Phys., 47^, 1887 (1976).

3. J. G. Kelly and L. P. Mix, J. Appl. Phys., 46, 1084 (1975).

4. D. J. Johnson, Shyke A. Goldstein, Roswell Lee, and W. F. Oliphant (to be published).

5. Shyke A. Goldstein and Roswell Lee, Phys. Rev. Letts., 35, 1079 (1975).

6. E. W. McDaniel, "Collison Phenomena in Ionized Gases," (John Wiley and Sons, Inc., New York, 1964), p. 62S.

7. R. Genuario (to be published).

8. T. H. Martin, IEEE Trans. Nucl. Sci., 20_, 289 (1973).

9. С Eichenberger, S. Humphries, Jr., J. Maenchen, and R. N. Sudan, J. Appl. Phys., 48_, 1449 (1977).

10. D. J. Johnson and Shyke A. Goldstein, J. Appl. Phys., 48, 2280 (1977).

11. А. Г. Blaugrund, G. Cooperstein, and Shyke A. Goldstein, Phys. Fluids, 20_, 1185 (1977).

12. J. M. Creedon, J. Appl. Phys., 46, 2946 (1975).

13. R. D. Genuario, et al., Phys International Internal Report PIIR-4-77, February 1977.

14. G. Yonas, K. R. Prestwich, J. W. Poukc. J. R. Freeman, Phys. Rev. Lett., 30, 164 (1973).

15. J. W. Poukey, First International Topical Conf. on Electron Beam Research and Technology, Albuquerque, NM, November 3-5, 1975, 1_, p. 247 (1976).

16. J. W. Poukey, et al., 12th Electron, Ion and Laser Beam Technology Symposium, Cambridge, Massachusetts (1973); Shyke A. Goldstein and R. Lee, Phvs. Rev. Lett., 35, 1079 (1975).

17. A. E. Blaugrund and G. Cooperstein, Phys. Rev. Lett., 34_, 461 (1975); A. E. Blaugrund, G. Cooperstein, and Shyke A. Goldstein, First International Topical Conf. on Electron Beam Research and Technology, Albuquerque, NM, November 3-5, 1973, 1_, p. 233 (1976).

18. Shyke A. Goldstein, et al., Bull. Am. Phys. Soc., 22_, 1063 (1977).

19. J. P. Quintenz and J. W. Poukey, J. Appl. Phys., 48, 2287 (1977).

20. Steven A. Goldstein and С W. Mendel, Jr., "Pinch Optimization on Proto I," Electron Beam Fusion Progress Report, SAND77-1414, Sandia Laboratories, Albuquerque, Ш (October 1977).

21. D. J. Johnson, et al., "Time Dependent Impedance Behavior of Low Impedance REB Diodes During Self Pinching," submitted to J. Appl. Phys. (1977).

22. D. W. Swain, et al., J. Appl. Phys., 48_, 118 (1977).

122 23. L. I. Rudakov and M. V. Babykin, Proceedings of the Seventh European Conf. on Controlled Fusion and Plasma Physics, Lausanne, Switzerland [1975).

24. P. A. Miller, et al., Phys. Rev. Lett., 3£, 92 (1977).

25. T. K. Martin, IEEE Trans. Nucl. Sci., 20_, 289 (1973).

26. A. F. Aleksandrov, et al., Zh. Eksp. Teor. Fiz., 61_, 1841 (1972).

27. A. I. Pavlovskii, et al., Zh. Tekh. Fiz., 45_, 286 (1975).

28. S. L. Thompson, Improvement in the CHART-D Radiation-Hydrodynamic Code II, SC-RR-71-0713, Sandia Laboratories, Albuquerque, MM (1971).

29. L. Spitzer, Physics of Fully Ionized Gases (Wiley, New York, 1962).

30. T. P. Wright (to be published).

31. J. R. Freeman, M. J. Clauser, S. L. Thompson, Nuclear Fusion, 17, No. 2, 223 (1977).

32. M. J. Clauser, Phys. Rev. Lett., 35, 48 (1975).

33. J. Shearer, Nuclear Fusion, 15, No. 5, 952 (1975).

34. ERDA Summer Study of Heavy Ions for Inertial Fusion, Oakland/Berkeley, California, July 19-30, 1976. Final report LBL5543 (December 1976).

35. T. H. Martin, K. R. Prestwich, D. L. Johnson, SC-RR-69-421, Sandia Laboratories, Albuquerque, NM (October 1969).

36. D. J. Johnson and S. Stephanakis, private communication based upon unpublished data obtained at Naval Research Laboratory 1976.

37. S. С Luckhardt and H. H. Fleischmann, Appl. Phys. Lett., 30_, 182 (1977).

38. R. J. Lanter and D. E. Bannerman, The Silver Counter: A Detector for Bursts of Neutrons, Los Alamos Report LA-3498-MS (1966).

39. E. A. Burrill, Neutron Production and Protection, High Voltage Engineering Corporation Bulletin.

40. Model NPM-54, EG§G Inc., Las Vegas, Nevada.

41. M. Greenspan, S. Humphries, Jr., J. Maenchen, R. N. Sudan,. Phys. Rev. Lett., 39, 24 (1977).

42. S. Humphries, Jr., to be published, J. Appl. Phys (January 1978).

43. M. J. Clauser, Phys. Rev. Lett., 35, 848 (1975).

44. S. Humphries, R. N. Sudan, and L. Wiley, J. Appl. Phys., 4_7, 85 (1976).

45. ERDA Summer Study of Heavy Ions for Inertial Fusion, edited hy R. 0. Bangerter, W. B. Hermannsfeldt, D. L. Judd, and L. Smith, Lawrence Berkeley Laboratory Report No. LBL-5543 (1976).

46. S. Humphries, to be presented at the Topical Meeting on Inertial Confinement Fusion, San Diego, California, February 1978.

123 47. S. Humphries, Proceedings of the Second International Topical Conference on High Power Electron and Ion Beam Research and Technology, Cornell University, Ithaca, New York (October 1977).

124 . *•• . «ft

TARGET INTERACTION

OVERVIEW peak pressures of 10 Mbar in tungsten. Lower pusher temperatures (2 to 3 eV) have been Target interaction and implosion physics re­ correlated with nonuniform loading, in parti­ search has continued on the Hydra and Proto I cular target configurations. Ablatively driven accelerators. Diagnostic development of opti­ cylindrical targets (brass] have been driven cal X-ray, XUV, and neutron detection methods up to 1.3 Mbar pressures, and optical holo­ was also carried out as part of the target graphy indicates jet formation during the interaction program. final stages of implosion due to nonuniform loading. The formation of ejecta (fluff) Measurements of the interaction of a tightly from shock loading of the pusher free sur­ focused REB with planar and spherical gold face has been observed in experiments with targets were made on the Proto I accelerator cylindrical targets. [2 beams, each typically 1.8 MV, 200 kA, 20 ns). Irradiation symmetry to better than 10 percent Development of time-of-flight neutron diagnos­ has been routinely obtained with - SO percent tics has continued. The thermonuclear neutron of the electrons reaching the target region. "signature" from an ICF target has been estab­ Due to the low v/y of the Proto I beam, only lished using a fast plastic scintillator- a small fraction of the incident flux is ab­ photomultiplier combination shielded by several sorbed in the target in good agreement with cm of lead in conjunction with a 2-ns-wide theoretical calculations. Preliminary calcu­ neutron source which directly simulates the lations for a focused 1.5-MV, 2-MA Proto II expected thermonuclear neutron pulse from a beam, indicate that order of magnitude in­ REB imploded target. In addition, neutron creases in the energy deposition are likely Monte Carlo calculations have been carried due to recirculation of electrons in the out to predict the response of a SPM to a intense self-fields of the beam. The Proto 1 burst of thermonuclear neutrons. In both experiments underscore the necessity of both cases, the thermonuclear neutron pulse shape considering the electron focusing efficiency is found to have a sharp leading edge of a and the efficiency with which the electrons few nanoseconds, a rather narrow FWHM of the couple to a target when estimates of target order of some twenty to thirty nanoseconds, energy deposition are made. and a long tail that can extend for up to one hundred nanoseconds. Radiation temperature measurements associated with XUV (soft X-ray vacuum ultraviolet) photo­ Details of our research on preheated fuel electric diodes or, commonly called, X-ray targets which rely on magnetic thermoinsula- diodes (XRD's) imply that electrons incident on tion to permit the use of laser beam power 6 micron thick, than a single pass through the are also reported here. We have carried out foil. Defining the concept of enhancement as experiments which have produced - 10& DD that of a given electron making more than a neutrons and our analysis indicates that a single pass, we found enhancement factors of 2 fuel temperature ~ 1 keV and a fuel density for the above foils. We have found a geometri­ of 1.6 x 10"2 g/cmS has been achieved. Since cal dependence on the enhancement, namely, the our analysis is one dimensional, this con­ diameter of the hole behind the foil. Indepen­ clusion must await scaling experiments dent of geometry, we found no enhancement for for verification. gold and aluminum foil thicknesses greater than 12 microns. Current density and energy deposi­ tion are not inconsistent with a beam stagna­ BEAM-TARGET INTERACTION MEASUREMENTS ON tion model. PROTO I A series of experiments which were designed to Research on implosion dynamics of ablatively assess the feasibility of implosion studies driven targets has resulted in the development on DT-filled spherical pellets were performed of experimental techniques and optical diag­ on Proto I (2 diodes, each 1.6 MV, 200 kA, nostics to measure implosion time, pusher tem­ 20 na). Preliminary measurements of the perature, and implosion symmetry. By utiliza­ Proto I pinch on thick-flat targets indicated tion of shock convergence, pusher temperatures that about SO percent of the current could be of 5 to 10 eV and pusher velocities of ~ 3 cm/us focused to the target region (r < 0.8 mm) have been obtained on Hydra, corresponding to which was in agreement with PIC calculations

I2S of J. P. Quintenz. If all this energy were EXPERIMENTAL GEOMETRY uniformlv deposited in a DT-filled thin gold sphere, calculations! indicated thermonuclear neutron yields greater than 10^ would be ob­ tained. The experiments described in this re­ Lexan port investigated the uniformity which could be Prcpulse Switch obtained on spherical pellets with the tightly focused beams of Proto I, and the coupling effi­ . Brass Cathode ciency of the focused beam to the targets. The 2.5cm OD x 1.25cm 1С diagnostics included quantitative X-ray pinhole photography, visible radiometry f5070 ± 50 Л) .— Gap - 06 СЛ1 and VUV radiometry (8 to 80 eV). By comparison with PIC diode simulation which included an r>o<: __—Anode approximate model of the beam-target interac­ tion as well as Monte Carlo calculations which coupled the pinch profiles from the PIC simula­ tions witli a more exact treatment of tile beam- Gfl target coupling, a clear picture of the beam- target interaction was obtained. The theory as well as experiment indicated that insufficient energy was coupled to the target to obtain neu­ X-ray pin hole photography tron yields of 10б even though irradiation Visible Radiometry (5070 ЛI symmetry of 10 percent could be routinely ob­ VUV Radiometry ( 8 eV 80 eV) tained. The details of these experiments will X-ruy Radiometry ( 30keV 180 keV) now be considered.

Fig. 1. Experimental geometry for dual diode Irradiation Svmmetrv Measurements target measurements on Proto I. The first measurements were designed to evaluate the dependence of irradiation symmetry on the Figure 3 shows an X-ray pinhole photograph target mounting geometry. For these experiments and a microdensitometer scan across the 1.5 mm diameter gold pellets with both 25 м and image. The enhancement of the X-rays at the 50 u wall'thicknesses were mounted in 0.8 mm edge of the shell is due to limb brightening.- thick aluminum anodes and positioned in the mid- On this shot, the symmetry is better than plane between two 2.54 cm diameter tapered hollow 5 percent around the circumference of the cathodes having dielectric flashover switches in sphere. For both geometries described above, their base as shown schematically in Fig. 1. uniformities of 5 percent to 15 percent could The cathodes were aligned with the pellets to be frequently obtained, however, the total an accuracy of 0.2 mm. The gold pellets were X-ray intensity tended to be higher for the mounted in the anode'using several geometries.2 first geometry which provided for better Two examples are shown in Fig. 2. In the first, grounding of the target. Although this the central portion of the aluminum was tapered technique provided good data on the variation over a distance of 3 mm to a central hole the of deposition with azimuth around the target, diameter of the pellet. The pellet was then the variation from pole to equator could not placed in the hole and connected to the alumi­ be detennined except by very crude estimates num using conductive exposy around the equator based on the ratio of the X-ray amplitude of the pellet in the small gap between the at the edge to that at the center. Since, gold pellet and the aluminum anode. To insure as explained later, the total electron deposi­ uniform return current paths, the entire assem­ tion appeared to be significantly less than bly was then coated with 1-u gold plating over that required for measurable thermonuclear the central 1.2 cm diameter of the anode. The neutrons, this aspect of the symmetry was second geometry involved supporting a pellet in not investigated further. the center of 1.2 an diameter hole on three 0.2 mm diameter stainless steel tubes. As in the first case, a 1-y gold plating was placed Energy Deposition Measurements over the entire central portion of the anode. In assessing the feasibility of doing implo­ The primary diagnostic for determining irradia­ sion studies, not only must the irradiation tion symmetry was X-ray pinhole photography. symmetry be determined, but also the

126 (a) Conducting epoxy suspension in chamfered (b) Spoke suspension in 1.2-cm hole in A/anode. A/anode.

Fig. 2. Two of the pellet geometries investigated on Proto I.

—| 2mm \—

Fig. 3. Typical X-ray pinhole data. Top is a microdensitometer scan across the diameter of X-ray pinhole photo shown below.

127 efficiency which the electron energy can he and optical diagnostics, only the front suri delivered to the target from the cathode. To of the target is observed.• The 2:1 variatic answer this question, both the spherical tar­ observed in the deposition profile of the gets, described above, and more easily fabri­ Nfcnte Carlo calculations would change the cated, planar targets were used. Optical factor of two effect to a 50 percent effect. radiometry, VUV radiometry and quantitative The failure to observe this change could be X-ray pinhole photography provided estimates due to eitheT inadequate statistics on the of the energy deposition in the targets. The deposition or to a 30 percent reduction in radiometry data was converted to an equivalent the pinched current. blackbody temperature which was compared with the electron temperatures predicted by the coupled radiation-hydrodynamic code CHART-D. Theoretical Calculations Figure 4a shows a comparison for typical optical data and Fig. 4b shows similar data The experimental results have been modeled for the VIIV radiometry. In both cases, the using two main calculational approaches. diode power pulses were modeled with a trap­ Particle in a cell calculation? (PIC) have ezoid and the maximum amplitude adjusted to been performed by J. P. Quintcn? (5241) and obtain agreement with the experimentally Nfonte Carlo calculations have been performed determined electron temperatures. For these using several Sandia electron/photon transpo data, good agreement is obtained when peak codes. The Monte-Carlo calculations relied specific power deposition of 3 and 7.5 TlV/gm heavily on the results of the PIC calcula­ are assumed in the calculations for the tions, gave good agreement in the limits optical and VUV data, respectively. The where both codes are believed accurate, and measurements depended heavily on the deposi­ provided additional theoretical predictions tion or temperature in the outer regions of in the intermediate regime where the approx­ 4 the plasma with pr-^lO" . The measured tem­ imations in the PIC scattering model are peratures were found to be independent of the believed to be invalid. target thicknesses, to within ~ 30 percent over the range of 6 у to 1Q0 u in agreement The PIC calculations are described in some with theoretical calculations reported in the detail elsewhere.6 The calculations not onl) next section. treat the self-consistent electric and magnel ic fields but also treat the scattering and The total dose to the pellet was also inferred reflections from the anode with a macroscopic from the intensity of the images on calibrated approximation written by M. M. Kidner (55.41). X-ray film The calibrations were performed The results for Proto I parameters indicate at Sandia'- P- liti-on facility. Renormaliza- that for a thick anode, no effects are tion was t'••:..: ._ii by comparison of the X-ray observed clue to the scattering in the anode. dose to th - :W \ v.ith predictions of Х-тау The current density averaged over a 0.4-mm 2 generation using coupled electron photon Monte radius was calculated to be 13 MA/cm with Carlo codes.* The pinhole images were fil­ half the energy deposited in a 0.8 mm radius. tered to provide 80 percent of the response The thin target results for 6-u gold targets (10 percent to 90 percent) in photons with indicated a radial diffusion of the deposi- energies of 30 to 160 keV. The Х-тау intensi­ tron energy deposited in the 0.8-mm radius ties from the gold targets indicated depositions target region. of order 5 TW/gm. The X-ray intensities were observed to scale with the thickness of the Coupled electron-photon Monte Carlo calcula­ target for thin targets, indicating constant tions were performed to look at the effect specific energy deposition (for the filtered of scattering on the deposition in planar pinholes, self-absorption was not a significant targets which are neither very thick nor very effect). thin compared to a csda range. Both one- dimensional (TIGER)4 and two-dimensional No significant differences in the deposition (CY1EM)S calculations were performed. For were observed between two-sided illumination all the calculations, a cosinusoidal angular of spherical targets and single-sides irradia­ distribution for the incident electrons and tion of planar targets. The reasons for this a radial distribution consistent with the apparent discrepancy are twofold. In the case PIC calculations of Quintenz were assumed. of the X-ray diagnostics, Monte Carlo calcula­ A r_l variation in the current density was tions, described below, indicate little electron assumed for each bin of the PIC simulation recirculation within the target region and a in order to smooth the discontinuities in bremsstrahlung spectrum strongly peaked in the the profile. The two-dimensional calcula­ forward direction. In the case of the VDV and tions included magnetic fields consistent

118 Theory (peak power 3TW/gm)

fa)

12 **—\ Vxp (50n Au foil

/7 chart Dy^TW/grn-* — // '/ If 11 1J l/ Powe r АЛ С 20 40 A /1 0 Timelns) 30 60

(b)

Fig. 4. Analysis of typical radiometry data. a. Visible data at 5070 ± 30 A. b. VUV data at 8 to 80 eV. with trie current profile and electric fields in fact, is consistent with the average elec­ which were roughly equal to the self-consistent tron making about two passes through the fields calculated in the PIC simulations. foil before being lost at large radii. Variation in the electric field by factors of 2 to 3 in the CYLEM resulted in negligible changes (~ 10 percent) in the energy deposited Summary in the target region (r < 0.75 mm). Experiments to measure the interaction of the The two-dimensional calculations, with 51 per­ superpinched electron beam on the Proto I cent of the incident electrons striking the accelerator have been completed with both target region, indicate that 0.8 percent, 2.2 planar and spherical targets. Azimuthal percent, 10.4 percent and 40 percent of the symmetry to better than 10 percent has been incident energy will be deposited in target observed for a number of target configura­ region on 6, 12, 50, and 250-u gold foils, tions and the specific energy deposition is respectively. As In the PIC calculations, measured to be independent of target thick­ the fields were chosen to be symmetric about ness for thin gold targets and to range from the anode for all but the 6-u case. The rea­ 3 to 9 TW/gm depending on the particular son for the slightly lower value (0.8 percent diagnostic technique. The energy deposition vs. 1.1 percent for the PIC simulations) for predicted by the theoretical calculations and the 6-u foil can probably be explained by the the experimentally determined values are difference. In contrast to the PIC calcula­ summarized in Pig. 5. The specific energy tions of 100 percent efficiency, an efficiency deposition is a factor of 2 to 4 less than of 80 to 90 percent was observed -for energy that predicted by the theory and about a deposition in thick anodes. Por both the factor of 3 to 5 less than that required for 6-u and SO-u foils, with no fields at the measurable yield of TN neutrons. The scaling rear surfaces of the targets. of deposition with target thickness is an excellent agreement with calculations althougf One-dimensional calculations were performed the experimental pinch profile is slightly with the №mte Carlo code TIGER for the case broader than predicted by the PIC simulation. of 6-u and SO-u gold targets and compared Proto II, with powers of 8 TIV and V/Y ~ 50 with two-dimensional calculations in which (vs. 0.7 TW and \>/y ~ 2) is nearing readiness. the fields at the rear of the anode were Preliminary estimates of the energy deposition removed. The TIGER calculations were per­ in a V/Y ~ 10 pinch suggest that an order of formed both by allowing all transmitted and magnitude increase in the eneTgy deposition reflecteu particles to escape and by allowing over that measured in Proto I due to recir­ specular reflection of the reflected particles culation of the electrons in the intense self- at the front surface of the target. The arith­ fields of the beam. Due to the enhanced metic mean of the two calculations predicted capabilities of Proto II, DT-filled target 0.8 percent and 9.2 percent compared with experiments will not be performed on Proto I. 0.8 percent and 9.7 percent for the two- On the other hand, Proto I parameters may dimensional calculations. lend themselves more readily to ion beam/ target experiments, thus ion diode experi­ ments are presently being planned. The very good agreement between the PIC cal­ culations and Monte Carlo results for thin anodes suggests that the incident profile is not seriously modified by the reflexing ELECTRON DEPOSITION IN THIN GOLD AND ALUMINUM of electrons in the anode. The good agree­ TOILS HEATED BY A RELATIVISTIC BEAM ment between the one- and two-dimensional codes in the case where half the back scattered Radiation temperature measurements associated electrons are reintroduced indicate a relative­ with XUV (soft X-ray vacuum ultraviolet) ly small enhancement due to recirculation in photoelectric diodes or commonly called, X-ray the diode field; about 80 percent of the dose diodes (XRD's) imply that electrons incident is obtained if no recirculation occurs. The on б-micron thick, unbacked, gold and aluminum lack of recirculation is apparently a result foils make more than a single pass through the of the relatively low value of beam V/Y, foil. We found electron energy depositions namely, of order 1. The observed lack of consistent with theoretical calculations which reflexing occurs only that the reflexing include the effects of self electric and mag­ electrons tend to strike the anode at radii netic fields in thin targets. larger than the pinch region. Other data

130 .о • Thick T.irget and 6.35 micron thick aluminum foils on the Proto I accelerator with an unbacked hole radius of 1 cm. Particular emphasis was placed on XUV radiation temperature measure­ ments for describing energy deposition and hard X-ray measurements for determining total cliarge and electron density incident on the foils. Other sections of this report will describe optical temperature measurements, two-dimensional effects and detailed theore­ tical modeling of the above experiments.

Experimental Procedure

Figure 6 shows the experimental setups foT the three seriej of experiments on the Hydra 1 10 100« lODOji and Proto I accelerttors. Tapered annular Fig. 5. Comparison of theoretical and experi­ cathodes with inner and outer radii of 1.6 mental data on Proto I. cm and 3.8 cm, and a cone half-angle of 80° were used with a cathode spacing of 0.27 cm; the diameter of the hole behind the foil. In­ corresponding to an initial geometrical R/d dependent of geometry, we found no enhancement of about 5.7 on the Hydra accelerator. Taper­ of gold and aluminum foil thicknesses greater ed annular cathodes with inner and outer than 12 microns. Current density and energy radii of 0.64 and 1.27 cm and a cone half- deposition are not inconsistent with a beam angle of 75" were used with an anode-cathode stagnation model. spacing of 0.32 cm; corresponding to an initial geometrical R/d of about 2 on the Proto I accelerator. On the Hydra experiments, Current densities in these experiments were either optical photodiodes for optical tempera­ 2 rather modest, less than S MA/cm for the ture measurements on the grazing incident Proto I accelerator (v/y < 3) and less than spectrograph for time-integrated, high- 0.6 MA/cm2 for the Hydra accelerator (v/V < 7). resolution XUV spectra was used on a given Introduction shot. On the Proto I experiments, only single- sided operation was used. On the series (2) During the last six months we have been experi­ experiments on Proto I, the bottom diode was mentally investigating electron energy deposi­ used. On the series (3) experiments on Proto tion in thin, flat, anode foils. As has been I, the top diode was used. This was done to proposed, if there were some means of effic­ allow both front and rear radiation tempera­ iently coupling beam energy to thin targets, ture measurements to be made. Figure 7 shows then targets with less mass could be used for typical accelerator operating characteristics electron beam fusion targets and power require­ for both the Hydra and Proto I accelerators. ments from electron beam accelerators could be Typical parameters were 0.8 MV, 300 kA, 80 ns reduced. Recently Clauser, et al. have pulse width for the Hydra accelerator and reported efficient coupling to a spherical 1.5 MV, 200 kA, 30 ns pulse width for the shell extending from the anode on a stalk. Proto I accelerator. The mechanism for coupling was beam stagnation which is a combination of magnetic stopping and electrostatic reflexing. In these series Radiation temperature measurements were made of experiments we wanted to investigate a with a triplet array of XRD's° and/or visible simpler target design with particular emphasis (~ 507 cm) photodiodes. Figure 8 shows the on the effect of target thickness on energy cathode response of XRD's as a function of deposition. In this section of the report, incident photon energy and the sensitivity we describe a series of three experiments that of XRD's to a blackbody radiation source as were carried out to observe enhanced deposition: a function of blackbody temperature. Between (1) SO.8, 25.4, 12.7, and 6.35 micron thick 3 and 7-eV blackbody temperature, the response unbacked gold foils on the Hydra accelerator of the XRD varies only 20 percent, indicating with an unbacked hole radius of 1 cm; (2) 50.8, its strong T4 dependence in this temperature 25.4, 12.7, and 6.35 micron thick gold foils Tegime. The detectors are insensitive to and 6.35 micron thick aluminum foil on the radiation temperatures below 2 eV. Filters Proto I accelerator with an unbacked hole were placed in front of two of the XRD's to radius of 0.5 cm; and (3) 205.7, 88.9, 38.1, view the shot on spectral regions of 30 to 70 eV (l-мт aluminum) and 70 to 284 eV (0.33-um

131 Au 254 cm 0PT,CAL PHOTO 1.9 ' DIODES : . ' GRAZING Ll! H INCIDENCE j \ у 13.97 cm f—\ SPECTROGRAPH,. A' ANODi Au = .006 to Г .051 mm

Hydra:

STREAK] I\ / CAMERAS—У V PIN DIO f (GRATING]' C2ERNY -TURNER SPECTROMETER

in Diode Array Film

I _/ 3.23 mm Proto I: 9=f f~ 3 _ ., 6.35 mm , 3affles Anode Foil Meters - -I 9.53 mm Vuv Filters HV Anode XRD Vll Cathode

Fig. 6. Experimental setup on Hydra and Proto I.

132 HYDRA 5706 100 т—г E 8.0 I ° 6.0

2.0 0.0 0.4 0.8 1.: 1.6 2.0 0,0 0.4 0.8 12 1.6 2. X 102 Hydra: VCOR 4.0 1.0 w 3 3.0 0.6

? 2.0 > 0.2 X 104 X 106 1.0 •02 0.0 0.6 •1.0 •1.0 _l_i_J__L 0.0 0.4 0.8 1.2 1.6 2.0 0.0 0.4 0.8 1.2 1.6 2. X 10 POWER= 0.3503E+12 X 1°2 FNERGY» 0.2308E+05

4.0r v x io620i" 0.0i-W •*• I 0.0 4.03.0 X 1P1 joe 1К1*в JOaOUS Proto I: -Mr A X 10.° a 0.0- •1.0" л 0.0 4..*±^-0 8.0 Tnsec X 10' Proto 1 , » 20B 1135 !ПП6Н»10i в О H X10 M s

Fig. 7. Machine characteristics for Hydra and Prcto I.

133 R = j;"R(E)S (E,T)dE - 10' BB

Al Ln.m(.073keV) fSBB(E.T>dE \jM L.j(.118keV) ю- \,CK(.284keV} R E s E T dE 4\ OK(.532keV) X'" < > < ' ) 10' oAT4 XRD Response vs. Ml K(1.56keV) Blackbody Temperature о. s 10l ее 0.001 0.01 0.1 1.0 10 Energy (keV)

T lev) Blackbody

Fig. 8. Response of X-ray diodes (XRD's) to photons and as a function of blackbody temperature.

134 Parylene, CgHg). In measuring the blackbody temperature from a radiating disk at normal incidence, one finds that the power on the of incident particles per unit time on the detector is given by foil. To determine the total current in the pinch, we utilized high band-pass filters in front of absolutely calibrated PIN's. The PIN's were absolutely calibrated using a DC >вв • ™A£) electron beam source which produced normally incident monoenergetic beams between 0.4 and 1.1 MeV on 50-um gold plus 0.5-cm-thick and graphite targets and a one-dimensional elec­ tron photon transport code TIGER4 to calculate the resulting photon spectrum. The PIN's were calibrated behind 150-um-thick uranium dfl _ "ddet and 250-pm-thick erbium photon filters to allow an essentially flat response for inci­ dent radiation. Figure 10 shows the spec­ tral and total energy response to the PIN's. In the case of the thinnest aluminum foils, where da(j_t is the detector area, R is the distance from the source, and A is the the total current was estimated from abso­ source area. The aperture system for the lutely calibrated photographic film. ° XRD's (see Fig. 6), allowed a maximum viewing area of 2.8S cm2 on the Hydra accelerator and Temperature measurements were compared with 0.5 cm? on the Proto I accelerator. In esti­ calculations using the Lagrangian hydro- mating the temperature, the diameter of the dynamic code CHART-D.11 This code, which energy deposition profile was chosen to be includes complete equation-of-state, Saha 2 mm greater than the FWHM of the hard X-ray ionization and thermal electron and radia­ profile.^ This is an extremely conservative tion transport was used to calculate temp­ estimate of the temperature, because of the erature and density as a function of time T* dependence. and position on both sides of the thin foil. CHART-D is a one-dimensional code. By In measuring radiation temperature as a func­ inputting the average energy deposition per tion of time, one is actually measuring the particle and electron number density, the energy deposition in the foil as a function of radiation temperature as a function of time time. In order to determine whether the energy was found. We varied the electron number deposition in the foil is due to multiple density holding the pulse length constant passes of an electron, one must assume average until it matched the experimentally measured deposition energy for a given electron in a temperature. A comparison was made with the single pass. Figure 9 compares different integrated electron flux to the target as angular electron distributions for a given inferred from hard X-ray measurements. In photon flux incident on 6-pm thick gold. The all cases the two values of electron number number of electrons incident per unit solid density were in agreement. In the case of angle chosen to be a constant gives the most 6-um thick targets the experimentally deter­ reasonable energy deposition for pinched beam mined values of rig when compared to diode diodes. This means an average electron angle current implied ,tnat the electrons made more of incidence of 45°. Also, we have allowed than one pass through the target. the electric field in the diode to return all scattered electrons back to the anode at the same point of scattering, a one-dimensional Results analysis. This deposition profile is what we call the average deposition per electron. Figure 11 shows a one-dimensional analysis for current density for 30, 25, 12, and 6-um- thick gold foils and 0.32-cm range thick It is clearly the maximum possible energy de­ carbon as a function of radius from time- position per particle, and probably over­ integrated pinhole photographs for the series estimates the deposition per particle by as much of experiments on the Hydra accelerator. Peak 2 as 30 to 40 percent.10 current density never exceeded 0.6 MA/cm for the gold foils. The pinch profile was nor­ Similarly, in order to estimate the total malized to the total current as given by deposition in the foil, we must know the number the dB/dt monitors assuming 90 percent of

135 PIN DIODE tO'MHSLE OF INCIDENCE .25 mil COLO TIGER - ID ELECTRON - PHOTON

.7 .1 .t ELECTRON ENEROr (M.v)

Fig. 9. Photon energy production for different incident electron angular distributions on 6-pm gold foils.

136 Pin Detector Response High Band Pass Filter (.15 MM Uranium » .26 MM Erbiun

Photon Lnimjv 'n IMtiV) Monoenerqetic Electron Епеп)у(М'Л')

Fig. 10. Response of high band-pass PIN's to incident photon energy. The buildup factor is the difference between experimental measured photon flux to the theoretically predicted photon flux.

137 Fig. 11. Shot 5691 0.32-cm-thick carbon Shot 5692 50-M-thick gold . . . Shot 5693 25-y-thick gold I Shot 5694 12-u-thick gold Shot 5706 6-p-thick gold

138 the current was in electrons (ДО percent in maximum possible temperature profile to be ions). Total current in the pinch as a function expected from experimentally measured peak of time was also inferred by the absolutely current densities. In the case of aluminum, calibrated apertured PIN detector as shown in we found temperatures consistent with a Fig. 12. In the case of the 6-um thick gold, current density of greater than 4 MA/cm2, the inferred current appears to exceed the total whereas peak current densities inferred frqm current as measured by the dB/dt monitors. hard X-ray profiles never exceeded 2 MA/cm'- Figure 13 compares the experimentally measured for 6-ym aluminum. Therefore, for 6-um temperature as a function of time to that of the aluminum, more than a single ps " was required temporal profile of temperature with a maximum to explain the observed energy u-position. current density of 0.5 MA/cm2. Indeed, the experimental temperature profile corresponds to a profile with a maximum current density of Conclusion 0.4 MA/cm2 or 1 TW/g. From Fig. 11 we see that the maximum current density is 0.2 MA/cm2 when In a set of three experiments: CI) 50.8, the total current is normalized to the dB/dt 25.4, 12.7, and o.35-um-thick gold foils on monitors for 6.35-urn gold. This corresponds Hydra with a 1-cm-radius holder; (2) 50.8, to a peak power of 0.5 TW/g, where dE/dx =2.54 25.4, 12.7, and 6.35-um-thick gold foils MeV-cm2/g. From this data we conclude that and 6.35-vim-thick aluminum foil on Proto I electron energy coupling to thin 6-um gold with a 0.5-cm-radius holder; and (3) 205.7, targets was increased over that expected from 88.9, 38.1, and 6.35-um-thick aluminum foils electrons making a single pass in the target, on Proto I; we observed a general broadening by a factor of 2. Figure 14 shows the time of the hard X-ray pinhole images as the foil integrated XUV spectrum. Very few emission was made thinner. If one assumes that the lines appear in the spectral region (greater total diode current is included in the pinch than 600 A), suggesting that most of the radia­ with a current density profile given by the tion is due to continuum. The spectrograph is X-ray rinhole data, then the pinhole data astigmatic and is therefore insensitive to would imply lower peak current density or continuum radiation. Figure 15 shows absorp­ axis with thinner foils. However, we have tion lines appearing in the visible. However, shown conclusively on the Hydra experiments these appear late in the power pulse. During that the number of electrons per unit time the main part of the power pulse, the visible incident on the diode side of the foil is spectral region is dominated by continuum. twice the current as measured by the inte­ Therefore, the assumption of a blackbody grated dB/dt monitors for 6-pin gold. Also radiating source appears to be valid. in the case of G-um-thick gold foils on Hydra and 6-um-thick aluminum foils on Proto I,,the measured radiation temperatures are a Figure 16 shows a one-dimensional analysis for factor of two greater than what can be expec­ current density for 50, 25, and 12-u-thick ted when assuming the maximum possible current gold foils and 25-um-thick gold plus 0.635 cm density incident on the foil. The mechanism range thick carbon with 0.63S-cm gap between for enhancement is consistent with the beam the carbon and the gold foil. Current density electrons making more than a single pass was inferred from apertured PIN detectors on through the 6-pm.thick Au and Al targets. the image plane of a pinhole camera. Current densities for the 6-um gold and aluminum foils were taken from time-integrated pinhole photo­ graphs. The unbacked hole radius behind the foils were 0.5 cm. Peak current densities for 50-u, 12.7-u and 6-u gold foils were 4.8, 1.8, 1.0 MA/cm2, respectively. As in the Hydra case, there was a general broadening of the current profile, the thinner the foil. Peak current densities for 6-um aluminum foils were approximtely 2.1 MA/cm2. Figure 17 shows experimentally measured temperature profiles compared with theoretical tempera­ ture profiles with a peak current density less than or equal to that experimentally measured. In the case of gold foils, we found no temperatures greater than the

139 TIME (IO"'SEC)

Fig. 12. Total current in the Hydra diode from the integrated dl/dt loops and the number of electrons/sec incident from the diode side of the foil during the pinch phase.

Shot 5706 6-y gold foil Shot 5694 12-y gold foil -.-.-.-.- Shot S693 25-p gold foil Shot S714 50-y gold foil

Pig. 13. Comparison of experimental temperature profile with the CHART-D temperature profile for a peak current density of 0.5 MA/cm^. The experimental temperature is closer to that of a peak current density of 0.4 MA/cm2.

140 (ф) Sandia laboratories

ш

ОШ Б16.3 А

ОП 741.3 А 01748 Д OS 164.2 Л ОПТ 182.7 Л

ОШ236.2 л

Fig. 14. Time-integrated spectra between 10 to 80 pm from a 2.2-m grazing incidence spectrograph. Many of the emission lines coincide with the spectral region of the 1-um aluminum signal, these emission lines appear during impedance collapse, implying a low-density plasma.

Ml Saiulia Latintainiics

6м FOIL

WAVELENGTH LINE (Л) 1. 2954.2 2. 2822.6 3. 2748.2 4. 2675.9 5. 2641.5 6. 2616.4 7. 2616.4

50^

240 TIMF

Fiq. 15. Time-resolved optical spectra from rear surface of gold foils. Wavelength range is 2450 Л - 5150 A, spectra] resolution is 4 A, temporal resolution i: ' lis.

142 ^ 25 micron Gold + .635 cm Carbon with .25" Gap 025 micron Gold

rbit r A < i'&\ i, \\ .>s и \>

iten s a \ > to ж %,ч CC !•''• '': i i I i I l * Ч x4 3 2 1 0 1 2 3 4

J= ' ', ^ • 50 r.iicron Gold >• , i О 25 micron Gold E / ' X 12 micron Gold

bi t .' i П 6 micron Gold < / o. \ >• 1 '•• \ '« 1 .'• \ ?, ^чЧ '

(-ra y Inte n p*r H ^432101234

Fig. 16. Relative current density profiles for 50, 25, and 12-y-thick gold foils and 25-um-thick gold plus 0.635-cm range thick carbon with a 0.635-cm gap between the carbon and the gold foil.

143 си

•CHART О 400-

3QD- 13.6 TWg PEAK WEP. гов-

100 - о — J 'V \ CHART О го «о ее so юо

TliVIE (ns

11МЕ(|0"В«с)

//

1 5 ! t

Fig. 17(a). Temperature measurements of 25, 12,7, 6.35-nm-thick gold foils on Proto I. Comparisons were made with CHAKT-D11 with a temporal profile as given by a trapezoid normalized to the total current density during the pinch.

144 at',;

300 гот л\ ICPL / •• \ г 'CHART D Г с го ад 60 во 100

IIME(ns)

А г/

ьП Л) <Ю

Fig. 17(b). Temperature measurements for 6-pm aluminum foils compared with CHAKT-D profiles. This corresponds to peak current densities of 3.8 and 5.4 MA/cm . X-ray pinhole photographs current densities on the 2 and 2.7 MA/cm2, respectively.

MS MEASUREMENT OF THE TEMPERATURE OF A THIN FOIL critical; a Questar telescope is used to image USING OPTICAL AND VUV DETECTORS the central 0.05 cm2 of the prnch Tegion throu. a pinhole to the optical diode so that the Temperature measurements of thin gold foils emitting area is accurately known. heated by the Hydra relativistic electron beam have been carried out using optical and The beam deposition was characterized by (1) vacuum ultraviolet detectors in a blackbody an X-ray pinhole camera to measure deposition assumption. The goals of the program were area, (2) an X-ray PIN diode to measure total twofold: CI) show that blackbody radiation pinch current using a Monte Carlo X-ray emis­ in these spectral ranges could be used as a sion simulation, and (3) conventional 6 and ^ reasonable diagnostic, and (2), compare observed machine current and voltage monitors. The PIN temperatures with hydrodynamic simulations diode was absolutely calibrated using a DC based upon classical or greater than classical electron beam as a function of beam energy and deposition rates.12 To this end, it was neces­ angle. A 1-D electron-photon ttansport code sary to measure accurately the beam pinch (TIGER)4 was used then to calculate a total enhanced current density. This was done with response to a pinched electron beam in terms an absolutely calibrated high energy PIN detec­ of pinch current at a given beam voltage. tor and time-integrated X-ray pinhole camera The current density approaches 1 MA/cm2 for using a 1-D photon transport code. Peak thick foils (25 to 50 urn) and falls to 0.2 temperatures of 5 to б eV were obtained for MA/cm2 for the thinnest foils, б да. the thinnest foils, 6 pm, consistent with classical deposition at 1 TW/gm. A summary of the measured temperatures is pre­ sented in Fig. 19, where only the highest The diagnostics are sketched in Fig. 18 for temperature (in 5 shots) observed at each foil shots on the Hydra accelerator CI MV, 30D kA, thickness is given. Peak values of 6 eV and 80 ns). The foil backside temperature is 5 eV are found by the VUV and optical diodes, measured by (1) an optical diode, filtered respectively. The large disagreement for the for hv =_2.5 eV, (2) a vacuum ultraviolet 2S-vim-thick foil has not been explained. It diode, hv = 20 eV, and [3) an ultraviolet may be rel-ired to aligning the optical system streaking spectrometer, 4 < hv < 5 eV. to where a pinch is expected; for thinner foils the pinch was broader and alignment An experimental detail concerning the emitting would be less critical. source area, A, which will affect the tempera­ ture via the blackbody emission Agreement between the diagnostics is considered acceptable, considering the number of calibra­ tions involved. In the optical system, for example, substantial deviations from expected SM = ^rhA[ex p (hv/kT) . j- performances were caused by near-field vignett­ ing in the telescope and degraded sensitivity of the diode (as revealed by a tungsten lamp should be clarified. In the vacuum ultra­ calibration at 507 nm1*). violet, the diode sees only hv >^ kT and in­ tegration of the source diode response func­ Validity of the blackbody assumption is a cri­ tion, R(v), for a signal voltage, V(T) tical question in the analysis of these experi­ ments. To this end, the foil heating and expansion was simulated with the CHART-D V(T) = JR(\>) S(V) dv hydrodynamic codecs and compared with opacity calculations.16 The results of a typical case are displayed in Fig. 20 for a 25-vmi-thick shows very nearly a T dependence. Thus an gold foil with a 1 TW/gm deposition rate. error in the emitting area will not seriously The temperature and density profile, measured affect the VUV result. (For example, a factor from the foil center plane, are given as of 2 in the area gives a 19 percent error.) simulated at the end of the peak of the poweT A source area is chosen somewhat larger than pulse with values corresponding to the left- the pinch size as determined by the pinhole hand ordinate. Photon escape functions, camera 13

The optical diode, however, is filtered to a 70-nm bandwidth at 507 nm and has a calculated exp K(p,T) dx V(T) ~ T1-9 for our temperature range. The /; accuracy of the emitting area becomes more

146 Optical Diode

X-ray Diode Grating Streak Camera

Fig. 18. Blackbody measurements on the Hydra accelera­ tor. Temperatures were measured by optical vacuum ultraviolet, and ultraviolet detectors. The electron beam was characterized by a pin­ hole camera, an X-ray diode, and (not shown) fi and V monitors.

л 6 > —vuv to -Optical a. 2 E a> \- 10 20 30 40 50 Foil thckness (jum)

Fig. 19. Foil temperatures vs. thickness. Only best values are shown; five or more shots were made at each thickness.

H7 12 , 4.5 TW/gm / 10 / / / / > 8 / u / -Si / 3 6 / , 1 TW/gm / /f"\ „Optical 4

Temper s / '/ I /A ^-'.2 TW/gm 2 - /1 A'" 20 60 100 140 Time (nsec) Fig. 20. Hydrodynamic simulation of 25-um gold foil uniformly heated at 1 TW/gm. Comparison is made to Fig. 21. Temperature time history as viewed opacity calculations. by optical and VUV detectors in comparison with surface temperatures simulated with various deposition right-hand scale, describe the percentage of rates, 0.2 to 4.5 TW/gm. optical (2.5 eV) or average VUV (20 eV) photons escaping from a given depth. It is clear that the expanded foil is still opaque for these for thick anodes so that the high v/y require­ photons and that both systems see into regions ment was not satisfied for a conclusive test. of the same temperature, although differing densities. These simulations also allow an Time-resolved ultraviolet and optical spectra estimate of the interior temperature, 8 eV, to were also obtained with the spectrometer/ be made based on surface measurements. streak camera combination sketched in Fig. 18. Spectral and temporal resolutions are 0.4 nm Agreement of simulated surface temperatures and 6 ns, respectively. In the ultraviolet and measured values provide an estimate of [240 nm < X < 300 nm), the spectrum consists the deposition rate, 1 TW/gm, appropriate of a continuum superimposed upon which numerous for these experiments. Fig. 21 shows the neutral and first ionized gold lines appear time history of several simulations at varying in absorption. The ion lines are more pro­ depositions in comparison with a typical lific for the thicker foils. These spectra 25-шп gold foil shot. It is seen that the are consistent with a model in which a cooler observed temperature is a sensitive indicator plasma of gold ions and neutrals exists bet­ of the deposition. Use of the TIGER election ween the detector and a much hotter blackbody transport code, at current densities given by source emitting the continuum. Given the PIN diode and pinhole camera analysis, shows energy levels involved, a vapor temperature that no unexpected phenomena are occurring; of 0.5 to 2.0 eV is estimated. The opacity deposition is in accordance with classical of this vapor in the spectral region viewed mechanisms. It had been hoped that these by the optical diode is largely undetermined shots would test a model of enhanced deposi­ Tentative results in the visible, however, tion in large self-magnetic fields in thin show no absorption lines and the photon escape foils.12 However, the pinches with thin functions of Fig. 20 also suggest that there foils were not as tight as previously measured*7 should be no absorption problem.

I« These experiments in thin foil heating are performance and stability of the targets will continuing on Proto I where a higher current provide a data base for fusion target design density, although at a lower v/y, should pro­ as well as an understanding of the roles of vide higher foil temperatures. In the Proto the above problem areas in ICF experiments. I situation, the VUV and optical systems are mutually exclusive and require separate It should be recognized that experiments per­ shots. Interesting results, suggesting formed in the pressure-velocity domain 1 to 10 enhanced deposition, have been obtained by Nlbar, 1 to 3 cm/ps on the Hydra accelerator the VUV system and are described elsewhere have practical value exceeding purely a demon­ in this report. Optical data are too prelimi­ stration of diagnostic capability. Severe nary for comment. multidimensional phenomena (from nonuniform loading). Rayleigh-Taylor instabilities and We conclude from the Hydra experiments that particle ejection from shocked surfaces are surface temperatures of 5 to 6 eV imply depo­ all known to occur at the 1 Mbar level and sitions of 1 TW/gm, in agreement with classi­ below. How exactly they affect the performance cal deposition models. However, decreasing (as defined above) of an ICF implosion has not current densities with thinner foils may been established at any level. In addition, indicate that the necessary high V/Y values equations-of-state presently used in hydro- for enhanced deposition processes were obtain­ dynamic - TN burn calculations have no experi­ ed. mental verification (for converging systems) in the megabar range or above.

IMPLOSION PHYSICS Previous experiments were conducted on the Hydra accelerator with single ablatively driven The performance of an ICF target can be hemispherical shells.18 In that work, the characterized by CI) neutron yield, [2) fuel electron beam target coupling efficiency was compression (pr), (3) temperature of pusher established by comparisons of experimental and fuel, and (4) velocity of pusher. Poten­ implosion times and material jet velocities tial problems for practical ICF target designs with the results of two-dimensional hydro- (including high-gain multiple shell configu­ dynamic calculations. More recently, pusher rations) which could result in significant velocity and temperature measurements have reductions of these figures-of-merit are been performed on Hydra for (1) double-shell (1) multidimensional phenomena, i.e., effects hemispheres, and (2) thick-walled single- of loading and structural (pellet) nonuniformi- shell (shock focusing) hemispheres, and ties, (2) Rayleigh-Taylor instabilities ori­ optical holographic data for cylindrical ginating at material boundaries, (3) particle implosions have been obtained. ejection from shocked interfaces, and (4) equation-of-state anomalies, i.e., viscosities, In Fig. 22 is shown the basic experimental melt temperature and opacities which lie out­ arrangement for measurement of the material side the normal extrapolations employed in implosion time and jet velocity. The figure calculations. depicts a double-shell target, with an outer shell Au (wall thickness = 0.32 mm, diameter = In closed spherical ICF targets, experiments 3.8 mm), and an inner shell of W (wall thick­ designed to measure any of the above figures- ness = 0.2 mm and diameter = 1.4 mm) with of-merit other than neutron yield are diffi­ water as the buffer material between the cult if not impossible to perform. We have, two shells. The inner shell and buffer thick­ therefore, chosen open (but converging) ness satisfy conditions calculated by Toepfer model targets in order to perform thermo- and and Tiffany for stability. The two thin hydrodynamic experiments. These model targets pellicles (~ 7 urn) are used along with a consist of (1) open hemispheres, and (2) cylin­ laser source and fast photomultiplier detec­ drical sections. Experimentally determined tor to measure the time of arrival of inner thermo- and hydrodynamic quantities are com­ shell material at the equator (first pellicle) pared with calculations for these geometries and arrival approximately one-centimeter using one- and two-dimensional hydrocodes. downstream (second pellicle). For the arrange­ Diagnostics include, at present, velocity ment shown in Fig. 22, the measured implosion and temperature measurements by optical time was approximately 300 ns and the material methods (four-pulse holography, streak and jet velocity was 1.6 cm/us for a total diode framing photography and fast photo-multipliers) energy of 22 kj. and in the future, flash X-radiography. The

119 • Jet Velocity Calculation and Measurement

brass anode tantalum

Fig. 22. Experimental arrangement for measure­ ment of target implosion, time and jet velocity. 160 240 TIME (ns) In addition to pusher velocities of double shells, we have performed time-resolved temperature measurements of the inner shell Fig. 23. Measurement of implosion time and of the double shell system during implosion. jet velocity for shock focusing In Fig. 23 are shown two oscillographs of a target. photomultiplier detector which recorded either (a) light with wavelength = 514 nm (a laser source) reflected from the inner Hydrodynamic code calculations have not yet tungsten suTface or (b) light with wave­ been performed for these double-shell con­ lengths centered at S14 nm (bandpass = 100 A) figurations . generated by the collapse of the inner wall. The purpose of illuminating the tungsten We have applied these techniques to the single inner wall was to detect the time of shock shell, thick-wall or shock-focusing target wave arrival at the inner surface, a useful system for which there are hydrocode calcula­ parameter in hydrodynamic analyses. A defi­ tions. Both gold and nickel ablators have nite reduction in reflectivity is observed been used in experiments. Figure 24 gives (a) and the occurrence of this change is the target dimensions. In Fig. 25 are shown interpreted to be the time of shock break­ the results of an implosion time-jet velocity through. Following the reduction in inten­ measurement similar to that for the case of sity, the rapid rise in light intensity is a double shell. The shock focusing single thought to be associated with the luminosity shell was multilayered, consisting of a thick of the converging shell. In (b), the shell outer layer of gold (thickness =0.6 mm, dia­ was not illuminated, and the intensity is meter =1.9 mm) and thin inner layer of solely associated with self-luminosity. In tungsten (thickness = 0.094 mm). For the shot this case, the calibrated optics and photo- depicted in Fig. 25, total diode energy was multiplier implied a peak temperature of 23 kJ. The first pellicle was placed slightly approximately 0.S eV. Although the complete downstream from the shell equator. The light structure of the light signal is not under­ intensity trace indicates an initial reduction stood, the occurrence of the initial maximum due to X-ray induced noise in the detector at ~ 300 ns is in agreement with previous followed by a change at 130 ns due to arrival measurements of the implosion velocity. In of material at the first pellicle. The initia. fact, the rise of the luminosity signal is change in reflected intensity from the second correlated with the time-resolved implosion pellicle occurs at approximately 255 ns result­ velocity for the double-shell system. ing in a jet velocity of 3.3 cm/us. Also shown in the same figure is the calculated

150 50 ns/div 50 ns/di

Ni-W 514 nm 5-10eV T; = 70-80 ns

100 ns/div 100 ns/div

Fig. 24. Shock focusing target temperature data. The negative signals are the output from calibrated photodiode detectors viewing optical emission at S14 nm from the pusher free surface. The positive signals are X-ray bremsstrahlung emission from the tar­ get.

> Au-W Double Shell (vac. buffer) ± Time (jus) —> Time {/JS> —>• 0 .1 .2 .3 .4 .5 .6 .7 Q Л .1 .3 4 .5 .Б .7

i Shock I Breakthrough

(A) Illuminated (A) Not Illuminated

Fig. 25. Measurement of implosion time and jet velocity for double shell target.

151 Cfrom the two-dimensional hydrodynamic code gm/cm^. The electron beam focused to a dia­ CSQl9) velocity-time history for beam and meter comparable to the target O.D. Optical target conditions similar to those of the holography of transversely mounted tubes gave experiment. The calculated implosion time simultaneous measurements of pusher and and pellicle arrival times correspond well ablator velocities. For the case of the to the experimental data. The calculated thicker tubes which were total stopping to pressure velocity and temperature profiles the _< 1.2-MeV electrons, an ablator velocity are shown in Fig. 26. of 1.5 cm/usec was measured (Fig. 28). The particle velocity at the time of shock break­ Temperature measurements were carried out through at the pusher surface was inferred employing both the Au-W and Ni-W shock from the pusher velocity to be 0.25 cm/usec. focusing hemispheres. Examples of the Using published Hugoniot data for brass, this results are shown in Fig. 27, where the corresponds to a shock pressure of 1.3 Mbar ranges in measured temperatures and implo­ and shock velocity of 0.73 cm/usec. In cer­ sion times are listed along with the oscillo­ tain holograms, distortion of the reference graph traces. A significant experimental fringes indicates the occurrence of fluff observation is that the temperature for the (particle ejection from the shocked inside Ni-W configurations (5 to 10 eV) are approxi­ surface). Asymmetric loading of the targets mately a factor of two higher than those for resulted in off-center convergence. Several the Au-W hemispheres (2 to 4 eV). One also shots were taken with air-filled cylinders notes that the measured values for Au-W are optical emission measurements were made of considerably lower than that predicted by the temperature attained from compressional the CSQ calculation (~ 13 eV). To further heating. Temperatures = 1 eV were observed. study these phenomena, one-dimensional spheri­ Future experiments will employ high aspect cal implosion calculations'^ were performed ratio pushers with low density buffers to for the actual experimental conditions for study stability, and various pusher materials both the Au-W and Ni-W cases. These results and surface finishes to study fluff. In indicated 10 to IS eV for Ni-W and IS to 20 addition, experiments will be performed to eV for Au-W. The significant discrepancy study the effects of altering the pusher between measured and predicted temperatures shape to improve convergence for nonuniform for Au-W may be associated with multidimen­ loading. sional effects and indicate the sensitivity of shock focusing target performance to uni­ formity of loading. Such effects might be SCIOTILUTOR-PHOTOMULTIPLIER RESPONSE TO A expected to be more severe in Au-W than Ni-W SIMULATED REB GENERATED THERMONUCLEAR NEUTRON because more smoothing of the implosion would PULSE occur for the thinner shell of Ni-W than for that of Au-W. In these experiments, target Recent experiments have reported evidence of loading was observed by X-ray pinhole cameras thernonuclear neutron production from relati- to vary by SO percent over the surface. The vistic electron beam (REB) imploded or heated long risetime in the temperature profile targets,^i-2.s the detection of thermonuclear observed in some Ni-W shots is probably attri­ neutrons from such targets is complicated by butable to preheating of the tungsten layer the possibility of background beam-target (by electrons) to thermodynamic states above neutrons being produced. To measure thermo­ vaporization. In general, however, preexpan- nuclear neutron production all experiments sion does not appear to be a significant to date have employed fast plastic scintillator problem for these targets (as evidenced by photomultiplier combinations (SPM) to determent the rapidly rising temperature profiles neutron time-of-flight. Typically, the SPM's and reasonable implosion times. require some 10 to 20 cm of lead shielding to attenuate the intense bremsstrahlung pulse Additional experiments to study ablatively present in these experiments to a level that driven cylindrical targets have been carried kept the SPM's from saturation. Such massive out on Hydra. Diode parameters were nominally amounts of lead shielding attenuate and modify 4 Si, > 0.3 TW, 30 kJ. TITO anode configurations the neutron pulse shape. were tested, with target axes either parallel to the diode axis through the anode center, or Published calculations of REB imploded targets perpendicular to the diode axis and above the all indicate that thermonuclear neutron pro­ anode surface. The targets were 6.32 mm O.D., duction should occur in £ 3 ns.24 This 1.6 mm thick and 4.76 mm O.D., 0.80 mm thick research report considers the response of a brass tubing with an average density of 8.50 SPM shielded with various amounts of lead to

152 1 MA/cm! «85 .05TWat1MV || 16 . (60 nsecfll 2 mm o.d. «65 I 90 1 80 / ?0 I 60 / I

45/ V .15ПЬ 7E

! -0.8 -.04 0 +.04 60 80 100 120 Axial Position (cm) Z —> Time (nsecl

Fig. 26. Calculated pressure, velocity, and temperature in shock focusing targets.

0.62 g/cm2

f/ T 0.140 mm

1.77 g/cm2

> \\ 7? 0.94 mm 1.143 mm \\

Fig. 27. Shock focusing target dimensions.

I S3 Fig. 28. Electron beam driven ablative implosion of the brass cylinder in transverse diode.

a beam target produced 3-ns-wide burst of NPM-54 SPM has a risetime of 2.2 ns and a Ш neutrons. In this way, the "signature" FWHM repsonse of 3.5 ns when used to observe of a true burst of thermonuclear neutrons a single instantaneous event such as a comic was established. Since background beam- ray. In Fig. 29a, the prompt X-ray brems- target neutrons are produced throughout the strahlung pulse of the Febetron as well as power pulse of a REB accelerator (typically the neutron burst is observed through 1.9 cm > 20 ns), it should be readily possible to of lead at a laboratory angle of 100°. The distinguish the background neutron "signature" neutron time-of-flight from this trace is from the true thermonuclear neutron "signa­ 46 ns consistent with 2.45 MeV DD neutrons. ture." The neutron pulse has a risetime of 3 ns and a FWHM of about 6 ns. In Fig. 29b, the X-ray bremsstrahlung pulse is completely attenuated Thermonuclear Neutron "Signature" by the 10.2 cm of lead, and the neutron pulse is observed to have a 4 ns risetime and a The pulsed power source used to produce the FWHM of approximately 20.С ns. In Fig. 29c, 3-ns neutron burst was a commercially avail­ the neutron pulse is observed to have a 4 ns able Febetron Model 706 electron beam risetime and a FWHM of approximately 32 ns accelerator. The Model 706 has a nominal when detected through 20.3 cm of lead. It voltage of 600 kV, an electron current of is clear that in both cases, the lead shielding 10 kA, an impedance of 60 й, and a 3-ns-wide is degrading the leading edge of the neutron power pulse. The neutron production was pulses to a small extent, but its most notable monitored by a standard silver activation effect is to create a large tail on the neutron counter25 and by an EG§G NPM-S4 SPM detector.26 waveforms. Consequently, it is possible over A typical shot produced some 1 x 10^ DD neu­ reasonable flight paths to use the leading trons in a 3ns burst yielding a neutron in­ edge of a neutron pulse as a timing marker to tensity of some 3.3 x 10l* neutrons/sec. to determine the time of arrival of a mono- energetic neutron group. It should also be noted that there is no need to make a shorter As an example of SPM response, Fig. 29 shows neutron burst since the fastest SPM detectors three DD neutron pulses detected through 1.9, have integration times of the order of 3 ns. 10.2, and 30.3 cm of leading shielding. The

IS4 a. 2,S volts/div, 20 ns/div. The detector was shielded by 1.9 cm of lead and the total neutron yield was 1.1 x 106. Note the prompt X-ray pulse.

b. 5.0 volts/div, 20 ns/div. The detector was shielded by 10.2 cm of lead and the total yield was 1.0 x ID6. The lead shielding completely attenuated the X-ray pulse.

с 2 voits/div, 20 ns/div. The detector was shielded by 20.5 cm of lead and the total yield was 8.8 x 10^. Once again the lead shielding completely attenuated the X-ray pulse.

Fig. 29. Oscilloscope traces comparing neutron scintillator - photomultiplier detector response to a 3-ns-wide DD neutron pulse when the neutron pulse is detected through 1.9, 10.2, and 20.3 cm of lead shielding.

155 As a further check on the neutron pulse shape A one-dimensional Lagrangian hydrocode POLLY^O presented in Fig. 29b, time-dependent Monte was used initially to calculate the foil motion. Carlo neutron transport studies were made In this code, the piston was the left boundary using the code SORS developed at Lawrence of the system and was given a constant velocity. Livermore Laboratory.2' Figure 30 shows The gas, which was assumed to be ideal with the good agreement between the neutron Monte Y = 5/3 was divided into fifty zones. The foil Carlo generated pulse shape for a 3-ns-wide was treated as the right boundary of the system source and that obtained from the Febetron with a mass determined by its density and thick­ pulsed neutron source. ness. The velocity of the foil, shock position, and state of the gas were calculated as a func­ tion of time. STABILITY OF MULTIPLE SHELL ICF CAPSULES An analytic estimate of the foil motion was With a marginal driver, one method to achieve made by assuming the gas to undergo adiabatic high pusher velocities is the utilization of compression, with initial conditions speci­ velocity multiplication in a multiple shell fied by the TOLLY calculation when the reflect­ target." In this type of a target, the outer ed shock had returned to the piston for the more massive shells, act on the inner shells first time. This model resulted in an analytic hydrodynamically through the mediun of a expression for the Rayleigh-Taylor growth. low-density buffer, which ideally acts as Foil motion as calculated by analytic model a perfect, massless spring to transmit was compared in various cases with the resul­ momentum and energy. A major problem with tant motion as calculated by the hydrocode. this concept is that during acceleration of Foil mass, initial gas pressure, and piston each inner shell by the buffer, the interface velocity were varied in the calculations. between the buffer and shell is Rayleigh- Piston velocity varied between 0.2 and 1 cm/usec, Taylor unstable. We have considered the initial gas pressure between 0.1 and 10 atmo­ spheres, and foil mass between 0.008 and 0.8 idealized problem of an infinitely massive 2 piston (outer shell) acting on a thin foil gms/cm . (inner shell) to estimate regimes of buffer density, shell separation, and thickness The analytic model consistently gave higher which are less susceptible to instability peak accelerations over shorter time inter­ during the acceleration phase. vals than the hydrocode. Peak foil velocities agreed to within a few percent. Since the Consider the problem in Fig. 31. An infini­ amplitude of the Rayleigh-Taylor mode (linear tely massive piston is assigned a constant regime) is velocity Vp, compressing the buffer gas, which in turn accelerates a thin foil. We analyze the acceleration history of the foil S(t) /ka A dr (1) to determine the Rayleigh-Taylor instability exp growth. For ICF applications, the piston velocity is greater than the speed of sound in the uncompressed gas so that a shock wave is where <50 is the initial perturbation, к the generated ahead of the piston. The shock mode wave number, а (т) the acceleration, travels to the foil where it is partially and A the Atwood number, it follows that using transmitted and reflected. As the gas is a (T) as calculated by the analytic model compressed further, the shock undergeos will result in a conservative estimate for repeated reflections between the piston and 6ft). Thus, to explore the regime of para­ the foil. After a few reflections, the gas meters' for which Rayleigh-Taylor instabilities is heated to a point where the piston velo- » could be come serious, we consider Й* city is subsonic and the gas undergoes an analytic model. adiabatic compression. After maximum compres­ sion, the foil attains a velocity greater than that of the piston, resulting in expansion of Y Y the gas. For an ideal buffer, the maximum Y r (2) PS - P0sQ

foil velocity obtained is 2 vp. Dissipation resulting from shock heating of the gas and foil results in less efficient momentum trans­ fer and lower foil velocities. p « ms (3)

156 SCINTILLATION NEUTRON DETECTOR SIGNATURE

— calculated Plj shield 10cm • experimental & detector m I Or

3ns neutron source mmmm

30. Comparison of the Monte Carlo generated neutron pulse shape for a 3-ns-wide source and that obtained from the Febetron pulsed neutron source. The smooth curve are the Monte Carlo generated data, and the dots are the data set obtained from the pulsed neutron source. The Monte Carlo generated data was peak normalized to the pulsed neutron source data.

PISTON FOIL

31. Geometry for model calculation of stability for accelerated foil. .2 2 The corresponding value of the parameter —j- ps + j ms T P S C4) .U x is 0.13 for which G(t,J = 1.8. Hence, Y-l о о 2 о to assure stable acceleration of the inner shell, this requires a shell uniformity Here s is the foil piston separation, p the between one and ten percent. gas pressure, m the (areal) mass of the foil, and dot denotes differentiation in time. The

initial conditions p0, s0, So are specified at some time to after the shock is reflected from the foil, with best agreement between the code calculations and analytic theory

obtained when t0 is taken to be the time at which the shock returns to the piston. The solution to Eqs. (2) - (4) can be parameterized by the ratio

= Vi/fr-DW•Лo (5) of initial foil energy in the piston frame of reference to the initial energy stored in the gas. For the problem at hand, s =• v, 0 P" The resulting expression for S(t) is

<5(t) 5 exp {G(t)v/A s /X } о o (6)

The function G(t) can be evaluated in terms of elliptic integrals F(n,v) and E(n,v).31

Let tm denote the time of closest approach between the foil and piston, then

3/2/

G(tm) = Vo¥ (-^-) {j2 F[n,Tr/4)

2^2 E(n,ir/4) + 2(x + x*) ) (.7) where ti = sin"1 Vl-[x/(l+x)]4, (Fig. 32).

At t = 2tm the foil has essentially escaped from the piston and G(2tm) = 2G(tm). NEUTRON PRODUCTION FROM ADVANCED REB FUSION TARGETS The above theory has been applied to calculate the stability of double shell targets designed During the past two years we have been inves­ for EBFA (cf. pp. 48, 49). For the best de­ tigating an advanced REB fusion target concept sign (neutron yield 1.5 x 109), the piston consisting of targets which contain preheated (outer shell) velocity is 0.827 cm/vs, and fuel and magnetic fields.32 The purpose of the reduced areal mass density of the piston the preheat is to place the fuel onto a higheT and driven shell is 0.433 gm/cm2. The adiabat and the purpose of the magnetic field pressure behind the reflected shock is is to reduce fuel thermal conduction losses. 1.3 MBar and the distance between the inner An advantage to this approach is that high shell and the outer shell when the reflected fuel temperatures can be obtained for relative] shock returns to the piston is * 0.008 cm. slow implosion velocities, implying increased

I Si Fig. 32. The function 6(1^), where t^, refers to time of closest approach between piston and foil, vs. x, the ratio of initial foil kinetic eneTgy in piston frame of reference to initial energy stored in buffer gas. neutron production for targets irradiated with low power REB accelerators. In particular, Widner and Farnsworth32 have predicted mea­ surable neutron yields (~ ID6 to 10") for modest REB accelerators (power ~0.2 TW). Additional 1-D calculations32 indicate this approach may also permit lower beam power for breakeven level targets, assuming negligible INSULATING SHELL thermal conduction losses and deposition of a energy in the fuel. Others33 have also in­ dicated the advantage of using preheat and magnetic fields in inertially confined systems in the context of thin shell REB targets for which the beam, passing through the target, provides the preheat and magnetic field in the fuel. Fig. 33. "Ф" target configuration.

Recently, we reported a target design32 which incorporated a preheated, magnetized fuel during the beam prepulse and during the main as is shown schematically in Fig. 33. Owing beam pulse until the electron range becomes to its resemblance to the Greek letter "Ф", greater than the collector thickness. This it was designated as the "Ф" target. The charge accumulation causes electrical break­ target consists of an insulating spherical down to occur along the fuel wire and subse­ shell which contains the fuel (in this case quent filament explosion. The resulting a CD2 wire), a thin metallic collector and current flow provides ohmic heating of the two electrodes, one of which connects the fuel and a magnetic field inside the shell. collector to the shell, and the other connects In this way, the "Ф" target makes use of the the shell to the anode. The thin metallic electron beam not only to drive the shell collector serves to collect electric charge implosion but also to precondition the fuel.

159 The simplicity of this target design permits high fuel temperatures can be obtained for easy experimental implementation. relatively slow implosion velocities, implyjj increased neutron production for targets The "Ф" targets used in these experiments con­ irradiated with low power REB accelerators. sisted of plastic (polystyrene) shells 0.3 cm In particular, Widner and Farnsworth32 have I.D. and 0.03 cm wall thickness, which con­ predicted measurable yields (~ \Ф to 108) tained a CD2 filament ~ 25 to SO urn in diameter for modest REB accelerators (power ~ 0.2 TW). as shown in Fig. 33. These targets were irra­ Additional 1-D calculations3' indicate this diated by a single REB from the Hydra leg of approach may also permit lower beam power the REB accelerator (called Rehyd) (1 MeV, for breakeven level targets, assuming negli­ 250 kA, and 100 ns). Energy deposition gible thermal conduction losses and depositic symmetry of the shell, as demonstrated pre­ of a energy in the fuel. Others33 have also viously-" with spherical targets without indicated the advantage of using preheat and collectors, was preserved by the choice of magnetic fields in inertially confined systen aluminum collectors thin to an electron range in the context of thin shell REB targets for at full diode voltage. In the experiment, which the beam, passing through the target, these targets were shot both with and without provides the preheat and magnetic field in th machine prepulse. fuel.

In addition, a number of null targets were Recently, we reported a target design32 which shot. These include targets with large por­ is incorporates a preheated, magnetized fuel tions of one or both halves of the spherical as is shown schematically in Fig. 33. Owing shell cut away (Fig. 34a and b); targets with to its resemblance to the Greek letter "4>", a thick plastic cylinder around the shell to it was designated as the "Ф" target. The prevent uniform energy deposition (Fig. 34c); target consists of an insulating spherical and targets with the electric conduction path shell which contains the fuel (in this case interrupted by an insulating plug between a CD2 wire), a thin metallic collector and the CD, wire and the bottom electrode (Fig. two electrodes, one of which connects the 34d). * collector to the shell, and the other connect the shell to the anode. The thin metallic collector serves to collect electric charge during the beam prepulse and during the main beam pulse until the electron range becomes greater than the collector thickness. This charge accumulation causes electrical break­ down to occur along the fuel wire and subse­ /////У/// ///////// quent filament explosion. The resulting current flow provides ohmic heating of the fuel and of the neutron pulse was measured. Additional diagnostics, other than the usual current and voltage monitors, included an optical streak camera to measure the target blowoff velocities and a target current monitor measuring the current collected by the target. A schematic diagram of the experimental set up is shown in Fig. 35. '//У/У/ УУ УУУ У У/У A total of 39 target shots were made, of which 24 were with null targets and 15 were with complete targets. No 2.5 MeV neutrons were produced by the null target Fig. 34. Null target configuration. shots. However, some of these have generated lower energy neutrons (< 0.5 MeV) apparently from the Cl2(d,n)N13 reaction3* arising from The neutron diagnostics included a silver the deuterium in the diode and carbon on the activation counter and two scintillation anode. Of the IS complete target shots, 7 neutron time-of-flight detectors35 placed at shots failed to produce 2.S MeV neutrons; 4.28 m and 6.35 m away from the diode and 4 were due to low diode output, and the . at 10° and 32.7° respectively measured from failure mode of the remaining 3 have not the diode axis. With these detectors the been identified. neutron yield and the time of arrival that

160 -A*

It-,- - '«чей симсм мгшго, 'll

Fig. 35. Experimental setup.

Eight target shots produced from S x 10 - signal was recorded by the far detector. 2.5 x 10' neutrons as measured by the AG These arrival times are much later than those activation counter. The neutron energy are of curves a and b. reduced from the time-of-flight data (see Fig. 36). It is clear two groups of neutrons The rate of expansion of the shell, as mea­ are produced and they are well separated by sured by the optical streak camera, has been the far detector. The slow group is attribut­ compared to СНАКГ-D computer code3' results to ed to background Cl2(d,n)Nl3 beam-target reac­ estimate the specific power absorbed by the tions and the leading edge of the fast neutron shell. This comparison, which assumes the pulse is analyzed. These arrival times are light front approximately corresponds to a plogged in Fig. 37 against the detector dis­ constant a real electron density of ~ 10*-' tance away from the diode. Curves a and b cm"2, indicates approximately 3 to 5 TW/g show TOF data of two different arrangements absorbed in the shell. of detector position, they indicate 2.5 MeV ±0.2 MeV neutrons were produced during tin Neutron production from exploding shell tar­ last 30 ns of the beam voltage pulse. The gets without preconditioning of the fuel have corresponding implosion time for curve b is been investigated by others.39 Measurable derived from optical streak data of the shell neutron yields from such targets have been and shows agreement with the extrapolated predicted, but have been shown to require neutron formation time. The total 2.5 MeV substantially higher deposition levels than neutrons produced in these shots are bounded the •" 4 TW/g observed here. We find this on the low side by the time-of-flight counter conclusion verified by the present result sensitivity, ~ 10°, and on the high side of that the production of 2.5-MeV neutrons the Ag activation counter measurements, ~ 10', requires prepulse current. which includes both high and low energy neu­ tron contributions, as is evident in Fig. 36b. Our present interpretation of the precondition­ Also plotted are the neutron arrival times ing of the fuel is that prepulse current ohmi- at the near detector of shots.made with incom­ cally heats and expands the CD fuel filament plete spherical shells, and due to the low 2 6 during the 1 usee, 5 to 10 kA prepulse, creating neutron yields (~ 10 ) of these shots, no a nonuniform fuel density distribution. The

161 [a) (b)

Fig. 36. Time-of-flight neutron detector signals for neutron detectors located at 0.S6 m (a) and 4.45 m (b) from the target. Arrows indicate reference time and time of arrival of neutrons. The signal near time zero is from the X-ray pulse.

radiative transport and Spitzer resistivity are in general agreement with this interpre­ tation.'! For example, at a point in time just before the target starts to implode, calculations indicate central region condi­ tions are: p ~ 2 x НГ6 gem"3, T «20 eV, В « 10t gauss. During the main beam pulse, the fuel and magnetic field are compressed, increasing the ют of the low density central region. This leads to a substantial reduction of electron thermal conduction losses during the high temperature compression phase when such losses would otherwise dominate. Using the calculated preheat temperature, and fuel density, and assuming a thermal conductivity reduction factor consistent with the expected final magnetic field from flux conservation considerations, we have performed 1-D hydro- Fig. 37. Position vs. neutron arrival time. dynamic and thermonuclear neutron calculations Time is referenced to the end of using CLYDE. These calculations show that the voltage pulse. after an implosion time of ~ 80 ns the fuel reaches ~ 1 keV and 1.6 x 10"2 gem"3 in temperature and density and produces ~ 10^ lower density regions obtain higher tempera­ neutrons in S to 10 ns. We find that the ture for a given current density and there­ calculated implosion time is consistent with fore, become less resistive, providing a the observed time of origin of the 2.5 MeV preferential channel for current flow. The neutrons, and the predicted neutron yields are result is that a fraction of the wire mass is consistent with those observed. heated to a high temperature and contains suppression. Calculations made with a two- dimensional Mfl) computer code40 including

1(2 TARGET PREHEAT EXPERIMENT line ratio gives a temperature of 3.9 eV if an electron density of 1018 cm"3 is assumed, A central question in the electron-beam fusion consistent with line broadening estimates.4* experiments with preheated-fuel targets43 is (Interfercmetric measurements also justify the exact condition of the fuel at the begin­ this; see below.) Variations of gas fill pres­ ning of the implosion pulse. Separate experi­ sure show only weak dependence of temperature ments are being carried out which simulate upon initial pressure: 3.8 eV for 50 torr the target preheating in a siller, more fill. accessible geometry. Fuel temperature is measured by spectroscopic comparison of OIII and Oil line intensities. Electron density has been obtained both by broadening of Da, Dg and various 0П lines and also more directly with a laser interferometer.

The experimental arrangement for spectro­ scopic observations is shown in Fig. 38. A two-stage, 80-kV Marx bank provides an 8 kA, 500 ns heating pulse which closely approxi­ mates measured prepulse in the Rehyd target shots. The target itself is a 2 to 4-mm- diameter quartz tube with "in-situ" gas mixing which allows each target to be used with fresh gas for several shots. Streaked spectra are obtained with a Czerny-Turner spectrometer coupled to an ultraviolet- sensitive streak camera. Windows and lenses are of fused quartz since a principal line is the OIII at 326.5 nm. Temporal and spectral resolution are 30 ns and 0.6 nm, respectively.

Fig. 39. Microdensitometer scans of ultra­ violet spectra; top, 4 mm target at 100 torr initial pressure and bottom, 2 mn target at 200 toTr. OIII (326.5 nm) and Oil (372.7 nm) were used for temperature estimates.

A slightly higher temperature, 4.1 eV, is found by decreasing the tube diameter to 2 mm. The spectrum in this case, lower trace in Fig. 39, shows continuum radiation domi­ nating all but the reversed OIII line. Such a reversed line might be explained, for example, Fig. 38. Experimental apparatus for time- by a three-zone emission model: (1) a high resolved spectra. The preheat temperature continuum-emitting core, (2) a pulse is characterized by current lower temperature, high-density annulus around monitors (B and CVR) and a volt­ the core absorbing the continuum in a 4-nm age monitor across the target. band at the OIII line, and finally (3) a lower density outer shell emitting the OIII X microdensitometer trace of the spectrum bet­ line in a 1.2-nm band. Spatial streaks with ween 310 nm and 390 nm is shown in the top the slit transverse to the target axis do show •.race of Fig. 39 for a 4-mm target filled to a luminous front structure; however, more 00 torr of H + 10 torr of 0 . The OIII/OII quantitative results are found with the inter­ 2 2 ferometer.

163 The interferometric arrangement given by Fig. The results to date include temperature and 40 is basically a Mach-Zender utilizing a electron density measurements at constant CW HeNe laser and a gated photomultiplieT.''-' current for variations in target diameter and Peculiar features are: (1) unequal path gas fill pressure. An low pressure (50 torr)

lengths, (2) a cylindrical lens to recolli- we measure Te = 3.8 eV in a luminous front. mate the beam after passage through the The front may be optically thin but the core cylindrical target, and (3] a divergence is too tenuous to be observed in comparison matching telescope in the reference beam. with the front. At higher pressure (200 torr) Photomultiplier traces for the 4-mm target, and smaller targets the temperature of the filled to 200 torr, are displayed in Fig. 41 front increases by 10 percent and evidence for probing across the target axis (top) and of a blackbody core is seen. 1 mm off of the axis (bottom). The axial result shows a steady increase in density Futther measurements, both spectroscopic and (2 oscillations), a stagnation, and then interferometric, are in progress to complete decay. The off-axis result, however, shows the fuel preheat characterization. Numerical a gradual buildup of plasma, passage of a high- simulations are being initiated by the Plasma density front (1-1/2 oscillations), and then Theory Division to investigate whether our slower variations. Qualitatively this is 4-eV luminous front is consistent with a consistent with the three zone spectroscopic significantly hotter interior as earlier model. The density of the front, 2 x 10l» postulated.4,5 cm"3, is also sufficient for absorption of the ОШ line from the continuum emission.

Fig. 40. Density measurements via laser-interfero- metry. Power in the two beams was matched by proper selection of beam splitters and insertion of neutral filters (0.0 to 0.3) as needed in the reference beam. The photo- multiplier is encased in Mi-metal and has a double electrostatic shield.

164 P.M. GATE к™..и

(b) OFF-AXIS

Fig. 41. Interferometer signals for axial and off- axis probe beam alignments. One !-• oscillation corresponds to a density line integral of 0.35 x lOl8 air2.

liS REFERENCES

1. M. J. Clauser, Electron Beam Fusion Progress Report - July-September 1976, SAND76-0711, Sandia Laboratories, Albuquerque, NM (May 1977).

2. All spherical targets were constructed and assembled by LASL, Division CMB-6.

3. John R. Freeman, IEEE Transactions on Plasma Science, PS-4, 194 (1976).

4. J. A. Halbleib, Sr. and W. H. VanDevender, TIGER: A One-Dimensional, Multilayer Electron/ Photon Monte Carlo Transport Code, SLA-73-1026, Sandia Laboratories, Albuquerque, NM (March 1974).

5. J. A. Halbleib, Sr. and W. H. VarDevender, J. Appl. Phys., 48_, 2312 (.1977).

6. J. P. Quintenz, Electron Beam Fusion Progress Report, SAND78-0080, Sandia Laboratories, Albuquerque, NM.

7. G. Yonas, J. W. Poukey, K. R. Prestwich, J. R. Freeman, A. J. Toepfer, and M. J. Clauser, Nucl. Fusion, 14, 731 (1974); M. J. Clauser, L. P. Mix, J. W. Poukey, J. P. Quintenz and A. J. Toepfer, Phys. Rev. Lett., 38, 398 (1977); M. M. Widner, J. W. Poukey and J. A. Halbleib, Sr., Phys. Rev. Lett., 38, 548 (1977).

8. E. J. T. Burns, Electron Beam Fusion Progress Report - October 1976-March 1977, SAND77-1414, Sandia Laboratories, Albuquerque, NM (October 1977).

9. D. J. Johnson, W. F. Oliphant, G. A. Doschek and U. Feldman, J. Appl. Phys., Dec. 1977 (to be published).

10. L. P. Mix, private communication.

11. S. L. Thompson, SC-RR-71-0713, Sandia Laboratories, Albuquerque, NM, 1972 (unpublished).

12. J. A. Halbleib, Sr., M. M. Widner, J. W. Poukey and J. P. Quintenz, Bull. Am. Phys. Soc., 22_, 1061 (1977).

13. D. J. Johnson, D. J. Nagel and N. F. Oliphant, J. Appl. Phys., 48, 1058 (1977).

14. M. A. Palmer, private communication.

15. S. L. Thompson, SLA-73-0477, Sandia Laboratories, Albuquerque, NM (1973).

16. Kindly provided by M. J. Clauser and A. V. Farnsworth, Sandia Laboratories, Albuquerque, NM.

17. P. A. Miller, private communication.

18. M. M. Widner, F. С Perry, L. P. Mix, J. Chang and A. J. Toepfer, J. Appl. Phys., 48, 1047 (1977).

19. S. L. Thompson, SAND74-0122, Sandia Laboratories, Albuquerque, NM (August 197S); M. M. Widner, private communication.

20. M. A. Sweeney, private communication.

21. S. Nakai, K. Imasaki and C. Yamanaka, 6th International Conf. on Plasma Physics and Controlled Nuclear Fusion IAEA CN-351F8 (1976); K. Imasaki, S. Miyamoto, S. Ohmaka, S. Nakai and С Yamanaka (to be published).

IM 22. S. L. Bolyubskii, B. P. Gerasimov, V. I. Liksonov, A. P. Mikhailov, Yu. P. Popou, L. I. Rudakov, A. A. Samarskii and V. P. Smirnov, JETP Lett., 24, 182 (1976).

23. J. Chang, M. M. Widner, A. V. Famsworth, JT., R. J. Leeper, T. S. Prevender, SAND77-2017, Sandia Laboratories, Albuquerque, NM (1977); J. Chang, M. M. Widner, A. V. Farnsworth, Jr., R. J. Leeper, T. S. Prevender and L. Baker, 2nd International Topical Conf. on Electron Beam Research and Technology, Cornell University, Ithaca, New York (1977).

24. M. J. Clauser, SAND76-0158, Sandia Laboratories, Albuquerque, MM Oky 1976).

25. R. J. Lanter and D. E. Barmerman, LA-3498-MS, LDS Alamos Scientific Laboratory, Los Alamos, NM (1966).

26. Available from EG§G, Inc., Las Vegas, Nevada.

27. J. Kimlinger and E. F. Plechaty, UCRL-50532, Lawrence Livcrmore Laboratory, Livermore California (October 1968).

28. W. P. Gula and R. С Kirkpatrick, Proc. of the Int'l. Topical Conf. on E-Beam Res. and Tech., SAND76-S122, Sandia Laboratories, Albuquerque, NM, Vol. 1, p. 158 (Feb. 1976).

29. G. I. Taylor, Proc. Roy. Soc., A201, 81 (1950).

30. M. J. Clauser, private communication.

31. M. Abramowitz and I. A. Segun, eds., HanJbook of Mathematical Functions, p. 589 (Dover, 1965).

32. This is documented in a number of internal memoranda involving M. M. Widner, A. J. Toepfer, A. V. Farnsworth, Jr., and J. Chang beginning in Feb. 1975 and continuing until the present time.

33. M. V. Babykin, Ye. K. Zavois'..-, A. A. Ivanov, and L. I. Rudakov, Nucl. Fusion, Supplement 1972, p. 75; D. J. Meeker, J. H. Nuckolls and R. 0. Bangerter, Bull. Am. Phys. Soc, 20, 1352 (.1975); F. Winterberg, Phys. of High Energy Density, p. 376 (Academic Press, 197ТУ.

34. J. S. T. Chang, M. M. Widner, G. W. Kuswa and G. Yonas, Phys. Rev. Lett., 34, 1266 (1975).

35. R. J. Leeper, L. W. Kruse and J. J. Ramirez, Bull. Am. Phys. Soc, 21_, 1196 (197Г;.

36. S. S. Nargolwalla and E. P. Przybylowicz, Activation Analysis with Neutron Generators (Wiley 4 Sons, NY, 1973).

37. S. L. Thompson, SLA-73-0477, Sandia Laboratories, Albuquerque, NM (1973).

38. S. A. Goldstein, D. W. Swain, G. R. Hadley and L. P. Mix, Proc. of Int'l. Topical Conf. on E-Beam Res. and Tech., SAND76-5122, Sandia Laboratories, Albvquerque, NM, Vol. 1, p. 262 (Feb. 1976).

39. M. J. Clauser, Thin Shell Targets for Neutron-Producing Implosions with Low-Power Electron Beams, SAND76-0158, SanlL Laboratories, Albuquerque, №1 (May 1976).

40. L. Baker, J. R. Freeman and S. L. Thompson, Bull. Am. Phys. Soc, 21_, 1146 (1976).

41. Initial results from a bench-type experiment designed specifically to study fuel pre­ conditioning also support this interpretation.

42. B. D. Meizner (unpublished); H. Smith (unpublished).

=67 43. J. Chang, M. M. Widner, A. V. Famsworth, Jr., R. J. Leeper, L. Вакет and J. N. Olsen, Prop, of the 2nd Int'l. Topical Conf. onHigh Power Electron and Ion Beam Res, and Tech., Cornell University, Ithaca, NY (1977J• 44. Hans R. Griem, Plasma Spectroscopy (McGraw-Hill Co., New York, 1964), Chapters 4 and 13. 45. J. J. Ramirez and L. W. Kruse, Rev. Sci. Instr., 47^, 832 (1976).

Itt APPLICATIONS

REP-RATE PULSE TOWER TECHNOLOGY STUDIES A dual resonance transformer was chosen for this repetitive pulser application because of Overview the high-energy transfer efficiency.'"0 In the present system, 94 percent of the energy The Rep-Rate Technology Program was initiated stored in the 1.5-yF capacitor is transferred in FY'77 to develop the pulse power technology to the PFL. The 6 percent energy loss is required for high average power drivers that dissipated in the 1.5-yF capacitor (~1 per­ will be necessary for either particle beam cent), the switch (~. 4 percent), and the or laser initiated ICF reactors. During this transformer (~. 1 percent). period, fabrication of the 350-kV. 300-J, 100- pps tests facility was completed and the faci­ Currently, the operating limits for the system lity is now being utilized to investigate the are set by the lifetime of the high current appropriate pulse power components. The effects switch in the transformer primary and by the of gas flow rates on the operation of high- ability to quench the output switch. The vo.'.tage and low-voltage spaTk gaps are being lifetime properties of the transformer switch investigated. Development of high-voltage, should be improved with installation of pulse transformers continued during this period. tungsten alloy electrodes and a ceramic body. Design of a 1.0 to l.S MV test facility was The output switch presently being tested initiated. Recent activities in our develop­ requires a higher gas flow rate than is ment of excitation sources for high pressure available from the laboratory compressors. gas lasers are included here also. Additional air supply or a design that utilize the air more efficiently will be required for stable operation. Development of a 350-kV, 300-J, 100-pps Test Facility Low Voltage Rail Gap Switch The 100 pulse per second test facility!>2 has now achieved half power operation for extended It has been clearly established through experi­ runs and full power operation for periods of ments with the 100-pps modulator and later in a few minutes. The facility is designed to testing of the 100-pps, 300-J accelerator generate 350-kV, 30-kA, 30-ns pulses at rates system that•flow patterns and gas velocity to 100 pps. Current extended runs have reached are the key factors in effective scavenging 400 kV, 14 kA at 70 pps and 350 kV, 12 kA at of residual ionization and debris in rep- 100 pps. A total of more than 10' shots have rated spark gaps. It is further indicated been fired yielding data on performance of that high velocity gas flow over the working high, time-average power, repetitive spark electrode surfaces provides surface cooling gap switches and component lifetimes. Indivi­ between shots and consequently prevents or dual runs have exceeded 5 x 10s shots. The at least inhibits the occurrence of hot spots PFL has been charged to 1 MV on single shot which enhance the probability of successive tests. breakdowns at those same points. The effects of hot spot formation are progressively lower An artist's drawing of the facility is shown voltage holdoff and significantly increased in Fig. 1. It consists of a low-voltage modu­ electrode erosion at the hot spots. lator section, a voltage step-up transformer, a pulse-forming line (PFL)» a high-voltage To investigate this problem in the rail gap switch and a load resistor or diode. Fig. 2 switch on the low-voltage, high-current dis­ is an electrical schematic of the system. The charge stage of the 100-pps modulator, a modulator converts DC power at 10 kV to pri­ variety of slotted and perforated trigger mary pulsed power at 20 kV by resonant charging tubes were tried. The arrangement that pro­ a 1.5-uF capacitor from 14.5-nF capacitor. duced the widest electrical operating range When the 1.5-yF capacitor is subsequently dis­ and most efficient use of gas is shown in charged through the primary of the voltage Fig. 3. This trigger electrode has a tubular step-up transformer, the PFL is charged to base with a 6-mm square rib bonded to its 700 kV. Near the peak of the charge cycle, side. A line of 0.5-mm-diameter holes were the output switch closes and energizes the drilled into the tube on each side of the load. At full voltage and 100 pps, the rib base such that air delivered to the tube average power output of the system 4s 30 kW. from each end was forced through the gap

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Fig. 2. Schematic of 100-pps accelerator.

170 HIGH VEIOCITV JETS

'HIGH PAE5S4JRE »IR IN

Fig. 3. Electrode configuration for tlie rail gap switch. region from a series of high velocity jets produced by the small perfformations. When triggered, the breakdown of the gap occurs between the main electrodes the top edge of the rib. With the high velocity air streams below the breakdown region, the plasma and debris is expelled into the top region of the gap housing where it is carried away * i 1 1 J 1— through a hose. With this arrangement, air 0 10 20 30 40 SO 60 flows directly over the breakdown areas and provides rapid cooling of both the trigger electrode and main electrodes. The relative PRESSURE (PSIG) effectiveness of the various purging schemes was judged primarily by postmortem examina­ tion of erosion uniformity on the electrodes Fig. 4. Self-breakdown voltage and minimum after extended runs and by visual observa­ trigger voltage or pressure for tion of shot-to-shot arc movement during the rail gap switch. operation.

Ihe rail gap switch is typically operated spark gap shown in Fig. 5. (Air was chosen in a range of 10 kV to 25 kV with a total for reasons of economy.] When operating gap space of 8 mm. The switch is triggered into a matched load, it conducts a peak зу a cable discharge trigger source operated current of 35 kA for 30 ns with a 10-ms it 25 kV. The DC self-break level and trig­ recovery period between pulses. In eight gering threshold characteristics are plotted hours of running (about 3 x 10& shots), it л Fig. 4. Two points representing the must pass 3000 coulombs without degradation rynantic self-break voltage and the triggering in performance. Optimal power flow is achiev­ hreshold at 50 pps are superimposed on ed with the switch operating as close as he DC data curves. These points lie very possible to the peak of the PFL charging wave­ lose to the DC data. The low triggering form. However ringovers, that is pulses on hreshold of the switch (approximately 30 which the statistical fluctuation in breakdown srcent of the self break) provides a wide voltage cause the switch not to close, must uige for stable operation. A further be minimized to avoid excessive heat dissi­ Ivantage of the wide triggering range is pation in the transformer and second stage le ability to vary the pulse rate and volt- capacitor. The spread in breakdown voltage ;e of the system without having to continu- determines the fraction of PFL full voltage ly readjust the pressure in the switch. at which the switch must operate.

gh Voltage Switch The switch operates at pressure up to 10 atm with a gap length of 3 to 6 cm. Its cylin­ e switch between the PFL and the load is drical acrylic housing (2) has a 20 cm inner э 700 kV self-breaking, air-dielectric, diameter and is 19 cm long. Dried and filtered

171 consecutive traces (spaced manually along the -s) © p time axis to display voltage stability). The I / / total voltage pulse lasts 1.65 Msec, the posi­ tive siring 0.45 psec. Illumination occurs shortly after the zero crossing and breakdown J^.:iElM- / „ occurs repeatably at 450 kV, the peak of the waveform. Figure 7 displays 19 equivalent shots vertically spaced to display time jitter The standard deviation of the breakdown time , ^^%й,, ... distribution is 15 ns.

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Fig. 5. High voltaic switch.

air jr- injected from t:it' output (larger) elec­ trode (-1) through S ports (5) with a rotni-y flow pattern which sweeps and cleans the housing.'' Lm'iust gas exits through the center of the same electrode passing through a copper cooling coil (Sj located within the .high voltage structure before entering nylon Fig. 0. Sequence of high voltage switch tests ]'-ncs through the high field region. Both showing voltage jitter, 100 kV/div, the input .'ind output gas lines are connected 0.5 us/div. to the output side of the switch to minimise the voltage standoff required. Because of a slight field enhancement, breakdown occurs over a 3(1 an- b;jiJ at the outer edge of the small electrode fl) during normal operation. Dispersion of the spark over large areas of the tungsten-copper electrode tips has re­ duced the electrode erosion for 2 x 1()6 shots to a negligible level.

The switch was operated for 2 x 10$ shots at 1 to 5 pps and 1 кЛ peak on a repetitive impulse tester with a dual resonance waveform prior to the 2 x 10^ shots in the present accelerator. Data from these experiments confirmed that the electric field vs. pressure curve for air is not sensitive to the first half cycle of the dual resonance waveform. The voltage breakdown distribution is observed to have a standard deviation equal to 3 to 5 percent of its mean. With ultraviolet illumi­ nation this deviation has been reduced below 1 percent. An arc from a needle positioned 1 cm from the smaller electrode and resistively coupled through 25 кй to the larger electrode Fig. 7. Sequence of high voltage switch tests generated the UV. Figure 6 shows 30 showing time jitter - SO ns/div.

172 Experiments conducted at repetition rates above 5 pps have used a switch without UV illumina­ tion installed in the 100-pps facility. The charging waveform on the output switch is displayed in Fig. 8. It is bipolar, the peak 'У,-,"- positive voltage being approximately 60 per­ cent of the peak negative value. Total pulse ll length is S.6 usee. The positive swing last 5 usee and the negative 2.6 usee. In the figure, the output switch fires near the peak of the negative voltage excursion.

Fig. 9- Л sequence of high voltage switch tests on the 350-kV facility showing voltage jitter 160 kV/div, 2 ps/div.

tSOO - KOW vs REP RATE

SS0 kV. U kA 2000 - / /440 kV. II kA Fig. 8. Charging voltage waveform on the PFL 300 kV/ div, 2 us/div. I50D - / / For a given gas flow rate and a given pulse

there is a maximum pulse repetition frequency 1000 (PRF) at which the switch operates stably. # Instability may be marked by dispersion of the breakdown voltage on the negative voltage swing or by the appearance of breakdown near ЫЮ the peak of the positive voltage excursion - J? with a stable negative breakdown level. On­ set of jitter in the negative breakdown level may be observed from multiple shot photographs as in Fig. 9 which shows a sequence of shots. With sufficient gas flow, the switch stability at 20 to 30 pps with no UV illumination is MAXIMUM STABLE REPETITION BATE (PPS) comparable to the 1-pps stability with UV illumination. This is presumed to be due to Fig. 10. Flow rate vs. repetition rate for the presence of remnant ionization and metas- stable operation of high voltage tables which simulate the effect of UV pre- switch. ionization.

The appearance of positive voltage breakdown constant gap spacing, voltage, and peak current. is a more common sign that the switch is not The proportionality between flow and maximum adequately purged. Figure 10 presents a pulse rate indicates that the product of time plot of the flow rate needed to surpress between pulses and the flow rate is equal to positive breakdown vs. repetition rate at some function of the other variables in the

173 problem, e.g., voltage, peak current, and geo­ metry. Experiments to establish this func­ tional relationship are currently in progress.

Miltichannel Surface Discharge Switch Experiment

Repetitively pulsed high energy systems will require multichannel spark gaps to avoid excessive electrode wear and in some cases achieve low inductance. In anticipation of this requirement a simple experiment was conducted to evaluate the merits of a surface discharge switch for these applications.

The experiment consisted of a 0.04 pF capaci­ tor connected to a two-electrode rail gap (15 c;:i long) with an insulated trigger electrode placed between the two main electrodes (Fig. 11). The trigger electrode was made up of a 2-cm copper tube inserted inside a 2.5-cm alumina ceramic tube with the ends sealed off to prevent breakdown from the inside. The '.rigger pulse was supplied by a charged co­ Fig. 11. Surface discharge switch. axial cable winch was connected to the trigger electrode through an isolation capacitor and a pulse shaping spark gap.

The gap was unprcssuricud and operated over a range of 10 to J1' kV with visual observations :icide of the breakdown channels. Preliminary results showed that with a dry ceramic surface and a trigger voltage of at least 20 kV, the average channel population was no greater than .S :n A. However, when the ceramic surface was lightly wiped with transformer oil, the average channel population increased to greater than in (Fig. 12). The high channel number would remain for 2U0 to л00 shots then fall to the original 3 or 1 within about 50 shots.

Tr renew the surface it was only necessary to dry wipe the surface of the ceramic and the channel number would again increase to greater than 10. In wiping the ceramic, the residue removed was barely perceptable which indicated that fouling of some sort was probably not. responsible for the eventual reduction of breakdown channels. In fact, the role of the thin cil film itself in controlling the channel population is not known. Further experiments will be required to understand Fi". 12. Open shutter photograph of multi­ the surface effects. channel surface discharge switch.

If the appropriate operating conditions can be found, a surface discharge gap may offer a wide trigger range without trigger electrode erosion and be an attractive candidate for future high current multichannel switches.

174 Spark Gap Gas Flow Experiments volumes up to 35 SCFM. At the same flow rate the coaxial switch model cleared within 0.8 As repetitive spark gaps ате required to pass ms. In all of the above cases the air was higher energy and power levels it becomes delivered to the open volume of the housing increasingly important to utilize the gas and exhausted through the electrodes in the flowing through the switchc • more efficiently. conventional manner of switch purging. Switch configuration, flow volume, flow pattern and gas velocity all have pronounced Л more efficient sweep out of the spherical effects on switch performance. The present electrode model was achieved by placing a experiments were conducted to study the flow tubular center electrode with a simple nozzle characteristics through a variety of switch in the plasma gun and supplying the nozzle configurations and determine the relative with 100 psi air. The resulting jet velocity merit of different scavenging methods. was approximately 335 m/sec (mach я 1). The jet was directed into the convergent section Conventional repetitive spark gaps typically of a diffuser in the opposite electrode which have in-line cylindrical electrodes with formed a single stage jet pump. With 15 hollow centers through which gas delivered to SCFM delivered through the nozzle, an addi­ the housing is exhausted along with the shot- tional 5 SCFM was drawn into the exhaust to-shot ionization and dehris. Occasionally, stream through the housing ports for a com­ gas injected into the housing is given rota­ bined flow of 20 SCFM. Tins volume with the tional motion to improve the mixing and clean high velocity jet at the center reduced the out process, hut in practically all cases the total clean out time to about 1 ms. Compared gas velocity is quite low (well below sonic to the original arrangement which required speed) until it passes between the electrodes. a full 2 nis with 33 SCFM. Consequently, plasma from a high current dis­ charge can expand freely into the low velocity The high speed vortex switch and the linear region outside the electrodes and mix with rail gap models represent two new techniques the approaching cool gas. Total clean out of scavenging which showed marked improve­ of the ionization products requires displace­ ments over all other methods. Kith the high ment of a sizeable gas volume outside the speed vortex model the plasma did nut expand electrode region in addition to the volume beyond the electrode diameter of 2.5 cm with between the electrodes. The scavenging pro­ a total flow of 4 SCFM. Total clean out was cess would be considerably more efficient if estimated to he less than D.75 ms. At higher the plasma were prevented from expimding into flow rates the expansion diameter was less and the approaching gas stream and the gas cooled was blocked from view by the electrodes. How­ as rapidly as possible by turbulent mixing ever, in the spherical, tubular, and cone in the regions betwr 1 the electrodes and in frustum models the plasma expanded to at least the exhaust ports, jhe present experiments 5 cm with flow rates up to 35 SCFM which illus­ included both conventional purging schemes trates the effectiveness of the high speed and three new tccliniques involving high velo­ vortex. city jet purging. The linear jet purged rail gap also showed A total of six types of switch models were good clean out efficiency. Kith less than tested. These were configured as transparent 3 SCFM the clean out time was no greater two dimensional models through which gas flow than 0.6 ins. The exceptionally fast clean could be observed in a schlieren optical field. out time of this gap is attributed to the The switch models are shown in Fig. 13. Л test supersonic air '.tream through the gap regions. run typically consisted of flowing air through Furthermore, in this and the high speed vor­ the model within the schlieren field and firing tex model the high velocity was generated a plasma gun (20 J) into the gap region of the outside the gap region and projected through model to simulate a switching arc. The plasma it which prevented the typical back blast of gun discharge was synchronized with a high expanding plasma into the open volume of the speed framing camera which recorded the dis­ switch. charge arid subsequent sweep out of the plasma cloud by the flowing air. A six-frame se­ At tiiis point, the results of these experi­ quence of one such test is shown in Fig. 14. ments must be regarded as largely qualitative. However, the dramatic improvements in switch In summary, the results showed that the spheri­ scavenging with the new low volume, high cal electrode, tubular electrode and cone velocity models illustrates that substantial frustum models were fully cleared of ioniza­ gains can be made over conventional designs. tion products within 2 to 2.5 ms with flow Further experiments are planned with actual

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Fig. 13. Gas flow experiment spark gap models.

174 t = 800 ys = 1000 us

Fig. 14. Six frames from gas flow experiment.

177 prototype switches and flow models as ;m on­ going effort to improve high power switch performance and efficiency.

High Voltage Transformer Development

Л new high voltage transformer (Fig. 15) has been designed and constructed. It is a spiral strip design with a single turn pri­ mary and a 48-turn secondary winding. The margins of the transformer on both the core and case are graded with split rings similar in principal to most previous designs. It lias, however, several features which reduce the cost of the unit and improve its per­ formance in repetitive pulse applications.

Cost reduction features include use of stan­ dard tube sizes for all cylindrical sections, commercially available duct transitions for the end flares and insulated No. 12 gauge electrical wire for grading rings on the core. The case rings are inserted in grooves Pig. 15. Pulse transformer. in the case and do not require a vacuum en­ capsulation process to secure them in place. power, it will be necessary to closely cont This transformer has an externally exposed energy losses in the Marx generator. Resis primary turn which facilitates heat removal tive isolation between stages, resulting in in repetitive pulse applications. The exposed significant energy loss during the Marx cha primary has flanges connecting the two case must be replaced by inductive isolation. sections which simplifies the assembly of the unit and allows it to be easily disassembled As presently envisioned the Marx would have for winding changes or service. 16 stages with plus/minus charging. A charj voltage of approximately 60 kV per stage The transformer has undergone preliminary would be required for one megavolt operation testing up to 800 kV on the REPIT facility1 The 60 kV would be achieved by LC ring-up and has been operated at approximately 10 from a 30 to 40 kV primary energy source. pps for shot periods at approximately 500 Capacitance per stage for 10 kJ operation at kV. No internal problems have been detected. 60 kV would be 0.35 иF for a total Marx char; The transformer will be tested to 2 MV in the time capacitance of 5.6 uF. To charge the near future to determine whether windings can Marx in 10 msec the input inductance would Ы be operated with voltage stresses in the range 1.8 H. The maximum charge current would be of 400 kV/cm. Before the tests can be com­ only 106 A. Thus, ring and hold charging pleted, however, the load capacitor and using semiconductor diodes is entirely rea­ switch in the REPIT tank must be modified sonable. Studies using the SCEPTRE network to accommodate the higher voltages. analysis program** demonstrate that the Marx capacitors can be uniformly charged in this fashion. Marx Generator Design Studies Upon erection, the Marx generator energy must Conceptual design of a Niarx generator for a be delivered to its load. The equivalent high-voltage, high-power test facility (1 circuit for the erected Marx and its load is Ml', 10 kj per pulse, 10 to 100 pps) has been shown in Fig. 16. An analysis of this cir­ completed. This facility will be used to cuit with SCEPTRE shows that the energy can study electron beam generator components at be transferred to the load in about 1 us. higher voltages and at average power levels Most of the energy loss goes into the inter­ approximately one order of magnitude greater stage inductances, Lj. Assuming Lj = 30 uH, than those of the present repetitive pulse the energy transfer efficiency to the load test facility. Because of the high average is 86 percent. Doubling Lj tuts the loss in

178 Fig. 16. Marx generator cc|uiv;ilcnt circuit.

half giving a transfeT efficiency of 92 per­ initiate space charge limited electron flow. cent. In this model, the energy transfer The mean electric field of 17 to 50 kV/an efficiency may be increased to better than are required to achieve the low-current den­ 99 percent with appropriately large inductor?; sity, space-charge limited electron flow. however, the effects of stray capacitance not With this type of cathode it is difficult included in the simulation will luiiit what to achieve uniform current densities at may be achieved in practice. the anodel" and substantial temporal fluc­ tuations in tiic current density are caused The peak output current from the erected Marx by cathode plasma instabilities.! "•'--> For generator will approach 50 kA. The development efficient operation of these diodes, it is of a 50 kA, 120 kV, 100 pps spark gap switch necessary to suppress.emission from cathode suitable for this Marx represents a major support structures. challenge. Eight such switches must be evict­ ed simultaneously and quenched in 10 msec. During this period, investigations of the For comparison, the primary input and output cathode plasma instabilities, emission switches of the prototype facility currently enhancement, and emission suppression tech­ operate at 20 kA, 20 kV and 13 kA, 600 kV, niques have been underway. Development of respectively. a 400-kV, 1-psec electron beam source for these studies was completed. The primary energy store for this test facility will be a DC power supply. Specifications for Cathode Plasma Instability Studies the device have been written and switch gear for connection to the 4160-V power mains is In certain operating regimes electron beams being installed. from diodes of the type commonly used for direct excitation of gas lasers or to sustain discharge lasers (blade cathode, planar anode) LOW CURRENT DENSITY DIODE STUDIES show large local current density fluctuations. '. Since these fluctuations could have a negative Overview effect on the efficiency and operation of the laser, experiments to study the phys ics under - 2 lying these instabilities and to search for Large area low-current density (1-40 А/ал ) stable diode operating regimes have been per­ electron beams are required for direct exci­ tation of low-gain lasers or photolytic formed during this period. These fluctuations sources. Generally sharp-edged electron are characterized by current density increases emitters are used to provide the field enhance­ to several times the ambient space charge ment necessary to form the- cathode plasma and limited value in bursts lasting a few tens of

179 nanoseconds. In general, the current density after the burst is unaffected by the occurrence of the burst, and subsequent bursts may occur aperiodically. A similar phenomenon has been studied exten­ sively in the single needle cathode geometry where aperiodic ci.-rent bursts exceeding five times the ambient space charge limited current have been observed^ together with microbeam formation, ion acceleration to energies in excess of the diode voltage,12,13 and disper­ sion of the electron energies.14 Assuming that the instability arises because the ex­ panding cathode plasma can not sustain the diode current density, a model relating the current density just before the instability, JCR, to the onset time of the instability, trR, has been derived and tested for 102 A/cm2 £ JC.-R < 10* A/cm^ with good agreement with data.15- Data collected during this period indicate that within certain limits Fig. 17. hlectriciil schematic of the LPN the model may also be correct for JCR as accelerator. Superimposed wave­ small as 2 A/cm2.lf,>17 This model further form corre^;x>nds to an open circuit postulates a turbulent plasma between the at the diode. cathode and the anode which disper-\:s the electron energies. Microwave emission from the diode at the time of the instability has been found to be consistent with such turbu­ INSUIAIOR STACK lence. 1R Kith the modifications described below, this needle cathode model also seems to apply to blade data. This is reasonable since the fluctuations in current density are qualitatively similar in both cases. The blade data shows that the instabilities on the average occur later in time for a given current density than the needle instabilities which is 'onsistent with the model. However, microwave emission has n't been observed in t>r ,.--ie .•. " rin.?:.ts, indicating the absence ."-..]•-.*, mid-gap plasma. This has yet .'. ii' xl theoretically.

•'•': _ng the LPN accelerator, a i ..: . -J-I!, lumped-constant, pulse- fi, - • .".• к (PFN).1-16 An electrical set.. : • ; с LPN is shown in Fig. 17 together with Luc Oj^en circuit (no-load) output pulse. The no-load pulse is 1.1 usee long with 0.2 usee rise and fall times (Because of the 30-J2 resistor in parallel with the diode, the no-load condition actually corresponds to the generator driving a matched load.) FOT the data presented the no-load voltage would have been 200 kV, but the actual peak diode volt­ age was typically somewhat smaller due to Fig. 18. LPN diode schematic. loading of the generator by the diode.

The cylindrical 75 nH LPN diode, Fig. 18, was adapted to mount needle, blade, or dome cathodes with anode-cathode (A-K) spacings

ISO from 0.5 to 6 a on a variable length shank. than 50 percent gap closure at tr$. As dis­ Current density and total diode current were cussed below, a comparison of the blade cathode r.onitorcd with the Faraday cup and CVR shown impedance data with a model based on the Child- in Pip. 18. Typically the Faraday cup was Langmuir theory for a cylindrical diode, indi­ covered by a copper foil more than an elec­ cates that closure speed is constant and in tron range thick with a 2-cm-diaineter hole reasonable agreement with the zero voltage on axis for current density measurements. calculation. Sucli detailed comf >risons have Diode voltage was monitored with a copper failed thus far for the case of the needle sulfate resistive divider at the output of data and indeed an inspection of the total the PFM. Microwave measurements were made current waveform of Fig. 19b, suggests that through three ports 90° apart in the diode the closure speed may increase following the sidcwall as shown in Fig. 18. Ml microwave initial instability. I!" it does the increase d:>ta were taken at 90° to the diode axis. might be lue to the generation of anode plasma by anode heating during the instability as Л typical needle event is shown in Fig. IS described in Ref. 15. where Fig. ISA is a combination of the volt­ age waveform (upper trace), and current den­ Plasma motion ,~as been monitored for a needle sity (lower trace), and Fig. 19b is the cathode with a 2.6-cm A-K gap using intcrfero- total current. The maximum mean stress is metry on forward scattered 23 Gil- microwaves. 5S kV/cm over a 2.7 cm gap. The peak current, Л constant gap closure by a dense (> 7 x 1012 after the diode has shorted is 12.Ь кЛ. The an"5) plasma is observed but the interfero- C\K current spike reaching 75 percent of the mctric data does not indicate any structure short circuit current at 20 ns into the associated with the current bursts. Microwave pulse is accompanied by a "notrh" in the emissiun from the diode plasma, however, shows voltage pulse and an instability going off a burst structure simultaneous with the current scale in the current density measurement. density bursts (Fig. 21). The microwave data Fig. 20 is a plot of jrjR vs. tr-g averaged are consistent with a model in which a sheath over 10 shots per point typically. The is created in the cathode plasnr, .11 A beam dashed curve is a fit to the data of is formed in the sheath which rtnets with the 15 Proskurovskii, et al. at higher JCR. Note plasma on its anode side to produce plasma that the points at lower JCR fall on the oscillation (turbulence1 ~ч! thus microwaves. curve while the points above 20 A/cm^ are Assuming that the sheat •;•••• ' in the den­

significantly above it indicating that the sity of the expanding cai„uje .ma becomes instability does not occur on the rising por­ too small to support the diode current den­ tion of the waveform. At smaller A-K gaps sity, the time at which instability occurs, the instability tends to occur at the peak trjR, should be proportional to JCR'^-S. To of the loaded generator waveform, a time understand this relationship, assume that at slightly earlier than the no-load peak (Fig. t = 0 a whisker on the cathode tip explodes. 17) but significantly later than the theore­ A cathode plasma forms when debris from the tical prediction. This Tesult combined with explosion is ionized by electrons emitted observation ;hat a small instability often from the cathode. Subsequently, additional occurs at the time of the small voltage drop material is vaporized from the cathode by 80 ns into the voltage pulse (Fig. 17,, leads the action of joule heating and ion bombard­ to the speculation that the diode, in general, ment. This material is fed into the cathode will remain stable as long as the voltage is plasma at a rate of R atoms per second. The increasing. A corollary '.rould be that a cathode plasma generated at the needle tip constant falling voltage eventually produces expands into the evacuated diode volume rath an unstable current regime. assumed spherical symmet-v at ^ plasma front speed v. As the pla, т. m!; its total mass increases as Rt .. : its volume increases The initial instability occurs before losure of the A-K gap by the expanding cathode plasma. even more rapidly, as it (vt)~. т,..,-, the In the event of Fig. 19, the initial instabi­ -^ lity occurs at trjR equal to 200 ns whereas gap mean plasma density, p, decreases as does the closure as measured by the fall of the gap volt­- maximum current density, j, which the plasma age to zero occurs at tciose equal to 420 ns. Assuming constant closure speed during the can stably conduct - pes (e is «\ . electron pulse, the original 2.7 cm gap has been reduced charge and s is the electron thermal speed to 1.3 cm at 200 ns. For the total data sample in the plasma). Simultaneously, the current 0.3 < tp-j/t , < 0.5, corresponding to less density being drawn from the plasma increases

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112 CURSENT DENSITY VS TIME FOR INSTABILITY ONSET

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TIMS (|D0 btftu)

Fig. 20. Current density vs. instability on­ Fig. 21. Microwave bursts correlated with set time curve from blade and needle total current bursts for a needle cathode data. shot.

due to closure of the A-K gap. A critical A/cm2 and 2 A/on2< JcRi.2 0 Von2. At in­ condition is reached when termediate values the data are ambiguous. Blade data were taken in the same apparatus with а 50-1ЛП thick, 10-cm-long stainless J = 1 steel foil blade with edges rounded to reduce (F^?) field enhancement. Voltage and current traces for a typical shot are shown in Fig. 22. Open shutter photographs showed a relatively uni­ Solving for t, the instability condition be- form density of emission sites along the blade. The peak mean stress is 120 kV/cm over a 1-cm comes ta . „ r . For future reference note gap. The instability in current density that if the plasma were to expand cylindri- (lower trace, Fig. 22) 250 ns into the pulse cally, as from a blade (line) source, the is not observed in either the voltage wave­ volume term would be TtJ!.(vt)2, S. being the form (Fig. 22a, upper trace) or total current blade length. Instability occurs at a time (Fig. 22b). Thus, neither voltage nor total t a 1/j. Finally for a parallel plate geo­ current may be relied upon to detect instabil­ metry where the only volume increase is due to ity from blade cathodes. linear expansion across the gap the volume

increases in proportion to time. The time Both tcR and the time to diode closure, tr factors cancel in the instability equation are significantly longer for a given A-K "-close > and instability at constant j is the predic­ gap and voltage тЛеп the cathode is a blade tion. compared to a needle. The relationship bet­ ween tro and JcR for blades is also shown in -0 4 1-3 It has been noted that tro a j ' fits the Fig. 20. The t^R a j™" behavior correlates 2 data on the intervals 10Z A/cm <: jcpi< 10 reasonably well with the model used to explain

183 (a]

fl>).

Fig. 22. Typical blade cathode shot, time base 100 ns/cm.

a. Diode voltage, 87 kV/an (upper trace). Current density 60 .Vcm2/cm (lower trace). b. Total current, 3 kA/cm.

184 the needle data. If the cathode plasma expan­ sion is cylindrical (as from a blade) Tather than spherical (needle) the theoretical rela­ tion is tq, a JcR1-0' 'which for the sophis­ tication of the'model is adequate agreement. В1Д0Е CATHODE IMPEDANCE MS TIME Nothing can be said based on the blade data concerning the affect of a rising voltage on stability because in the parameter range accessible, t^R always occurred on the flat portion of the LPN voltage waveform. Micro­ wave emission is very seldom observed in the \ blade experiments. It is never associated with the first instability and when seen occurs about the time the diode shorts.

As with the needle data, the initial instabil­ ity occurs well before gap closure. Fig. 23 shows the diode impedance for a blade shot 1IMF {|п4п»*/мт'| with a 2-cm gap and a maximum mean stress of 70 kV/cm. The dots are a rough fit to a Fig. 23, Impedance characteristic for a model based on the Child-Langmuir formula for blade shot (solid curve) compared space charge limited flow between coaxial with Child-Langmuir cylindrical cylinders. Two variable parameters were used, diode model calculations (dots). the overall normalization and the cathode plasma closure speed. It is not remarkable that a two parameter fit can match a rela­ tively linear curve, independent of the func­ tion being fit. However, the closure speed ft-K ClDSUni SPEED deduced from the fit is sufficiently close • NEEDLE. VOLTAGE ZERO •HADE. VDLIAGE ZERO to that computed from the diode voltage •11A0E. FIT collapse (as shown in Fig. 24) to lend credence to the model. It is interesting that the fit implies a constant closure speed across rhe entire А-К gap.

Data were also taken from a dome cathode (radius, 2 cm) at A-K spacings < 1 cm. This is a good approximation to a parallel plate geometry with enough field enhancement to insure that breakdown occurs on axis where it can be seen by the Faraday cup. Fig. 25 shows a typical shot; note the presence in the current density of bursts similar to those seen from blades and needles. Micro­ wave emission is not observed. The data are not, as yet, good enough to esta­ blish a relation between tCR and jrR. Note, however, that the model which has been success­ ful in reproducing the relationship between К CAP (еж) tCR and JCR for needles and blades predicts trR to be independent of JCR for a parallel Fig. 24. A-K closure speed vs. gap spacing plate geometiy. for blade and needle data.

185 Fig. 25. Typical dome cathode shot, time base 100 ns/cm. Voltage at 87 kV/cm (upper trace). Current density at 300 A/cm~/cm (lower trace).

Emission Enhancement Experiments nanoseconds. As the mean electric field w; increased to 45 kV/cm, the current turn-on The electron emission characteristics of three time decreased as shown in Figs. 28a and 2S materiils have been investigated as part of however, Fig. 26e indicates that the same the continuing study of low current density, discrete cathode plasma patterns were produ one microsecond diodes. The test cathodes Radiochromic dosimetry indicated that the were subjected to low electric fields spatial anode current density variations we (20 to Л4 kV/cm) while observing the emis­ ± 25 percent from the mean value. Uniformr sion quality and uniformity across the of the current density should be improved ii cathode surface. Diagnostic measurements the same electric field is applied to a larj included open shutter photographs of the A-K gap spacing. A 2.5-cm-diameter graphite cathode, radiochromic dosimetry at the felt cathode behaved in a similar manner wit anode, and diode voltage and current wave­ the exception of showing a higher impedance. forms. Л 10.2-cm-diameter cathode of solid graphite A 10.2-diameter cathode with a graphite-felt cross-hatched with 0.16-cm-wide and 0.16-cm- emitting surface was tested on the LPN deep grooves spaced 0.16 cm apart, was testec accelerator^ Д*> using the parameters in Fig. under the same conditions. With fields of 17. Graphite felt is a material of density 25 to 35 kV/cm, the electron emission was non packed graphite fibers i^ith the fiber ends reproducible with long current turn-on times position in a random direction. Mean elec­ or no turn-on at all. Above 35 kV/cm, the tric fields of 20 to 45 kV/cm were applied current turn-on time was reduced from hundred: between the cathode and anode. At 20 kV/cm, of nanoseconds to tens of nanoseconds. Open cathode plasma formed at discrete locations shutter photographs of the cathode showed a as shown by the open shutter photograph of relatively small number of emission spots Fig. 26a. These photographs are reflec­ which were randomly located (shot-to-shot) tions of the cathode from a metallic anode. across the cathode face. The current and voltage waveforms shown in Figs. 27a and 27b indicate that the current A small sample of metal-oxide matrix^ materia] lagged the voltage rise by a few tens of was tested on the LEGS accelerator (described

186 (a) a

W (c)

(d) Ce)

Fig. 26. Open shutter photographs of a graphite felt cathode with varying electric fields.

20 kV/cm. 25 kV/cm. 30 kV/cra. 35 kV/cm. 45 kV/cra.

187 The experimental setup and two photographs of the surface with magnification of 6,000 and 1,500 are shown in Fig. 29. As indicated by the photographs, the material consists of an array of tungsten needles less than 1 micron in diameter that protrude from the surrounding surface 10 to 15 microns. The needle density is approximately 4 x 10б cm"2 of sample surface area. A brass sample of the same macroscopic geometry was used to compare the emitting properties of the array. AD electric fields of 48 to 72 kV/cm, emis­ sion was not detected by a Faraday cup and radiochromic film dosimetry. With the elec­ tric field 114 kV/cm, the sample emitted Fig. 27. Diode waveforms for a low electric at 300 ns into the voltage pulse and the field [20 kV/cm) graphite felt diode had developed a short by 800 ns. The cathode. emission from the brass sample was similar to the tungsten array. Possibly, the high den­ Voltage at 500 ns per division. sity of the tungsten needles causes the array Current at 500 ns per division. to appear as a smooth surface thus reducing the field enhancement and making it comparable to a smooth brass surface with its macroscopic surface perturbations.

Emission Suppression Experiments

For efficient operation of electron beam diodes for laser excitation, methods must be developed to suppress electron emission that occurs from cathode support structures of low-current den­ sity electron beam diodes with a one micro­ second pulse duration. Initial experiments involving electron emission suppression using dielectric coated cathodes were performed on the LPN accelerator and reported in Ref. 1. Fig. 28. Diode waveforms for a high electric field (45 kV/cm) graphite felt Additional experiments have now been performed cathode. using the LEGS accelerator. Bare metal and dielectric coated cathodes were investigated Voltage at 500 ns per division. with the test setup shown in Fig. 30. All Current at 500 ns per division. cathodes had a 32 machine finish with the surface polished with 600 grit sandpaper and diamond polish (15-1 micron paste). Dielectric elsewhere ir. this report] using the parameters coatings were applied to this substrate by given in Table I. either aerosol spray or dipping. The thickness of the coating was not controlled; however, they were typically 50 microns thick. Instru­ TABLE I mentation was adjusted to detect current emissions of 0.1 amp/cm or greater being LEGS Pulser Parameters emitted by the 45 cm2 cathode.

Output Voltage 460 kV Tables II, III, and IV show the results of 10/90 Risetime 40 ns these tests. The electric fields shown in Pulse Width 1 us the tables are mean fields at which no detect­ Impedance 40 ohms 2 able emission occurred. It should be noted, Cathode Area 1000 cm however, that the fields are taken at the

Itt TABI£ II

Emission Suppression Resulf.i 1.15 cm A-K Spacing

Electric Field Cathode Type (kV/cm) Remarks

Unpolished brass > 63 lx>wcr machine limit Polished brass 94, 67, 72 Silicone oil-aluminum 119 Silicone oil-brass 125 Polished aluminum 156, 127 Nitrocellulose-aluminum 165 Polystyrene-aluminum 175 Acrylic #2-aluminum 194 Teflon-aluminum 203 Anodized-aluminum 211, 228, 197 Clyptal-aluminum 233 Epoxy-alimnnum 275 Acrylic Sl-anodized aluminum > 330 Upper machine limit Acrylic SI-aluminum > 330 Upper machine limit Acrylic fl-brass > 330 Upper niachine limit

TABLE III TABLE IV

Emission Suppression Results Emission Suppression Results 0.48 cm A-K Spacing 1.78 cm A-K Spacing

Electric Field Electric Field Cathode Type (kV/cm) Cathode Type СкУ/епр

Acrylic Sl-anodized-almiinum 206 Polished brass 66 Acrylic #l-brass 271 Polished aluminum 73 Acrylic SI-aluminum 349 Silicone oil-brass 147 Silicone oil-aluminum 162

IB* METAL-OXIDE MATRIX FIELD EMITTER

rf CMTEI IMIt ШИМЕ

Fig. 29. Metal-oxide matrix experimental cathode.

average applied voltage and does not reflect the higher voltage excursion at the beginning of the pulse (refer to Fig. 34]. The cathodes actually experienced 10 to 15 percent higher fields during the first 200 ns of the pulse. When emission occurred, it was observed during L the first 400 ns for the majority of the cathodes.

All dielectric coatings with the possible exception of silicone oil,'acted to suppress electron emission to some extent. Silicone oil, the pToduct employed by most researchers 7«CM CMHODt f***D»T С U to inhibit emission, gave the poorest results. It seemed to improve the brass cathodes but h its value on polished aluminum cathodes at 1 cm spacings is questionable until further tests are conducted. One coating, acrylic #1, was far superior to the others and could be used to repair damage and imperfections in other coatings without interfering with test results. Acrylic S2, of different manufacture, f could only be classified as average in its suppression properties.

Fig. 30. LEGS diode test setup. In vacuum breakdown, an А-К gap spacing depen­ dency is known but the extent of the depen­ dency is unknown. For this reason, it is difficult to correlate the results

IW between different spacings as shown in Tables TABLE VI II, III, and IV. Discrepancies between re­ sults at different spacings can also be attributed to sample preparation. Attempts Nominal Values for PFN Elements will be made to understand the somewhat ambiguous results that were obtained when 2nd Stage: Cll = CI2 = C13 = 2.8 nF the A-K spacings were changed. LI = 3.0 уН 3rd Stage: C21 = C22 = C23 = 2.8 nF LEGS, A Test Facility for Large Area, L2 = 0.7 uH Microsecond Diode Studies 4th Stage: C3] = C32 = 4.1 nF A design study for a microsecond e-beam L3 = 0.2 uH generator suitable for large area (~1000 cm2), low-current density (~l-20 A/cm2) diode investigations was reported in the In testing the generator, the values of LI, last semiannual report. The equivalent L2, L3, and Ш were adjusted to obtain the circuit for the pulser CLEGS) is shown in desired waveform. Substantial flexibility Fig. 31 and consists of a four-stage Type-A in the shape of the output waveform was Guilleman network (PFN). The triggered crow­ demonstrated by making changes in the values bar switch can be used to obtain diode pulse- of these elements. Fig. 34 shows the pulser widths shorter than the PFN pulsewidth; it output waveforms into a matched resistive also reduces the pulse fall time to ~ 60 ns. load (40 ohms). The 10/90 risetime is 40 ns The pulser was assembled and has undergone with a full width at half maximum (FWHM) of considerable testing. 1.2 us. .

The Marx generator, shown in Fig. 32, consists Small area cathode studies are underway. A of sixteen, ± 50-kV, 0.22-uF capacitors and large area diode chamber that will allow the eight low inductance, low jitter 100-kV study of beams up to ~ 1000 cm^ has been de­ spark gaps with a midplane trigger electrode. signed and ordered. Delivery is expected in The first two gaps are triggered by an exter­ early December. A Hibachi anode and gas nally applied pulse. The midplane electrode chamber has also been ordered. It will be in the remainder of the gaps are coupled to used for beam transport and energy deposition ground via 10-kfi resistors. The output para­ measurement. meters of the Marx generator are given in Table V.

TABLE V ^wmm_ -rmnw^ rioieieei . С1Э ,. «3,.

C M;.. "II Marx Generator (Xitput Parameters M - J. Cll ,. S 1 en I cti.. 1 C3i tl j 1

Capacitance: 13.8 nF p Inductance: 1.5 uH i u«w

The remaining stages of pulse-forming network (PFN) can be seen in Fig. 33. Also seen in this figure are the crowbar switch, the Marx generator and the diode vacuum feedthrough. Fig. 31. Equivalent circuit for LEGS pulser. The capacitance inductance values for these last three stages are given in Table VI.

191 Fig. 32. Photograph of Marx generator

Fig. 33. Photograph of PFN.

192 Fig. 34. Output voltage for LEGS pulser.

RADIAL ELECTRON BEAM LASER EXCITATION densitometer, a spatially resolved measurement of the energy deposited in the film was ob­ Total and Spatially Resolved Energy Deposition tained. Measurements The experimentally determined dose delivered Measurements of energy deposition by the radi­ to the radiochromic film was compared with ally converging e-beam from the REBLE accel- detailed Monte Carlo code calculations using eratorZD were conducted for nitrogen, argon, the CYLEM code.21»22 CYLEM is a derivative krypton, xenon and SFg gases at fill pressures of the CYLTRAN23 Monte Carlo code and allows ranging up to three atmospheresД6 The REBLE the inclusion of externally applied, static diode and typical diode current, voltage, electromagnetic fields. The applied e-fields power and impedance waveforms are shown in are restricted to void regions. Fig. 37 Fig. 35. The 20-cm-diameter, 30-cm-long shows the geometry used for the code calcu­ anode was a 0.005-cm-thick stainless steel lations. Only the radial electric field in foil. For these measurements the peak voltage the diode gap was used in the calculations. was nominally 750 kV. This allowed electrons which had random- walked out into the diode gap to be reflected Spatially resolved deposition measurements back into the material region. By sampling were made using the radiochromic film package the incident electron spectrum obtained from shown in Fig. 36. The package was ~ 1 cm the diode voltage and current waveforms, both wide and 30 cm long; it was placed at a given the initial energy of the primary electron radial position inside the gas chamber and and the radial electric field applicable ran the length of the volume. The black paper during that primary electron history were on either side of the film assured that the obtained. The incident electron axial dis­ deposition in the film was from the high tribution was measured and approximated by a energy electron beam. After placement of triangular source distribution symmetric about the film package, the chamber was evacuated, its midplane. All primary electrons were pressurized and irradiated by the incident started normally incident to the anode foil. radial e-beam. By scanning the film with a Because the code assumes a crylindrically

193 Transmission Lines ^V [ Voliayp Power Current ^v>> v i V ::: SI= -1* "^ Monitor \. f ^47 ^Cr^^ О •A о CO- 2 - и to en. 1 » 13 1 nsec 'iliv 13.1 nsec'rjt Anode ^SSTTP Cathod \ Current •=[ lm|)i>(Jancr> _^f^^V/ Г с

O-3**^- Cu.ro,,, Yonitnr iri -. . '- •- 13 1 asec rtiv 13 1 nsoe'div

Fig. 35. Radial diode and typical diode waveforms.

poiychlo-ostyreiii

Fig. 36. Radiochromic film package.

• Only E field in Diode Region • Incident Electron Spectrum — I. V waveforms • Diode Potential -» Incident Electron Energy • Axial Incident Electron Distribution • Mormat Electron Incidence • Symmetry about source midplane ^ В earn Diode Gap End-plate v RadiDchromic Package mm Source ( Midplane

Fig. 37. CYLEM code geometry employed for calculations. symmetric materials geometry, the radiochromic The total energy deposited in the gas as a package was modeled by a complete cylindrical function of pressure was determined by mea­ package. Since the measurements were con­ suring the pressure rise inside the chamber ducted using ~l-cm-wide strips, an experiment after exposure to the radial e-beam. These was conducted and verified that the dose mea­ measurements are compared to the Monte Carlo sured with the strips was not significantly results in Fig. 41 for nitrogen, xenon and different from that which would be obtained SFg gases. For the cases or argon and krypton using a complete cylindrical package. gas fills agreement similar to that shown for xenon was obtained. Figures 38a and 38b show a comparison of the experimental and calculated dose when the chamber was pressurized with xenon gas at Radial Diode Impedance Characteristics 1200 torr and 240П torr, respectively. The solid lines are the experimental dose and the Studies of various cathode materials and geo­ histograms are the Monte Carlo calculations. metries revealed that cathodes made of thin The code results were converted into joules/ annular metal foils exhibited the shortest gram by using the total incident beam energy turn-on times, gave more uniform current as obtained from the diode voltage and current emission, and had better shot-to-shot reproduci­ waveforms. The cross-hatched area indicates bility than any other cathode types investigated. the total error in the calculations including A study was made to determine the impedance both the statistical error and the uncertainty characteristics of the radial diode using in the determination of the total incident annular blades. beam energy. The disagreement between theory and experiment near the "wings" of the dis­ The diode impedance from a single annular tributions is primarily due to the slight blade cathode can be expressed as inaccuracy of the model used for the axial and angular distirbution of the incident beam electrons. Z(l-blade) = !±- (1) Quantitative agreement similar to that shown VIA in Figs. 38a and 38b was obtained for argon and krypton gas fills over the entire pres­ where X is the anode-cathode gap spacing in cm, sure range measured. Fig. 39 shows comparisons 2 for nitrogen gas fills of 400 and 600 torr. A is the area in cm covered by the beam at the In Fig. 40 comparisons are shown foT gas fills anode surface, and V is the diode voltage in MV.24 The area is 2 Roh where Ro is the of 400 and 1000 torr SF6. Although reasonable agreement was obtained at 400 torr of SFg, radius of the anode and h is the height of the calculated dose is about four times higher the beam at the anode surface. The height of than the measured dose at 1000 torr and R " the beam was experimentally determined to be 1.4 cm. This discrepancy in calculated and within 10 percent of twice the А-К gap. Sub­ measured dose delivered to the film at the stituting and rewriting R = 1.4 cm position occurs for SFg at all gas fill pressures above 600 torr. ZVb(l-Made) = 4Д- X . (2) Since agreement between experiment and cal­ culations was obtained for all gases and pressures except SFg, one can postulate that this discrepancy is due to the strong electro­ Impedance data obtained for a single blade negative nature of SF&. Electrons are attached cathode is shown in Fig. 42. The dots are leaving low mobility negative ions with an the experimental points and the solid line associated space charge buildup which prevents was obtained using equation 2 with К = 675. the beam from penetrating to the center of the This constant gave an accurate representation chamber. Although other mechanisms cannot be of the data for both a 10 cm and 5 cm radius ruled out, this scenario described above anode. It was also determined that, when no seems to be a reasonable explanation for the significant overlap existed between the disagreement between experiment and the Monte individual beamlets, the impedance of an Carlo calculations since collective effects, N-blade cathode was given by Z(l-blade)/N. such as space charge buildup, cannot be When significant overlap between the indivi­ included in the calculations. dual beamlets existed, the impedance of the

I9S 1200 Torr R = 9.7cm Xenon _е5я_ 241-

(a)

2400 Torr Xenon R = 9.7cm

-12 + 12 Z Axis (cm)

24 R = 1.4 cm

'16

3 •a s

-12 +12 Z Axis (cm)

0>)

Fig. 38. Comparison of experimental and calcu­ lated dose for xenon gas fills of 1200 torr and 2400 torr.

1*6 400 Torr Nj Г4: Е 24- •л 16

б "Vl'2 12 0 •!? Z А к к Ion)

| гв!

*ftIf f ^

-17 0 *12 -1? О 42 ZAxislcml ZAxislLml

Fig. 39. Comparison of experimental and calcu­ lated dose for nitrogen gas fills of 400 torr and 1600 torr.

400 Torr SF „, ri0O0T„.rSF 21 6 R!97c B I.B.

Fig. 40. Comparison of experimental and calcu­

lated dose for SF6 gas fills of 400 torr and 1000 torr.

197 400 800 1 ?001 GOO ?l)0(J Pr-vu to iTorr)

Fig. 41. Percentage of total incident beam energy deposited in gas as a function of fill pressure.

X(cm)

Fig. 42. Impedance of a single annular blade cathode of radius Rg + X.

198 Fig. 43. Hibachi support structure.

multiblade cathode was not easily calculable of the structure was 78 percent. Measurements but was always larger than that obtained with of the beam energy transported through this no overlap of the beamlets. anode structure were made using a 0.005 cm stainless steel liner in order to compare with measurements using a self-supporting Hibachi - Anode Investigations anode of similar construction to the liner. In the e-beam excited lasers Che maximum Measurements using the Hibachi anode indicate working pressure of the gas chamber is deter­ that the holes in the support structure had mined by the strength of the thin-foil anode. little or no effect on the impedance charac­ For maximum energy transfer through the anode teristics of the radial diode for A-K gap foil a high strength, low-Z metal foil is spacings of 2.5 cm or larger, i.e., the desirable, e.g., a high strength titanium diode impedance behavior was essentially the alloy. In order to extend the working pres­ same as that obtained with a solid anode of sure beyond the capabilities of a self- diameter equal to that of the support struc­ supporting foil, a stiff support structure ture. (Hibachi) may be used. Here some transport efficiency is lost due to absorption and The beam transport efficiency through the scattering of incident electrons by the ribs Hibachi anode was obtained by measuring the of the support structure; furthermore this maximum energy delivered to SF5 gas as a lost efficiency is not necessarily given by percentage of the total incident beam energy; the otpical transparency of the structure. this is compared to the equivalent measure­ ments using a self-supporting foil anode. The Hibachi-type anode shown in Fig. 43 was Measurements were conducted for various A-K designed for the REBLE facility to investi­ gap spacings and for support structures with gate the effect of the structure on the diode wall thicknesses of 0.32 cm and 0.64 cm. operating characteristics and to measure the Results are summarized in Table VII. transparency of the structure to the radial e-beam for various anode-cathode configura­ tions. The calculated optical transparency

199 TABLE VII

Beam Energy Transport Through Hibachi Structure With O.OOS cm Thick Stainless Steel Foil

A-K Gap Wail Thickness Maximum Percentage of Total Incident (cm) (cm) Energy Deposited in SFfi Gas 1.59 0.655 28 2.54 0.635 35 3.81 0.635 3G 2.54 0.318 47 3.81 0.318 57

These results are to be compared with a self- the pellet explosion to expand within the supporting 0.005-cm-thick stainless steel foil generator coil. In a charged particle beam which gives a maximum of - 70 percent of the reactor the burst-heated atmosphere within total incident beam deposited in the SFg gas. the pellet chamber could be allowed to drive a metallic fixed-radius piston axially throug As was expected, the structure with the thin­ the coil. In either case, electric energy is ner wall transmits more of the beam energy. generated with relatively high conversion Another feature evident from this data is efficiency compared to that achieved with that more of the beam energy is transmitted steam-turbine generators. at the larger gap spacings. This is related to the effect of the gap spacing on the elec­ Energy partition for the pellet explosion tron trajectories as they approach the would determine the fraction of plant power apertures. It can be seen from the last two produced by this direct and inexpensive means rows in this table that under these condi­ If enough energy could be converted to drive tions the structure approaches the calculated the Pulsar generator, the reactor could pro­ optical transparency. vide its own operational pulse power plus an additional amount for commercial use. A The Hibachi anode with 0.32- wall thickness relatively small steam-turbine "bottoming and O.OOS-cm stainless steel foil was pres­ stage" could then provide the remainder of sure tested and determined to be capable of the commercial power generated. operating at pressures up to five atmospheres. The self-supporting foil of equivalent dimen­ An experimental program has been conducted sions failed consistently at ~ 3 atmospheres utilizing a 200-kJ superconducting magnet gas pressure. with a 0.45-m bore. Experiments carried out to date with metallic pistons or plasma as the active elements (armatures) have confirmed AN INDUCTIVE GENERATOR TOR FUSION POWER expectations. A one-meter diameter 400 kJ GENERATION superconducting magnet is being prepared to test scaling predictions Zor larger si2es. Introduction During this period work was carried out to An inductive generator called Pulsar is being develop explosively generated plasma arma­ developed for possible future application as tures suitable for use in both the 0.4S-m and a prime power source of "topping stage" for 1-m magnets and preparations were made for pellet fusion reactors. This generator employs the refrigeration system which is needed for a direct current superconducting magnet to the 1-m magnet. Progress was made on an ШВ provide magnetic flux for a generator coil. code to simulate operation with plasma a~.a- In a laser-driven reactor this could be tures. accomplished by allowing plasma produced by

2M Plasma Armature Code Development For more accurate estimates of the effect of scale on performance of plasma armatures a To date, the computer code used to model per­ one-dimensional magnetohydrodynamic (MiD) formance with expanding metallic armatures has code is being developed. The code is based also provided the only means to estimate per­ on the CHART-D radiation-hydrodynamics code formance with plasma armatures (МГО code). with magnetic fields included self-consistently. This is a lumped parameter circuit model where The basic geometry includes a massive cold pis­ the armature is segmented into n elements to ton (high explosive detonation products) which simulate diffusion and proximity effects in expands at high velocity into the gas which the induced current distribution. By inputting can be either preionized or neutral. A shock measured plasma dimensions and velocities from forms in front of the piston and compresses actual experimental shots into the code and the magnetic field against a perfectly con­ changing the plasma conductivity until the ducting generator coil. The radiative diffu- computed load current agrees with the experi­ sivity will be turned off for the first runs mental measurement, the average plasma tem­ but will be added later. Other features to be perature achieved during an experiment can added as the code develops are: magnetic be estimated. By this means it has been diffusion into the metallic walls, additional established that experimental plasma armatures circuits such as the load and superconducting produced with high explosive have had tempera­ magnetic, and explosive debris simulation. tures up to 4 eV. Fig. 44, constructed from The code will eventually model output waveforms this lumped parameter model, compares Pulsar so that direct comparison with actual experi­ generator performance with lossless, metallic, ments may be carried out. Test cases for and 4-eV plasma armatures. It shows how the magnetic compression and diffusion which can ratio of energy in the pulse to energy stored be compared to analytical solutions should be in the superconducting magnet (Е/Ед^) depends completed in January. on the radius of the generator coil. In view of the assumptions implicit in the application of this code to plasma armatures the plasma The 1-m Magnet curve must be considered to be a crude esti­ mate. Still, with fusion reactor scale of a The next phase of the Pulsar study involves few meters and with plasma produced by a an increase in scale of the experimental nuclear burst one could expect that pulse generator. The original superconducting energy would exceed stored magnet energy, magnet with a warm bore of 0.45 m and 200 kj and that energy conversion efficiency (Jcinetic- storage is shown on the right in Fig. 45. On to-electric) would be very high (~ 1). the left is the superconducting winding for the next magnet. This new magnet will store only twice the energy but will have a warm bore only slightly less than 1 m. The dewar for this magnet is nearing completion. Experi­ ments with the larger magnet mil allow experimental verification of the effect of •A scale on Pulsar performance.

SYSTEMS STUDIES Overview The block diagram of a systems model that simulates the technical and economic per­ formance of .the conceptual reactor design is shown in Fig. 46. A computer optimization routine is utilized to select the optimal reactor configuration based on cost of the product. Both hybrid reactors and pure fusion reactors have been investigated and will be discussed here. Fig. 44. Scaled Pulsar output.

201 Fig. 45. The O.S and 1.0 meter magnets.

Fig. 46. Power generation characteristics for a REB fusion reactor.

202 The basis for the model is a power balance TABLE VIII analysis which treats the power production loop on a per pulse basis. The loop comprises REB Fusion Reactor - Baseline System the energy delivery system (Marx generator, transmission lines, and diode), the fuel pellet, the energy conversion blanket, and System the electrical power generating system. Characteristics Parametric curves for the efficiencies and capital costs for each of these subsystems Pellet Gain 100 are assumed and used to evaluate the cost Beam Energy 1.4 MJ of generating electricity with the system. Driver Efficiency 0.58 Focusing Efficiency 0.7 Blanket Gain 1.3 Pellet Cost •• $0.40 Parametric Analysis of a REB Fusion Reactor Circulating Power Fraction 9* Chamber Radius 5.3 m The results obtained for the fusion baseline Number of Vessels 5 system are shown in Table VIII. One megajoule Plant Capacity 1020 MWe of energy is assumed to be absorbed by the Pulse Rate per Vessel 4.5 Hz pellet, resulting in a 100-MJ thermonuclear ШС (1976$) 55 mills/kWhr yield. Five vessels are combined to arrive

at a total capacity of 1020 MWe. The modu­

larity provided by the five 200 MWe vessels appears to be a desirable characteristic. The sensitivity of BBEC to the pellet fabrica­ The system reliability should be enhanced tion cost is illustrated in Fig. 49 for several by the redundancy, and reactor development is pellet thermonuclear yields. The energy absorb­ possible in smaller steps that entail less ed by the pellet is fixed at 1 MJ and the risk. The pulse rate per vessel is less than pellet yield varies from 50 to 300 MJ. For 5 Hz, which is a desirable operating regime pellet fabrication costs of less than $0.10 per for the REB accelerator system. Pellet fabri­ pellet, all of the systems are economically cation costs are unknown so that a relatively viable. As the pellet yield is increased, high cost was assumed. We do not believe proportionally higher pellet fabrication costs that $0.01 pellets are likely although this are acceptable. In Fig. 50, the allowable area must be investigated further. The pellet fabrication cost is calculated as a resulting power generation costs are approxi­ function of pellet yield for the BBEC fixed mately twice that of current light water at 50 mills/kWhr. At a yield of 300 MJ, over reactor costs and are comparable to costs $1 per pellet is acceptable. A fixed pellet associated with alternate future energy gain of 100 is assumed for the second curve in sources. Fig. 50. The energy absorbed by the pellet is varied to change the pellet yield. The two systems are equivalent for,a pellet yield of Using the baseline fusion reactor as an 100. Pellet yield is the dominant factor in operating point, the sensitivity of the determining allowable pellet costs. As the levelized busbar energy cost (BBEC) to varia­ pellet yield increases, the requirements for tions in component cost and performance are other reactor parameters become less strin­ tested. In Fig. 47, BBEC is plotted as a gent. Pellet gain is a secondary effect pri­ function of the driver cost in $/J. Power marily because the high efficiency of the generation costs increase 25 percent when REB driver minimizes the effect that circulating driver costs are increased from 0 to $100/J. power fraction has on BBEC. Driver costs are currently estimated at $20/J. In this cost range, driver costs cannot appear to be a dominant factor in determ;ling plant Based upon the estimates presented, a pure economic viability. The BBEC is plotted as fusion REB driven reactor appears to be rea­ a function of the driver efficiency in Fig. sonable with the base case parameters. The 48. The curve is relatively flat above 30 driver costs if less than $100/J do not appear percent. Although the driver efficiency is limiting. The driver efficiencies should be arbitrarily taken to be 58 percent, there above 20 to 30 percent for the system to be is little penalty in choosing an efficiency economically viable with a 100-MJ pellet of 30 percent. If the driver efficiency is yield. Lower pellet yields and pellet gains less than 20 percent, the pellet yield must are acceptable if the pellet fabrication be higher than the 100 MJ yield proposed here. costs are low (< $0.10). The proposed pellet

MJ 25 50 75 100

DRIVER COST (»/J>

Fig. 47. Sensitivity of BBEC to driver cost - baseline fusion system.

20 DRIVER EFFICIENCY

Fig. 48. Sensitivity of BBEC to the driver effi­ ciency - fusion baseline system.

204 О ,10 .20 ,30 ,40 ,50 ,60 .70 .90 1,00 PELLET FABRICATION COST (1976$)

Fig. 49. Sensitivity of BBEC to pellet fabrication cost baseline fusion system.

~ 1.00

щ

100 . 200 300 PELLET YIELD (HJ)

Fig. SO. Sensitivity of allowable pellet cost to pellet yield - fusion baseline system.

MS costs of $0.40 per pellet is an upper limit driver efficiency has on power generation cost on acceptable costs, Modular systems, such is through the circulating power fraction. Tt as this one have attractive features but need circulating power fraction resulting from more study. The penalties associated with charging the Marx generator is low enough for this type of relatively low yield modular the base case (~ 5.5 percent) that doubling system are relatively unknown compared to that fraction does not have serious conse­ the cited advantages, such as higher reliabi­ quences. Pellet gains above IS are required lity and easier system development. for both systems and gains between 20 and 30 are desirable. Pellet gains above 30 do not significantly improve the system's economic Parametric Analysis of a REB Hybrid Reactor performance.

The pellet chosen for the baseline hybrid Pellet fabrication costs have a significant reactor system has a gain of 20 at 3 Ш of impact on the BBEC for the baseline hybrid absorbed energy. The driver efficiency is system, Fig. 52. Pellet costs can be as high assumed to be 58 percent with 70 percent as $1.00/pellet if one assumes 50 mills/kWhr of the beam energy effectively coupled to as a maximum acceptable power generation cost. the pellet to drive the implosion. A fast If the pellet fabrication costs are $0.10 each fissioning blanket that provides an energy the BBEC is cut in half to 25 mills/kWhr. multiplication of 7 and produces 1.4 fissile Reasonable estimates for pellet costs are atoms per fusion is used. The selling price currently unavailable so that each analyst of the fissile fuel is assumed to be $30 must rely on his own intuition. Pellet fabri­ per gram fissile. The chamber radius, which cation costs have a significant impact on the is 4.6 meters, is determined by the first choice of reactor design. If costs are negli­ wall X-ray loading. If the reactor vessel gible, an important constraint on high repe­ is at atmospheric pressure, the blast effects tition rate systems is removed. However, if size the vessel at approximately 4 meters. the pellet costs are not negligible compared The power plant consists of C-TV- reactor to the value of the power generated, these vessel and produces 990 Ше. The vessel systems are unacceptable. This area will be pulse rate is 7.7 Hz which is within the difficult to determine accurately because bounds of projected pulsed power technology. of the lack of experience in pellet mass pro­ duction and rapidly changing pellet design. The system operates with a circulating power The involvement of knowledgeable people in fraction of less than 10 percent of which the cost estimation is necessary even to make 4 percent is for auxiliary power requirements order magnitude estimates of pellet cost. and the remaining 6 percent is used to charge the Marx generators. Over 2000 Kg of pluto- A second hybrid system, described in Table nium are produced each year which account IX, was investigated to determine the effects for 25 percent of the system revenues. The of a very low gain pellet on reactor economics other 75 percent of the revenue is from The system uses a gain 3 pellet that requires selling power at a BBEC of 33 mills/kWhr. 1 MJ of absorbed energy to initiate the implo­ sion. A thermal fission blanket with a gain The efficiency requirements for the baseline of 25 is employed to compensate for the low hybrid reactor are similar to those of the pellet gain and yield. The fissile fuel baseline fusion system. Driver efficiency production of the thermal fissioning blanket increases above 30 percent have almost no is approximately 1 fissile atom/fusion, which effect on power generation costs while if is half that of the previously used fast the efficiency drops below 10 percent, the fissioning blanket. The pellet cost is chosen power generation costs quickly become un- to be $0.10. The plant consists of 4 vessels acceptably high. 1.4 meters in radius that combine for a total

plant capacity of 880 MWe. 400 kG of pluto- In Fig. 51, the relationship between BBEC nium are produced each year and the levelized and pellet gain are shown for two driver busbar energy cost is 46 mills/kWhr. and beam/pellet focusing efficiencies. Case 1 is the base case assumption where 41 per­ The sensitivity of the BBEC to pellet fabrica­ cent of the energy into the Marx generator tion costs is shown in Fig. 53 for several is effectively coupled into the pellet; in blanket designs. When the pellet costs are Case 2 that efficiency is 20 percent. The low (~ $0.01/pellet), the differences between power generation costs for the two systems the systems are minimized. The system with a are very similar. The main effect that gain 10 fast fissioning blanket produces power

206 1UU 11 l 1 1—

11 PELLET COST = $.50

11 CASE I - i) - .58, 80 D - \\ "F* 7 3 \\ CASE И- ч - .4 Я в 60 • 1

- 40 ^^^ CASE 11

CASE 1 -== i 20 -

n 10 20 30 40 50 PELLET GAIN

Fig. 51. Sensitivity of BBS? to pellet gain baseline hybrid system.

• 50 1.00 1,50 2.00 PELLET COST ($1976)

Fig. 52. Sensitivity of BBEC to pellet cost - baseline hybrid system.

207 TABLE IX

HYBRID SYSTEM - MAGNETIC PELLET

Systems Inputs

Pellet Gain 3 Beam Energy 1.4 MJ Driver Efficiency 0.58 Focusing Efficiency 0.7 Blanket Gain 25 Pellet Cost $0.10 Fissile Fuel Selling Price $30/GM

System Characteristics

Circulating Power Fraction 144 Chamber Radius 1.4 M Number of Vessels 4

Plant Capacity 800 MWe Pulse Rate per Vessel 10.3 Hz Fissile Fuel Production Rate 400 kg/yr BBEC (1976$) 46 mills/kWhr

.10 .20 .30 PELLET FABRICATION COST (1976»)

Fig. 53. Sensitivity of BBEC to pellet fabrication cost - magnetic pellet. that is only a few mills per kWhr more expen­ sive than the gain 25 blanket and both appear to be economically acceptable. As pellet cost increase, the low gain blanket systems quickly become unacceptable.

Conclusions Efforts are underway to construct the accel­ erators needed to achieve scientific break­ even from a particle-beam-driven, inertially- confined, fusion reaction. In the near term (by 1985), it is expected that net energy gain will be achieved from such a reaction if several physics questions are resolved favorably. The difficulty of achieving very high gain pellet physics is even more pro­ blematical and we have therefore concentrated our attention on gains less than 100. High efficiency, low cost, particle-beam drivers can be effectively utilized with these rela­ tively low gain pellets and hybrid blankets to construct an experimental or demonstration power reactor. Successful construction of such a device would provide impetus to the commercialization of fusion technology. A system having a driver efficiency of 20 per­ cent or higher, a driver cost of $40/J and a pellet cost of $1 each would be attractive for use with gain 20 pellets. If pellets can be manufactured in quantity for less than a few cents each, the high efficiency of the REB driver makes the system using a gain 5 Pellet economically feasible (as com­ pared with other long range energy solutions). The pure fusion systems appear economically acceptable when a few megajoules of energy can become coupled to a pellet to yield 100 to 200 MI of energy. This, too, appears as a reasonable extension of present technology given positive physics results. In the future, the decision of whether to build hybrid or fusion reactors will probably depend on the relative demand for energy or fissile fuel, rather than being determined by technical constraints. REFERENCES:

1. Electron Beam Fusion Progress Report - October 1976 - March 1977, SAND77-1414, p. 143, Sandia Laboratories, Albuquerque, NM (October 1977].

2. G. J. Rohwein, M. T. Buttram, and K. R. Prestwich, "Design and Development of a 350-kV, 100-pps Electron Beam Accelerator," Proceedings of Second International Topical Conf. on High Power Electron and Ion Beam Research, Cornell University, Ithaca, NY, October 3-5, 1977.

3. T. H. Martin, SC-RR-71-0341 (1971).

4. E. A. Abramyan, IEEE Trans. Nucl. Sci., NS-18 (1971).

5. D. Finkelstein, P. Goldberg, and J. Shuchatuwitz, Rev. Sci. Instr., 37_, N'. 2 (1966).

6. c. R. J. Hoffman, Rev. Sci. Instr., 46, No. 1 (197S).

7. C. Edwards, M. D. Hutchinson, J. С Martin, and T. H. Storr, AWRE Report SSWA/JCM/755/99, unpublished (1975).

8. J. C. Bowers and S. R. Sedore, SCEPTRE, A Computer Program for Circuit and Systems Analysis (Prentice Hall, Inc., Englewood Cliffs, NY, 1971).

9. G. K. Loda and D. A. Mesban, "Repetitively Pulsed Electron Beam Generator," Proc. of the First International Topical Conf. on Electron-Beam Research and Technology, SAND76-822, Vol. 2, p. 822 (Nov. 19?S).

10. S. Singer, J. S. Ladish, and M. H. Nutter, same as above, p. 274.

11. A. A. Plyutto, K. V. Suladze, E. D. Korop, and V. N. Ryshkov, "Discharges and Electrical Insulation in Vacuum," Proceedings of the Fifth International Symposium, Poznan, Poland, p. 145 (1972).

12. E. D. Korop and A. A. Plyutto, Sov. Phys.-Tech. Phys., 15, 1986 (1971).

13. E. D. Korop and A. A. Plyutto, Sov. Phys.-Tech. Phys., 830 (1971).

14. A. A. Plyutto, et al., Sov. Phys.-Tech. Phys., 2£, 1578 (1975).

15. D. I. Proskurovskii, V. P. Rotshtein, A. F. Shubin, and E. B. Yankelevich, Sov. Phys. 20, 1342 (1976).

16. J. J. Ramirez, K. R. Prestwich, R. S. Clark, and M. T. Buttram, "E-Beams for Laser Excitation," Proceedings of the 2nd International Topical Conf. on High Power Electron and Ion Beam Research and Technology (1977).

17. M. T. Buttram and R. S. Clark, Bull. Am. Phys. Soc, 5A13 (October 1977).

18. C. W. Mendel, Jr., and M. T. Buttram, Bull. Am. Phys. Soc, SA3 (October 1977).

19. R. K. Feeney, A. T. Chapman, and B. A. Keener, "High-Field Electron Emission from Oxide-Metal Composite Materials," J. Appl. Phys., 46_, 1841 (1975).

20. J. J. Ramirez and K. R. Prestwich, Proc. of the IEEE Int'l. Pulsed Power Conf., Lubbock, Texas, November 9-11, 1976.

21. J. A. Halbleib, Sr., and W. H. VanDevender, J. Appl. Phys., 48, 2312 (1977).

22. J. J. Ramirez and J. A. Halbleib, Sr., Bull. Am. Phys. Soc., 22, 1133 (1977).

210 23. J. A. Halbleib, ST., and W. H. VanDevender, Nucl. Sci. Eng., 61, 288 fl976). 24. T. H. ••lartin and L. P. Bradley, Bull. Am. Phys. Soc., 16, 1229 (1971).

211-212 PUBLICATIONS

PRELIMINARY DESIGN OF A 100 HZ, 350 KV SHORT PULSE GENERATOR, G. J. Rohwein and M. T. Buttram, SAND77-0174 (June 1977).

THEORETICAL CONSIDERATIONS OF MAGNETIC CUSPS FOR ELECTRON-BEAM FUSION SYSTEMS, T. P. Wright, SAND77-0S93 (April 1977).

NEUTRON DIAGNOSTIC SYSTEM FOR PARTICLE BEAM FUSION EXPERIMENTS, J. J. Ramirez, L. W. Kruse and R. J. Leeper, SAND77-0P16 (June 1977).

NEUTRON PRODUCTION #R0M ADVANCED REB FUSION TARGETS, J. Chang, R. J. Leeper, A. Farnsworth, M. M. Widner and T. S. Prevender, SAND77-0345.

SOME TIME DEPENDENT EFFECTS IN REB DIODES, J. P. Quintenz and J. W. Poukey, SAND77-0889 (June 1977).

DESIGN OF MAGNETICALLY INSULATED PULSED ION DIODES FOR SPHERICALLY FOCUSED BEAMS, S. Humphries, Jr., SAND77-1322 (August 1977).

IMPROVED FOCUSING OF ELECTRON BEAMS USING A DOUBLE-CONE DIODE, J. W. Poukey, J. P. Quintenz and C. W. №ndel, RS 5241/178, Internal Memorandum (May 1977).

PLASMA IMPLOSION EXPERIMENT ON PROTO II AND RIPPLE, J. P. VanDevender, RS 5245/022, Internal Memorandum (May 1977),

A MECHANISM FOR ELECTRON EMISSION FROM CATHODE PLASMAS IN Rf.LATIVISTlC DIODES, K. D. Bergeron, RS 5241/180, Internal Memorandum (May 1977). .

PERFORMANCE OF THE CODE 'LAXNEX' ON TWO HYDRODYNAMIC TEST PROBLEMS, J. R. Stark, RS 5241/186, Internal Memorandum (July 1977).

TRIANGULAR MESH POISSON SOLVER, G. R. Hadley, RS 5241/187, Internal Memorandum (July 1977).

THE RADIATIVE OPACITY OF AIR: COMPARISON OF SELECTED RUSSIAN AND AMERICAN WORK, L. Baker, RS S241/202, Internal Memorandum (September 1977).

CALCULATION OF NONCYLINDRICALLY SYMMETRIC SPACE-CHARGE FLOW, J. P. Quintenz and J. W. Poukey, Physics of Fluids, 2_0, 5, 862 (May 1977).

ONE- AND TWO-SPECIES EQUTIIBRIA FOR MAGNETIC INSULATION IN COAXIAL GEOMETRY, K. D. Bergeron, Physics of Fluids, 20, 4 (April 1977).

SUMMARY OF ELECTRON-BEAM FUSION EXPERIMENTS, G. Yonas, Nuclear Fusion, 16, 5 (1976).

THEORY OF THE SECONDARY ELECTRON AVALANCHE AT ELECTRICALLY STRESSED INSULATOR-VACUUM INTERFACES, K. D. Bergeron, J. Appl. Phys., 48_, 7, 3073 (July 1977).

ION CURRENT REDUCTION IN PINCHED ELECTRON BEAM DIODES, J. P. Quintenz and J. W. Poukey, J. Appl. Phys., 48, 6, 2287-2293 (June 1977).

213 EQUIVALENT CIRCUIT APPROACH TO LONG MAGNETICALLY INSULATED TRANSMISSION I.TNF.S, K. D. Bergeron, J. Appl. Phys., 48, 7, 306S (July 1977).

214 PRESENTATIONS

PINHOLE IMAGING TECHNIQUES FOR HARD X-RAYS, A. J. Toepfer, L. P. Mix and H. J. Trussell, Seminar on X-ray Imaging, Reston, Virginia, April 18-19, 1977. ELECTRON BEAM TECHNOLOGY AND ELECTRON-BEAM TARGET EXPERIMENTAL RESULTS, A. J. Toepfer, Controlled Nuclear Fusion Special Session 2.6, Inertial Confinement Fusion Concepts, ANS Meeting, New York, New York, June 1977. MHD EFFECTS IN ELECTRON BEAM FUSION TARGETS, J. R. Freeman, L. Baker and S. L. Thompson, The Third International (Kiev) Conf. on Plasma Theory, Miramare-Trieste, Italy, April 5-9, 1977. RELATIVISTIC ELECTRON MOTION IN CUSPED MAGNETIC FIELDS, T. P. Wright, 1977 Spring Meeting of the American Physical Society, Washington, D.C., April 25-28, 1977. TIME-DEPENDENT EFFECTS IN REB DIODES, J. P. Quintenz and J. W. Poukey, IEEE Conf. on Plasma Physics, Troy, New York, May 23-25, 1977. THEORY OF SHORT PULSES IN LONG MAGNETICALLY INSULATED TRANSMISSION LINES, K. D. Bergeron, J. W. Poukey and J. P. VanDevender, IEEE Conf. on Plasma Physics, Troy, New York, May 23-25, 1977. MIRROR FIELD EFFECTS IN RELATIVISTIC DIODES, J. W. Poukey, J. R. Freeman and J. P. Quintenz, Fourteenth Symposium on Electron, Ion, and Photon Beam Technology, Palo Alto, California, May 25-27, 1977. FIELD ENHANCED REB DEPOSITION AND BREMSSTRAHLUNG PRODUCTION, J. A. HalMeib, Sr. .and M. M. Widner, 1977 IEEE Annual Conf. on Nuclear and Space Radiation Effects, Williamsburg, Virginia, July 12-15, 1977. HYDRODYNAMIC INSTABILITIES IN INERTIAL CONFINEMENT FUSION, J. R. Freeman, Washington Meeting of the American Physical Society, Washington, DC, April 25-28, 1977. APPLICATIONS OF COMPUTATIONAL PLASMA PHYSICS IN PARTICLE BEAM FUSION, J. R. Freeman, Lecture at the College in Theoretical and Computational Physics, Miramare-Trieste, Italy, April 5-9, 1977. MICROCHANNEL PLATE RESPONSE TO HARD X-RAYS, K. W. Dolan and J. T. Chang, SPIE/SPSE Technical Symposium East, Reston, Virginia, April 18-21, 1977. CODED IMAGING OF REB TARGETS, J. N. Olsen, SPIE Seminar on X-ray Imaging, Reston, Virginia, April 20-21, 1977. HIGH-INTENSITY FOCUSED REB DISCHARGES INTO FLAT GOLD TARGETS, E. J. T. Burns, S. A. Goldstein and J. N. Olsen, The N.C. Christophilos International Summer School and Con. in Plasma Physics, Spetses, Greece, July 20-30, 1977. EFFECTS OF TURBULENCE ON THE RAYLEIGH-TAYLOR INSTABILITY, M. J. Clauser and L. Baker, 11th European Conf. on Laser Interaction with Matter, Oxford, England, September 19-23, 1977. FIELD ENHANCED REB DEPOSITION AND BREMSSTRAHLUNG PRODUCTION, J. A. Halbleib and M. M. Kidner, 1977 IEEE Conf. on Nuclear and Space Radiation Effects, Williamsburg, Virginia, July 12-15,

215 RECENT RESULTS IN SANDIA'S PARTICLE BEAM FUSION PROGRAM, G. Yonas, N.C. Christophilos Interna­ tional Simmer School and Conf. in Plasma Physics, Island of Spetses, Greece, July 20-30, 1977.

ELECTRON BEAM FUSION TARGET EXPERIMENT, M. J. Clauser, S. A. Goldstein, R. J. Leeper, C. W. Mendel, L. P. Mix, J. N. Olsen, F. C. Perry and A. J. Toepfer, 8th European Conf. on Controlled Fusion and Plasma Physics, Prague, Czechoslovakia, September 19-23, 1977.

SHOCK INITIATED INSTABILITIES IN ICF TARGETS, J. R. Freeman, Gordon Research Conf. on Laser Interaction with Matter, Tilton, New Hampshire, August 8-12, 1977.

DISCUSSION OF DIAGNOSTICS TECHNIQUES IN PARTICLE BEAM FUSION, G. W. Kuswa, 2nd Conf. on Hot Plasma Diagnostics, Kharkov State University, Kharkov, USSR, September 22-27, 1977.

SANDIA'S RECENT RESULTS IN PARTICLE BEAM RESEARCH, G. Yonas, 8th European Conf. on Controlled Fusion and Plasma Physics, Prague, Czechoslovakia, September 19-23, 1977.

216