Operational Characteristics of the Stabilized Toroidal Pinch Machine, Perhapsatron S-4

Home , Perhapsatron, Pinch (plasma physics)

P/2488 USA

By J. P. Conner, D. C. H age r m an, J. L. Honsaker, H. J. Karr, J. P. Mize, J. E. Osher, J. A. Phillips and E. J. Stovall Jr.

Several investigators1"6 have reported initial success largely inductance-limited and not resistance-limited in stabilizing a pinched discharge through the utiliza- as observed in PS-3. After gas breakdown about 80% tion of an axial Bz magnetic field and conducting of the condenser voltage appears around the secondary, walls, and theoretical work,7"11 with simplifying in agreement with the ratio of source and load induct- assumptions, predicts stabilization under these con- ances. The rate of increase of gas current is at first ditions. At Los Alamos this approach has been large, ~1.3xlOn amp/sec, until the gas current examined in linear (Columbus) and toroidal (Per- contracts to cause an increase in inductance, at which hapsatron) geometries. time the gas current is a good approximation to a sine Perhapsatron S-3 (PS-3), described elsewhere,4 was curve. The gas current maximum is found to rise found to be resistance-limited in that the discharge linearly with primary voltage (Fig. 3), deviating as current did not increase significantly for primary expected at the higher voltages because of saturation vçltages over 12 kv (120 volts/cm). The minor inside of the iron core. diameter of this machine was small, 5.3 cm, and the At the discharge current maximum, the secondary onset of impurity light from wall material in the voltage is not zero, and if one assumes that there is discharge occurred early in the gas current cycle. It no large change of inductance, the total resistance of was tentatively concluded that in the small PS-3 the discharge (R = Vjl) is computed to be ~28 torus the flux of energy to the walls had reached an milliohms. intolerable value. The resulting evaporation or sputter- ing of wall material then contaminated the discharge EXPERIMENTAL RESULTS causing the resistance of the discharge to rise. For this reason the next in the progression of toroidal Neutron Production machines, Perhapsatron S-4 (PS-4), was constructed The total average yield of neutrons is ~4 x 106 per (see Fig. 1). discharge at 30 kv, and neutron yields as high as 7 The machine has a quartz torus of 14.0-cm minor ~10 per discharge have been recorded. No cir- inside diameter, 218-cm mean circumference and wall cumferential asymmetry in neutron yield has been thickness 0.6 cm. An aluminum primary, 1.25 cm observed. thick, surrounds the quartz torus with minimum The average yield of neutrons is found to vary with spacing, ~0.4 cm, and can be energized by 90,000 pressure as shown in Fig. 4. The maximum neutron joules, 20 kv at two feed points, permitting a maximum yield is found at a lower pressure (~ 15 fx) than with electric field of 150 volts/cm. A two-ton iron core the smaller machine, PS-3 (~30 /¿). The neutron yield (laminations 0.002 in. thick) links the primary and also is observed to decrease more rapidly with in- secondary and has a rating of 0.2 volt-sec. A solenoid, creasing pressure than for PS-3 and is reduced to 1.6 turns per cm, wound around the primary is one-half of its maximum value at a pressure of 21.5 /¿. energized by a 2400 ¡d capacitor at voltages upjtoJS kv The dependence of the average neutron yield from and produces an axial Bz magneticTfield up to~4000 PS-4 on the initial axial Bz magnetic field (Fig. 5a) gauss. A split in the aluminum primary allows rapid exhibits a striking similarity to that reported for PS-3 Bz field penetration. with an optimum value of Bz giving the maximum yield. It is also found that the optimum value of Bz ELECTRICAL CHARACTERISTICS varies with the applied primary voltage as shown in Fig. 5b. Again a comparison with PS-3 can be made The typical behavior of gas current and secondary in that a smaller value of Bz is required in the larger voltage at 25 kv is shown in Fig. 2. The current- machine to optimize the neutron yield for a given voltage phase relation shows that the gas current is discharge current. It is of further interest to note that * Los Alamos Scientific Laboratory, University of Cali- for the Bz field adjusted for optimum neutron yield fornia, Los Alamos, New Mexico. at various primary voltages, the yield experiences 297 298 SESSION A-10 P/2488 J. P. CONNER et al.

350

25 5.0 7.5 I0.0 Í25 15,0 17.5 'вез Primary Bank Voltage (kilovolts)

Figure 3. Peak gas current as a function of primary voltage

1.0

Figure 1. Photograph of Perhapsatron S-4 showing the torus, iron cores, electrical feed points and a section of the vacuum system essentially a linear rise with increasing primary voltage. These data are summarized in Fig. 6. The average times of onset, cessation and duration of neutron emission are shown in Fig. la and 7b. As the primary voltage is increased the neutrons appear earlier in time and their duration grows shorter. The onset of neutrons at a fixed Bz field may well be associated with a critical value of discharge current (~180ka) so that the more the primary voltage is increased, with the resulting increase in the rate of rise of gas current, the earlier will this critical current be reached and neutrons emitted. The termination of the neutron burst is correlated with the appearance of radiation from wall materials. At the lower voltages (20 to 24 kv), neutron emission occurs almost sym- metrically in time about the current maximum with little or no impurity light (Si II) emitted in this time

10 20 30 40 50 60 70 80 90 Deuterium Gas Pressure (microns)

Figure 4. Neutron yield vs. deuterium gas pressure

Primary voltage = 30 kv, axial magnetic field Bz = 1700 gauss. Each point represents the average yield of three discharges; the standard error for this average is approximately ±20% interval. At the higher voltages (>24 kv), impurity light comes in earlier with an apparent quenching of the neutrons, thus leading to shorter burst duration. Figure 2. Multiple beam oscilloscope traces of (a) secondary voltage, and (b) gas discharge current. Initial condition: primary The region of emission of neutrons in the torus has voltage = 30 kv; axial magnetic field B2 = 1700 gauss, P = 12.5 ц been measured with a large paraffin collimator used OPERATIONAL CHARACTERISTICS OF PERHAPSATRON S-4 299

О 400 800 1200 1600 2000 2400 2800 3200 400 800 1200 1600 2000 2400 2800 2488.5o Вг Axial Magnetic Field (gauss) Bz Axial Magnelic Field (gauss)

Figure 5a. Neutron yield as a function of axial Bz magnetic field. Primary voltage = 30 kv Figure 5b. Axial Bz magnetic field for optimum neutron yield as a function of primary voltage. Deuterium gas pressure = 12.5 ¡L in conjunction with a fast phosphor and photo- multipHer detector. The results of the measurement are shown in Fig. 8. Within the experimental errors, the results for the horizontal and vertical scan across the minor diameter are the same, and it is concluded that the source is probably symmetric about the minor axis. From a knowledge of the resolution of the collimator, these data have been obtained with the aid of a 704 computing machine. The results of the calculation (Fig. 9A) show that the neutrons are produced in a hollow cylinder, ~2 cm id and ~3 cm od, with some contribution between 8- and 14-cm diameter. The data, however, are not inconsistent with neutrons being produced uniformly in a rod 3 cm in diameter with a small contribution between this rod and the torus wall, Fig. 9B. The energy distributions of the emitted neutrons have been measured in two directions tangential to the major circumference: one in the direction in which deuterons are accelerated by the induced electric field, and the other opposed. An expansion-type cloud chamber, 30 cm id, filled with CH4 at a pressure of 1.5 atmospheres placed one meter from the torus was used as the detector. A paraffin collimator between the cloud chamber and the torus limited the neutrons accepted to those emitted tangentially from the torus. To minimize X-ray background a 0.6-cm thick lead shield surrounded the cloud chamber. The high relative neutron yield from this machine has made it possible '0 8 12 I6 20 24 28 32 36 to obtain ~870 usable tracks in ~1600 expansions. 2488.6 Primary Voltage (kilovolts) The results with the machine operating at 27.6 kv, 1500 gauss B field and 12.5 [x D gas pressure, are Figure 6. Neutron yield vs. primary voltage z 2 Axial Bz magnetic field adjusted for optimum neutron yield at shown in Fig. 10 together with a calibration run made the individual data points. Deuterium pressure = 12.5 JU. 300 SESSION A-10 P/2488 J. P. CONNER et al.

I I I I I I I I I I I Г

•Time of cessation -22.4 kv of neutrons

ью- Í9 _ v29.8kv о le

I I I I I VI6 17 18 19 20 2l 22 23 24 25 26 27 28 29 32 33 34 35 i88 7e Primary Bank Voltage (kilovolts) 1б Figure 7a. Time, during gas current cycle, of onset and cessation of neutron emission, and time on onset of Si II 4128 Â line as a 34.0 kv function of primary voltage. Axial Bz magnetic field adjusted for optimum neutron yield at the individual data points 24 3- on a Cockcroft-Walton accelerator. It is seen that the 2- a Primary Voltage =34.0 kv_ neutron energies measured in the direction in which о Primary Voltage =29.8 kv deuterons may be accelerated in the machine are A Primary Voltage=22.4 kvi- higher than those in the opposite direction, with I J_ I _L pronounced energy peaks at 2.59 Mev in the forward О 700 1400 2100 2800 3500 24887 b direction and 2.37 Mev in the backward direction. Вг Axial Magnetic Field (gauss) The widths at half-maximum of the neutron energy Figure 7b. Duration of neutron burst as a function of primary voltage and axial B magnetic field. Deuterium pressure, 12.5 ¡j. peaks are 0.3 and 0.35 Mev and only slightly larger z than that of the calibration run, ~0.2 Mev. These This phenomenon has been observed in the linear energy shifts show that the center of mass of the discharge machine, Columbus S-4,12 and is of course reacting deuterons is moving in the direction of the 7 due to finite velocity of diffusion of currents through induced electric field with a velocity of ~5xlO the conducting medium. cm/sec. If the assumption is made that fast deuterons react with deuterons at rest, the peak of the neutron Pressure and current distributions were calculated energy distribution implies a deuteron energy of from a knowledge of the magnetic fields measured 10 kev. along a minor diameter of the torus. The equation used for calculating the pressures along this diameter

Magnetic Field and Pressure Distributions

The Be and Bz magnetic field distributions have been measured across a minor diameter of the torus in the horizontal plane. If the assumption of symmetry where R is the major radius to the point under of the discharge about the horizontal plane is correct, consideration, A is determined by the condition then only two components of the magnetic field, Be = 0 at R = A, and the arbitrary constant С is Be and Bz, need be measured as Br is zero. Subsequent determined at some point where the pressure is known. to these measurements, other results12 at Los Alamos The quantity (R2-A*)I2R is the minor radius of the in cylindrical geometry indicate a gross bodily move- toroidal coordinate surface through the point. The ment of the discharge with the resulting inference that equations for the current distributions along the a more detailed program of measurements including diameter are the third component (radial) in toroidal geometry is required. yie-¿^ + ^pBa) (2) The magnetic fields are measured by previously 4 and described techniques, and the resulting calculated jz 1 (3BZ В and je current density distributions at 30 kv primary (3) voltage and 1700 gauss Bz stabilizing field are shown in Figs. 11 я and 116, respectively. The discharge Typical results of the pressure calculations are shown appears to be confined by the pinch field and is in Fig. 12. concentrated about the axis with little or no current An examination of the pressure profiles reveals near the walls after the initial pinching at about one several outstanding features. (1) The center of the microsecond. discharge as defined by BQ = 0 does not move The discharge current along the axis is not zero appreciably during the first half-cycle of gas current when the total secondary current passes through zero. but is localized within less than ±1.0 cm along the OPERATIONAL CHARACTERISTICS OF PERHAPSATRON S-4 301

! I 1 I I I I I I I I I I I I

-7 -6 -5 -4-3-2-10 1 2 3 4 5 6 7 2488.8 Col I i ma tor Position Relative to Torus (cm) Figure 8. Results of neutron collimation experiment; A, vertical traverse (each data point is obtained from averaging 10 dis- -7 -6-5-4-3-2-1012345 charges); B, spatial resolution of col I i mated detector for a line 2488,9 Torus Minor Radius (cm) source of neutrons from a Pu-Be source Figure 9.-Summary of analysis of data presented in Fig. 8 axis. (2) A pressure minimum occurs on the axis and persists during most of the first half-period. Two 100 —BACKWARD pressure peaks of varying amplitude are present on DIRECTION each side of the pressure minimum at a radius of 90 (431 TRACKS) ~2 cm. (3) Appreciable pressures of varying ampli- —FORWARD 80 DIRECTION tude are present at the inside and outside walls. (441 TRACKS) These pressure distributions were unexpected as 70 they have not been obtained in toroidal machines 60 heretofore. The possibility exists that the assumptions (/>50 made in the derivation of Eq. 1 are not valid and a 840 complete mapping of the fields is required. It is ш interesting to note, however, that one possible configuration for the radial distribution of the emitted ¡ neutrons (see Fig. 9A) is a thin shell and that £20 consequently, this shell will be found on the pressure ¡ioj peaks at radius ~2 cm. Ш At. least three possible explanations exist for the m large pressures calculated at the walls: (1) The dis- g 140 charge moves bodily about the tube and large inward COCKCROFT-WALTON 120 CALIBRATION and outward accelerations are present as has been (431 TRACKS) observed radially in the linear system12 (the present 100 data are not sufficiently precise at this time to make a 80 more detailed analysis); (2) High-energy runaway 60- electrons are contributing to the pressure by their 40 centrifugal force; or (3) Errors in field measurements 20 exist near the walls or near the probe openings in the 4)6 08 10 12 1.4 L6 1.8 20 22 24 Z6 28 3.2 3.4 3.6 primary shell. 2488.10 NEUTRON ENERGY (Mev) Figure 10. Neutron energy distributions measured tangentially to Powered Crowbar Operation the torus circumference in the direction of the accelerated deuterons and in the direction opposite to that of the accelerated For some conditions it has been found possible to deuterons. Also pictured is a calibration measurement performed sustain the current maximum for times as long as with a monoenergetic source of neutrons from a Cockcroft- 30 /¿sec. The current is maintained by switching an Walton accelerator 302 SESSION A-10 P/2488 J. P. CONNER et al.

300

-7 -6 -5 -4 -3 -2 -I 0 I 2 3 4 5 6 -7 -6 -5-4-3-2-1 0 I 2 3 4 5 6 7 2488 il Torus Minor Rcjdius(cm) Torus Minor Radius (cm) 2 Figure 11a. Contour plot of axial jz current density (amp/cm ) as Figure 11b. Contour plot of transverse je current density (amp/ a function of radius and time. Operating conditions: primary cm2) as a function of radius and time. Operating conditions:

voltage = 30 kv, axial Bz field = 1700 gauss, P = 12.5 p primary voltage = 30 kv, axial Bz field = 1700 gauss, P = 12.5 ft additional 24 mf of capacitance (1.1 x 105 joules at operating conditions with 13 kv applied secondary 3 kv) in parallel with the primary energy supply at the voltage increases with crowbar operation at 13 kv to time of the current maximum. Because of the un- ~2xlO6 per burst with duration increased up to expected high impedance of the discharge, the dis- <60ju,sec (Fig. 13c). The rate of neutron emission charge current could be held constant by the crowbar remains relatively constant during this time. This bank only at the level corresponding to a total primary result apparently precludes explanations of neutron voltage ^of < 14 kv. If switching takes place at the production which use shocks originating by rapid rate current maximum with initially 7 kv and 3 kv on the of rise of discharge currents or sudden applications primary and sustaining crowbar supplies respectively, of high voltages. It appears necessary that the neutron- the results shown in Fig. 13 are obtained. With an producing mechanism be continuous for relatively initial applied secondary voltage of ~13 kv, it is long times and that it involve no permanent break-up observed that the discharge current is held relatively of the pinch structure. constant, ~180 ka, for ~30 /xsec by the crowbar During the sustained crowbar operation, a consider- system. able amount of energy (9.0 x 102 joules/^tsec) is being

The Вв and Bz magnetic field distributions have transferred to the discharge. No appreciable changes, been measured as a function of time with a sustained however, are found during these times in the pressure crowbar operation, and the corresponding current distributions. The conclusion is inescapable that

densities, jz andje, calculated (Fig. 14a, b). Pressure energy is being lost by the discharge as fast as it is profiles have also been calculated. It is surprising that being received. for times as long as 32 /xsec after the first current maximum the magnetic field distributions vary as Spectral Observations little as they do. It is apparent that the axial dis- The intensities of the deuterium D^ and silicon charge current remains separated from the walls for Si II (4128 Â) lines vary with time as shown in Fig. ~48 /xsec after the initiation of discharge current. 15. The hydrogen line is initially relatively intense. A neutron yield of ~2 x 105 per burst under normal Radiation from the discharge gas becomes much OPERATIONAL CHARACTERISTICS OF PERHAPSATRON S-4 303

-6 -4-20246 -6 -4 -2 О 246 -6-4-20246 2488.12 Minor Radius-cm. Minor Radius-cm. Minor Radius-cm. Figure 12. Calculated pressure distributions as a function of radius at t = 5.0, 7.5 and 10.0 /xsec weaker when ionization has taken place and then measured to be 8.4 kv, which gives from Ohm's law becomes very strong, presumably because the a total discharge impedance of 28 milliohms. From the ionization of hydrogen released from the walls. The jz and je current density distributions (Fig. lia, b) time of onset of the silicon light is a function of it would appear that there are no large fluctuations in primary voltage (Fig. la) and is associated with the the current density distributions at the current maxi- bombardment of the walls by particles or radiation. mum (t = 12.5 /¿sec) indicating that at this time there is little change in the secondary inductance and hence X-rays the voltage drop is purely resistive. The current For primary voltages above ~20 kv a short burst channel diameter, ~7.2 cm, is also defined by the jz of X-rays, duration ~2 /¿sec, is detected outside the distribution. With the assumption that the electric field is constant throughout the discharge, we calculate aluminum primary at the start of the gas current. 3 Little or no X-ray emission of energy >50 kev occurs a resistivity of 4.0xl0~ ohm-cm, 45° pitch of the during the remainder of the discharge cycle. The mean magnetic field lines, and a corresponding electron energy of the X-ray (s.) outside the aluminum primary is 68 kev, as measured with film badges. With an applied primary voltage of 30 kv, a maximum X-ray energy of 0.6 Mev was determined by lead absorbers. The X-ray intensity varies widely from discharge to discharge with an average reading of ~1 mr per discharge measured with a dosimeter placed alongside the torus primary.

DISCUSSION From the above data some conclusions can be drawn concerning the plasma confinement and heating in Perhapsatron S-4.

Electrical Resistance In the operation of PS-4, the electrical resistance of the discharge was found to be considerably larger Figure 13. Muitibeam oscilloscope traces of sustained crowbar than expected. At the discharge current maximum operation: (a) gas discharge current, (b) secondary voltage, 5 (c) neutron emission. Primary voltage = 14 kv, sustained crow- (3 x 10 amp), for example, the secondary voltage was bar voltage = 6 kv, Bz field = 700 gauss 304 SESSION A-10 P/2488 J. P. CONNER et ai temperature of ~6 ev, assuming validity of the a function of time. These calculations are now being resistivity-temperature relation p = Зх 10-6/Г3/2.13 made but no results are available at this time. A second measurement of the discharge resistance is In summary, the experimental results indicate that available from the performance of the discharge the apparent resistivity of the discharge is large during the sustained crowbar operation. In this case (4.0 x 10~3 ohm-cm) with a corresponding low, ~6 ev, the discharge current was held essentially constant electron temperature. It should be emphasized, how- for 30/xsec with little observed change in the jz ever, that these calculations use the simplified Ohm's current distributions (Fig. 14a). The discharge current law relation for the plasma,13 ignoring, in particular, was 1.8 x 105 amp with a secondary voltage of ~ 5.0 kv. velocity terms that would exist in the presence of From the foregoing crowbar operating conditions, the plasma waves or turbulent conditions. total discharge resistance is calculated to be ~28 milliohms in agreement with the calculation made at Energy Balance the higher voltages and currents. At the reduced It is instructive to account for the energy taken power level required for crowbar operation the dis- from the capacitor banks during the discharge. With charge remains remarkably free from impurity PS-4 operating at 15 kv, the energy deposited in the radiation for ~40 /¿sec. This enables us to reject various sections of the machine at a time of 6 /¿sec impurities as a serious contributor to the high has been calculated. The results are summarized in resistivity. Table 1. A more precise and meaningful measurement of the If the remaining energy, 7180 joules, were used to resistivity of the discharge can be made with the heat all the gas initially present in the torus, a plasma knowledge of the Be and Bz distributions. The electric temperature of ~760 ev would exist at 6 /¿sec in the fields and current densities parallel to the magnetic discharge cycle. This result is clearly incompatible field can be calculated throughout the discharge and with the ~6-ev electron temperatures calculated from from these data changes in the resistivity obtained as the resistivity.

i i i i i i i -7 -6 -5 -4 -3 -2 -I 0 I 2 3 4 5 6 7 Torus Minor Radius (cm) Torus Minor Radius (cm) 2 Figure 14a. Contour plot of axial jz current density (amp/cm ) as Figure 14b. Contour plot of preliminary transverse ¡в current a function of time and radius for sustained crowbar operation. density (amp/cm2) data as a function of radius and time for Primary voltage = 14 kv, sustained crowbar voltage = 6 kv, sustained crowbar operation. Primary voltage = 14 kv, sustained

Bz field = 700 gauss, and P = 12.5 ¿t crowbar voltage = 6 kv, Bz field = 700 gauss, and P = 12.5 /A OPERATIONAL CHARACTERISTICS OF PERHAPSATRON S-4 305

Table 1

Initial energy in condenser bank 50,600 joules Energy remaining in condenser bank at 6 ftsec .... 28,200 joules Energy in external inductance 2,760 joules Magnetic field energy associated with discharge, 2 B 2 S(Be l87T)dV+S{Bz2l87r)dV-S( zo l8<7r)dV 11,620 joules Resistance losses 540 joules Energy in primary inductance 300 joules Total 50,600 joules 43,420 joules Energy unaccounted for 7,180 joules

The sustained crowbar operation gives us additional wall, which indicates that all the gas may not be information as to the problem of heating the plasma. confined within the 3.5 cm radius. With a constant current of 1.8 x 105 amp the secondary A temperature of 220 ev is not sufficient to explain voltage remains essentially constant at 5.0 kv for the PS-4 neutron yield based on thermonuclear ~30 fisec. Energy is then being transferred to the reaction considerations although, as stated above, the discharge at the rate of approximately (1.8 xlO5) results of the collimated neutron observations across a (5 x 103) x 10"6 = 900 joules/jLtsec. The calculated minor diameter suggest that the neutrons are pro- current density distributions (Fig. 14a, b) indicate that duced in the measured high-pressure regions of the this energy is not appearing in the magnetic fieldsinc e discharge. these distributions do not change appreciably during this time interval. The energy must then be given to Neutron Energy Distributions the gas at the rate of 95 ev per particle per /¿sec. How- The energy shifts in the neutron energy distribu- ever, the calculated pressures during this time interval tions described earlier in this report show conclusively do not rise appreciably nor does the rate of neutron that most of the neutrons are generated by reacting production increase as would be expected from a deuterons having a center-of-mass velocity of steadily increasing temperature. ~5x 107 cm/sec. These arguments lead to the obvious conclusion Two possible mechanisms representing extremes of that there are mechanisms at work by which energy viewpoints can be considered by which the neutrons is being continuously drained from the discharge. The may be produced with the observed center-of-mass magnetic probe measurements indicate that the dis- velocity: (1) a simple acceleration process in which charge current is at least grossly confined away from 10-kev deuterons, which are a small fraction of the the tube walls, but the experimental results demon- total deuterons present in the torus, react with the strate that the energy transferred to the plasma is not remaining deuterons at rest, and (2) a streaming of confined. The nature of the loss mechanism is not yet deuterons of velocity 5 x 107 cm/sec reacting with known though it is suspected that loss by escape of each other by their relative velocities. energetic particles across the confining magnetic (1) In the acceleration process we can calculate the field, particle acceleration by the electric field, and current of 10-kev deuterons required to produce the loss by radiation generated by plasma waves and observed neutron yield. With an initial gas pressure of turbulence may be contributing factors. 12.5 /x and an assumed compression of ten, a deuteron current of ~ 370 amp is required to give the observed Pressure Balance Results rate of neutron production of ~5 x 1011 neutrons/sec. The pressure distributions calculated from the (If a smaller compression is assumed, the current measured magnetic fielddistribution s in PS-4 show, in general, a pressure maximum at a radius of ~2 cm. Time Dependent Spectrum With a pressure minimum on the axis it can be concluded that the pressure difference between the peak Hydrogen (H^) pressure and the axial pressure is not a result of X = 4861 Â forces such as those produced by runaway electrons. The pressure differences between the peaks and the minima at the axis vary up to ~2.5 atmospheres with corresponding energy densities of 0.25 joules/cm3. It is not clear whether this energy exists as gas Silicon И turbulence, shock waves, plasma waves, or as thermal X = 4I28 Д motion of the gas particles. If it is assumed that all the gas is confined uniformly in a rod of diameter 3.5 cm, and that an energy density of 0.25 joules/cm3 is expended in thermal motion of the gas particles within the rod, then a temperature of 220 ev is 40 50 60 80 90 100 deduced from the pressure measurements. This Microseconds Figure 15. Intensity distribution of deuterium (D^, 4861 Â) and temperature is perhaps low since considerable Si II (4128 Â) light as a function of time. Primary voltage = 30 kv, pressures are observed between 3.5 cm radius and the axial Bz field = 1700 gauss, P = 12.5 ¿t 306 SESSION A-10 P/2488 J. P. CONNER et al. will increase inversely as the compression.) The energy balance the reaction must proceed at large percentage of the total number of deuterons present relative deuteron velocities in a small fraction of the in the torus which would contribute to this current total gas. The necessarily high transient pressures would be small, ~0.02%, and the energy required to which would then exist would not be observed by the accelerate them to 10 kev quite reasonable, ~8 joules. present techniques. This highly localized source of Possibly the strongest argument that can be made neutrons receives some experimental confirmation in against this process of neutron production is that the that, during the collimated neutron measurements, at required deuteron current is too large. If conservation a given coñimator position, the numbers of neutrons of momentum is to occur in the discharge, then the detected on successive discharges frequently varies by ratio of the deuteron and electron currents should be a factor of three. inversely proportional to their masses. With a total Further experiments may reveal that the models discharge current of 300 ka, the deuteron current developed above are gross oversimplifications of the would be only 80 amp. true mechanism for neutron production in PS-4. The above calculation of the required deuteron current has assumed that all the accelerated deuterons SUMMARY have an energy of 10 kev, corresponding to the peak The preceding comments may be summarized as in the neutron energy distribution. This may well be follows: in error since comparable numbers of lower-energy deuterons may be present, which would not con- (1) The discharge has been stabilized in a gross sense tribute much to the total neutron yield since the (the discharge current confined from the walls) for cross section is such a rapidly increasing function of times ~48 /¿sec. energy. Unfortunately the cloud chamber data are (2) From the measured resistivity of the discharge not sufficiently precise to permit a deuteron energy the electron temperature is ~6 ev. distribution to be calculated. (3) By some as yet unknown mechanism, energy is (2) If it is assumed that deuterons are streaming being lost by the discharge at a rate of ~ 900 joules/ parallel to the axis of the torus it can be shown that /¿sec under certain operating conditions. the deuterons in the gas cannot all have the indicated (4) Maximum gas pressures of ~2.5 atmospheres velocity of 5xlO7 cm/sec (~2.5 kev energy). To are measured during the current cycle. accelerate all the deuterons to this energy, a total 7 energy of 1.2 xlO4 joules is required. In the energy (5) There is a center-of-mass velocity of ~5xlO balance consideration discussed above, only 7.2 x 103 cm/sec of the reacting deuterons which produce the joules are available for translational kinetic energy. neutrons. The mechanism responsible for the pro- One must then conclude that only a fraction, 60% at duction of these neutrons is not clear. most, of the deuterons could cooperate in this process. A difficulty arises in that this number of deuterons ACKNOWLEDGEMENTS traveling at a velocity of 5 x 107 cm/sec would produce We wish to thank J. L. Tuck for helpful discussions a current of 6.5 x 106 amp. In order to be consistent during the course of this work and to acknowledge the with the observed current and momentum conserva- cooperation of L. C. Burkhardt and R. H. Lovberg tion, the steady deuteron current must not exceed for the magnetic probe assemblies, of J. W. Mather 80 amp. The pressure distribution also establishes a for scientific assistance, of H. J. Longley, and T. L. limit on the maximum temperature and density which Jordan for assistance with the calculations, of R. S. can exist. We must then conclude if hypothesis (2) is Dike for the engineering design, and of R. Holm and correct that in order to be consistent with current and A. Schofield for the electrical design.

REFERENCES 1. P. С Thonemann et al., Nature, 181, 217 (1958). 8. M. Kruskal and J. L. Tuck, Los Alamos Scientific Labora- 2. N. L. Allen et al., Nature, 181, 222 (1958). tory Report LA-1716, Proc. Roy. Soc. (to be published). 3. L. С Burkhardt and R. H. Lovberg, Nature, 181, 228 9. R. J. Tayler, Proc. Phys. Soc. B, 70, 1049 (1957). (1958). 10. M. Rosenbluth, Los Alamos Scientific Laboratory Report 4. J. L. Honsaker et al., Nature, 181, 231 (1958). LA-2030, Proceedings of the Venice Conference (1957). 5. A. L. Bezbachenko et al., Atomnaya Energ., 1, No. 5, 26 11. V. D. Shafranov, Atomnaya Energ., 5, 709 (1956). (1956). 12. L. C. Burkhardt and R. H. Lovberg, Field Configurations 6. S. Colgate, University of California Radiation Laboratory, and Stability Studies of a Linear Discharge, P/2395, Livermore, California (private communication). Vol. 32, these Proceedings. 7. M. Kruskal and M. Schwarzchild, Proc. Roy. Soc. A, 348, 13. L. Spitzer, Physics of Fully Ionized Gases, Interscience 223 (1954). Publishers, Inc., New York (1956). OPERATIONAL CHARACTERISTICS OF PERHAPSATRON S-4 307

Mr. Phillips presented Paper P/2488, above, at the (1) uniform axial current density in the pinched Conference and added the following remarks : discharge and (2) a negative pressure gradient outward everywhere. The results show that a reverse The first theory is that, although the pinched dis- longitudinal magnetic field outside the discharge charge has been grossly stabilized, there still remain markedly increases the pressure that can be confined. local surface instabilities as observed by Burkhardt 12 The remaining theory is that the energy losses are and Lovberg. This would be expected to increase the due to plasma vibrations excited by the electron resistivity and give high local energy densities. current, as discussed by Akhiezer, Faynberg, Luchina Recently a possible stable configuration has been and Gordayev in the USSR and, recently, by Bune- derived, using the stability criteria of Dr. B. Suydamf mann and Tuck in the USA. The disturbing implica- for mixed BQ and Bz fields. The assumptions are tions are that, if such plasma vibrations exist in gas discharge systems, the stabilized pinch discharge is f Paper P/1364, Session A-5, Vol. 31, these Proceedings. in serious difficulty.