IMPLEMENTATION OF THE UNIVERSITY OF MISSOURI TERAWATT TEST STAND AND THE STUDY OF A LARGE, MULTICHANNELING, LASER TRIGGERED GAS SWITCH A Dissertation presented to the Faculty of the Graduate School University of Missouri – Columbia In Partial Fulfillment Of the Requirements for the Degree Doctorate of Philosophy Electrical Engineering By KEITH ROBERT LECHIEN Committee Dr. John M. Gahl, Chairman Dr. Ken Struve, Member Dr. Scott Kovaleski, Member Dr. Mark Prelas, Outside Member Dr. Bill Miller, Outside Member MAY 2006 Acknowledgements I would like to thank the members of the committee for their insight and assistance throughout this work. I extend special thanks to Dr. John Gahl whose door is always open and who is always willing to discuss research. I extend special thanks to Dr. Ken Struve of Sandia National Laboratories for support of this work. I extend special thanks to Rob Sharpe and Dr. Juan Elizondo at Sandia National Laboratories. Rob acquired the equipment used for this work and provided invaluable technical help. Juan’s correspondence was extremely helpful in designing the system used to test the switch. I extend special thanks to Andrew Benwell for much needed assistance and willingness to brainstorm. I extend special thanks to Shawn Bryan, Paul Glaser, Tim Evans, Zach Korenak, Ramasamy Ravindran, Mike Lombardo, Mike Hutsel, Brandon Morgan, Bill Baldridge, and Tyler Nickell. These undergraduate assistants made this research possible. I extend special thanks to my parents, Robert and Barbara. I have always understood the importance of education because of their support. ii Table of Contents Introduction 1 A. Purpose of Research 1 B. Dissertation Overview 3 Rimfire Switch History 8 Development of the University of Missouri Terawatt Test Stand 22 A. Phase I: High Energy Test Stand 22 B. Switch Charging Phase 29 C. Load Section and Switch Fields During the Conduction Phase 30 D. Intermediate Store Considerations 32 E. Output Section Inductance 33 F. Diagnostics 36 G. Gas Operating Curves 37 H. Marx Equivalent Circuit 38 Self-Break Data 42 A. Data Analysis 43 B. Multichanneling 45 C. Multichanneling Effect on Period, di/dt and Rise Time 47 D. Transitioning from Phase I to Phase II 52 Laser Triggered Data 54 A. Phase II: Test Stand Redesign 54 B. Output Section Inductance 56 C. Experimental Setup 57 iii D. Electrode Radius and Field Perturbations in SF6 59 E. Impedance Calculations 64 F. Switch Gas Type and Pressure 73 G. Impedance Discussion for Laser Data 86 SF6 Breakdown – Experimental Study 89 A. Impulse Gas Breakdown 89 B. SF6 – Argon High Pressure Breakdown Experiments 93 C. SF6 Microsecond Pulsed Breakdown 98 D. SF6 Sub-Microsecond Pulsed Breakdown 100 E. Electrode Roughness 104 F. Previous Researchers’ High Pressure Work 106 New Switch Study 109 A. Problems Plaguing Rimfire Switches Today 109 B. Reducing Switching Impedance: Reducing Radius and Reducing Length. 110 C. Shortened Design 113 Conclusions 121 Appendix 129 A. Inductance Formulas 129 B. Luminol Pulsed Breakdown 132 C. Intermediate Store Stress Analysis 134 D. Laser System Timing and Noise Issues 135 E. Electrical Software Description 138 F. New Switch Simulations 140 G. Self-break Photographs 142 iv H. Laser Triggered Photographs 147 References 169 v List of Figures Fig. 1-1. 2.5 MV Rimfire. 1) Load end (aluminum), 2) trigger electrode with hole for laser (stainless/Mallory), 3) trigger gap (gas dielectric), 4) cascade gap, ten total (gas dielectric, stainless electrodes), 5) cascade spacer (acrylic), 6) tie rod (nylon), 7) gas envelop (PMMA acrylic), 8) I-Store/high voltage end (aluminum). ...........................................................................4 Fig. 2-1. First multigap concept utilized on a large accelerator at Sandia consisting of one electrically triggered gap and fourteen self break cascade gaps [5]. ..............................................9 Fig. 2-2. The first version of Rimfire [14]. One laser triggered gap and sixteen self break cascade gaps. .......................................................................................................................................10 Fig. 2-3. HERMES III Rimfire implementation [16]. One laser triggered gap and ten self break cascade gaps. ............................................................................................................................14 Fig. 2-4. Runtime and jitter as a function of reduced trigger field [17].....................................15 Fig. 2-5. 6 MV alternative Rimfire designs [4]. .............................................................................18 Fig. 2-6. 6 MV Hybrid Rimfire [4]..................................................................................................19 Fig. 3-1. Phase I of the University of Missouri Terawatt Test Stand. .......................................23 Fig. 3-2. Simplified equivalent circuit.............................................................................................24 Fig. 3-3. Intermediate store charge voltage. ..................................................................................26 Fig. 3-4. SF6 DC breakdown curves. ..............................................................................................27 Fig. 3-5. Anticipated breakdown fields. .........................................................................................28 Fig. 3-6. Cascade gaps field at 2.7 MV...........................................................................................29 Fig. 3-7. Trigger field at 2.7 MV......................................................................................................30 Fig. 3-8. Maximum theoretical load voltage gradient and load field..........................................31 Fig. 3-9. Macroscopic drawing of load and switch (all dimensions in inches). The outer return conductor is listed here as 36”. This was for simulation purposes only. It was really 51”. .......................................................................................................................................................32 Fig. 3-10. (left) voltage potential lines for a straight return, (right) intermediate store surface electric field for straight coaxial returns..........................................................................................33 vi Fig. 3-11. (left) voltage potential lines for bowed returns, (right) intermediate store surface electric field for bowed coaxial returns...........................................................................................34 Fig. 3-12. Gas operating curves for the TG-70, MTG and Marx. ..............................................38 Fig. 3-13. Gas operating curves for the MUTTS Rimfire Switch. ..............................................38 Fig. 3-14. Equivalent Circuits. ..........................................................................................................40 Fig. 3-15: Circuit simulations............................................................................................................41 Fig. 4-1. Example of a 3-ball setup.................................................................................................43 Fig. 4-2. Breakdown voltage versus di/dt......................................................................................46 Fig. 4-3. Inductances obtained from ringing frequency. .............................................................48 Fig. 4-4. Breakdown voltage versus di/dt......................................................................................49 Fig. 4-5. Risetime versus channel factor. ........................................................................................51 Fig. 4-6. Period versus channel factor.............................................................................................52 Fig. 5-1. Test stand redesign (dimensions in inches). ..................................................................55 Fig. 5-2. Electrode cross section and a picture of a 6.67 cm electrode with holes (dimensions in inches)..............................................................................................................................................59 Fig. 5-3. Total cascade channels. Box denotes channel average..................................................60 Fig. 5-4. Multichannel breakdown synopsis. ..................................................................................61 Fig. 5-5. Simulated rise rate of voltage across each gap at time of breakdown.........................62 Fig. 5-6. Simulated breakdown sequence after presence of laser trigger at t=0 ns...................63 Fig. 5-7. System equivalent circuit. .................................................................................................64 Fig. 5-8. Switch impedance for a 6.67 cm electrode example.....................................................65 Fig. 5-9. Measured voltage and current plotted with solved voltage and current using the time varying parameters in Fig. 5-8. ................................................................................................67 Fig. 5-10. Total switch inductance..................................................................................................68 Fig. 5-11. Switch inductance by electrode configuration. ...........................................................69 vii Fig. 5-12. Rise time versus inductance...........................................................................................69 Fig. 5-13. Total switch conduction