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BREAKDOWN PROCESSES IN LASER TRIGGERED SWITCHING by ROGER ADELBERT DOUGAL, B.S., M.S., IN E.E. A DISSERTATION IN ELECTRICAL ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Approved Accepted May, 1983 C^oO'^-^ ACKNOWLEDGEMENTS I would like to thank the members of my committee for their critical review of my work, their guidance along the way, and for securing from the Air Force Office of Scientific Research the funds and equipment necessary for the execution of this research. A special "Thank you" goes to my dear wife. Sue, for allowing me to embark on this quest. 11 CONTENTS ACKNOWLEDGEMENTS i i ABSTRACT v LIST OF FIGURES vii I . INTRODUCTION 1 Objective 1 Background 2 11 . APPARATUS 9 Spark Gap Assembly 9 Diagnostic Equipment 21 III. EXPERIMENTS 26 Current Measurements 28 Photodiode Measurements 38 Spectroscopic Measurements 44 Streak Photography 53 IV. ANALYSIS OF DATA 82 Introduction 82 Self Breakdown 86 Transverse Triggered Breakdown 93 Recessed Coaxial Triggered Breakdown 97 Coaxial Mid-gap Triggered Breakdown 99 111 Coaxial Triggered Breakdown 102 Observations 102 The Model 105 Model Support 106 Comparison with Other Results 123 V. CONCLUSION 126 REFERENCES 129 IV ABSTRACT An investigation of laser triggered switching using a 1.06 y NdrYAG laser to trigger a uniform field gap filled with 800 Torr of N2 has been performed in the fol lowing triggering geometries: 1) Transverse not striking electrodes, 2) Coaxial not striking electrodes, 3) Coaxi al striking target electrode, and 4) Coaxial striking re cess in target electrode. Streak photography, time resolved spectroscopy, and current measurements reveal great differences in the breakdown processes for the various geometries. The major differences are attribut able to laser field interaction with the incipient arc channel. All breakdown processes except types 2 and 3 look quite similar to an overvolted breakdown. Diagnos tics of type 2 triggering have shown a laser assisted streamer propagating from the laser fireball to the oppo site electrode. Streak photography shows the streamer precursor of the breakdown channel initially proceeds 3 across the gap at a few times 10 cm/sec, but slows to 7 about 2X10 cm/sec as it advances in the focal cone to regions of lower laser intensity. The laser interaction with the streamer produces a uniform, high conductivity channel which emits intense continuum light once ohmic heating raises the channel temperature. When delay is V greater than the laser pulse length two distinct regions can be detected in the arc channel; one, laser assisted, showing the abrupt,uniform continuum luminosity, and the other not laser assisted, appearing much like a weakly overvolted breakdown process, exhibiting a glow discharge stage followed by thermalization proceeding from the ends of the glow region towards the middle at 10 cm/sec. VI LIST OF ILLUSTRATIONS Figure Page 1-1 Triggering geometries 3 1-2 Typical data from previous research 5 2-1 Spark gap housing 11 2-2 Electrical feedthrough, upper 13 2-3 Electrical feedthrough, lower 14 2-4 Resistive load and voltage divider 16 2-5 Load resister characteristics 17 2-6 Drawing of gap assembly 18 2-7 Equivalent circuit of gap assembly 20 2-8 Photodiode circuit 22 2-9 Streak camara operation 25 3-1 Recessed coaxial triggering configuration 27 3-2 Current profile. Self breakdown 30 3-3 Current profile, coaxial trigger 31 3-4 Graph of delay vs charging voltage 33 3-5 Current profile, transverse trigger 34 3-6 Current profile, coaxial mid-gap trigger 35 3-7 Current profile, recessed coaxial trigger 37 3-8 Photodiode trace, self breakdown 39 3-9 Photodiode trace, coaxial trigger 41 3-10 Photodiode trace, coaxial mid-gap trigger 42 3-11 Photodiode trace, transverse trigger 43 vii 3-12 Energy level diagram of N2 45 3-13 Collisional excitation cross sections in N^ 48 3-14 Example spectra 49 3-15 Example spectra 50 3-16 Example Spectra 51 3-17 Graph of N2 C-B vs continuum emission 52 3-18 Equipment schematic for streak spectra 55 3-19 Streak spectra near entrance electrode 56 3-20 Spectrally resolved streak 58 3-21 Faint streamer evidence 60 3-22 Streak photograph with bandpass filter 62 3-23 Streak showing arrested plasma growth 63 3-24 Streak showing absorption of laser energy 65 3-25 Streak showing laser interaction with streamer 67 3-26 Streak showing initial streamer 69 3-27 Streamer at lower voltage 71 3-28 Streak of width of arc channel 72 3-29 Voltage sequence of streaks, positive polarity 74 3-30 Voltage sequence of streaks, negative polarity 75 3-31 Streak of transverse triggered breakdown 76 3-32 Streak of coaxial mid-gap triggered breakdown 78 3-33 Streak of recessed coaxial triggered breakdown 80 3-34 Luminous front in recessed coaxial triggering 81 4-1 Summary of data for various geometries 83 4-2 Chalmers and Duffy overvolted data 89 4-3 Approach of glow to slit stop 96 viii 4-4 Diagram of laser triggered switching model 107 4-5 Field intensification by plasma on electrode 109 4-6 Graph of streamer velocity vs laser intensity 113 4-7 Diagram of equivalent avalanche 115 4-8 Variation of type "L" behavior with voltage 118 IX CHAPTER I INTRODUCTION Objective Many high power devices, including fusion machines, weapons systems, and high energy physics apparatus re quire switches capable of handling very high voltages and currents with nanosecond precision. Accurate timing is especially crucial in these devices; some multi-element systems can quite literally be destroyed by timing errors of only a few nanoseconds. Traditionally the switch best suited to the harsh environment of high power machinery has been the spark gap switch. Trigatron and field dis tortion gaps have been used for many years as high power switches, but they usually do not offer the temporal per formance required in the latest devices. Another type of spark gap switch developed in the middle 1960's, the laser triggered spark gap (LTSG), shows great promise for meeting the stringent switching requirements of ever greater precision operation. The terminal characteris tics of these switches have been thoroughly investigated by many people (see, for example, the review articles by Guenther and Bettis 1971, 1978), but the internal processes involved in initiation of the switching event have not been investigated in detail, and are inadequate ly understood. The goal of the work about to be described herein has been to investigate and model the physical processes occuring in the initiation phase of the switching of a gas filled laser triggered spark gap. Background The first demonstration of laser triggered switching (LTS) was performed in 1963 by Guenther and Griffin. Subsequently Pendelton and Guenther (1965) began an in vestigation of the switching characteristics of LTS resulting in the first publication concerning this de vice. They reported switching tens of kilovolts in a 1/2 cm gap on the nanosecond time scale with a Q-switched ruby laser as the trigger source. The transverse triggering geometry which was first used is illustrated in Figure 1-la, and will be referred to as raid-gap transverse laser triggering. The investigators noticed that actually striking the charged cathode reduced the switching time (delay), and soon the standard configura tion for laser triggered gaps became the coaxial geometry shown in Figure 1-lb, in which the laser impinges direct ly onto one of the electrodes. A third configuration is that of Figure 1-lc in which the coaxial geometry is o Figure 1-1. Standard LTSG configurations: a) transverse not striking electrodes, b) coaxial striking target elec trode, c) coaxial not striking electrodes. employed with hollow electrodes so that the laser is focused in the gas rather than on an electrode. Woodworth, Frost, and Green (1982) report that this third configuration is currently being incorporated into the Particle Beam Fusion Accelerator system at Sandia National Laboratory, a very large scale application. The coaxial laser triggering geometry in which the laser is incident on the target electrode has been the type most thoroughly investigated. Beginning in 1966 a flurry of publications appeared revealing data on switch ing delay time, (generally defined as the time between arrival of the laser pulse and the beginning of current conduction) and jitter (variation of the switching delay time shot to shot), as a function of gap parameters. Parameters of interest were: charging voltage, polarity, laser power, gas type, gas pressure, electrode material, and focal point in the gap. Typical data from these ex periments are shown in Figure 1-2. Following these quantitative studies of switching characteristics, demonstrations of switching in condi tions not previously investigated were reported: Zigler - overvolted liquid dielectrics (1966), Strickland - solid dielectrics (1969), Bettis - megavolt switching (1957), McKnight - 50 pps switching (1967), all with Guenther. These new switching configurations soon became well 25 20 15 la - s - 8 .31 .1 1.8 10 VOLTAGE CXSBVD CMU) 20 40 0 20 40 E/P CV/cm-torr) E/P CV/cm-lorr) Figure 1-2. Typical data from previous research (after Guenther and Bettis 1971). parameterized, though their operating mechanisms were not completely described. Very few of the investigations to the present date actually had as the main goal an under standing of the processes involved in the breakdown event. Those few that were so directed, Ujihara (1968), and Brumme (1974), did not address all of the pertinent questions. Ujihara observed the development of the spark chan nel in an air gap, triggered by a ruby laser, using a shadowgraph technique.

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