MURI CONSORTIUM on COMPACT, PORTABLE PULSED POWER

1. MURI CONSORTIUM onCOMPACT, PORTABLE PULSED POWER Consortium Team Members: University of Southern California, Martin Gundersen, P.I. University of Missouri-Columbia, William Nunnally Texas Tech University, James C. Dickens, Andreas A. Neuber, and Hermann Krompholz Research Concentration Areas: - III-V photoconductive and junction switching devices - Super-emissive cathode switches - Liquid breakdown for high voltage switching and energy storage

2. Size Comparison BLT 175 High Power Thyratron Purpose and Goals of the USC-Texas-Missouri MURI Consortium To explore new methodologies for III-V and other device switching leading to true optical hybrid architectures w/ vastly reduced size/weight. To study super-emissive gas phase switching, and liquid switching to advance understanding of underlying physics (such as the plasma-cathode interaction that enable super-emissive switches) To apply the recent advances in optoelectronics and in electronic device design, growth, & performance to key components necessary for future compact, repetitive, portable pulsed power. The USC-TTU-UM MURI team offers: - Advanced university test capabilities TTU - Liquid breakdown & switching experience TTU - Photoconductive, bulk III-V switching UM, TTU - Super-emissive cathode switching USC - III-V junction pulsed power switching UM, USC - Advanced III-V materials infrastructure USC

3. Payoff: Improved lifetime Higher current capability Optimum High voltage, high current switch Switching capability 1 GW/cm3 of material Approach: Linear Photo-switch Increase optical absorption depth by using long wavelength & interband doping Reduce current density in GaAs & increase max current Increase holdoff voltage by using multiple, stacked wafers & conducting layers Reduce optical closure energy Compact Pulse Power Photo-SwitchesUniv. of Missouri (Columbia) Bulk Cu:Si:GaAs Photo-Switches • Opportunity: • Picosecond closure, jitter • High Voltage, high current potential • Limited lifetime due to large current density in bulk, contacts • Current density limited by optical depth

4. Research Goals Understand the behavior of photoconductive switches (eg- GaAs) at 4 to 30 kV/cm Computational studies of breakdown and “lock-on” Approach Collective impact ionization theory Ensemble Monte Carlo simulations Personnel Prof. Charles W. Myles, Physics Ken Kambour, PhD Student Payoff High-power solid state switches Semiconductor Switch SimulationsTexas Tech University Photoconductive Semiconductor Switch GaAs phonon cooling rate vs. carrier temperature. Energy balance must occur in steady state. Thus, the Joule heating rate (dashed) must equal the phonon cooling rate (solid). However, the carrier temperature corresponds to a density which is too low to sustain a filament. Thus, the quasi-equilibrium assumption is not valid.

5. New lab apparatus will examine breakdown voltages of 200 kV. Focus: phenomenological picture of surface flashover and volume breakdown Evaluate LN2 as isolating material in cryogenic compact PP devices. Possible use of LN2 as switching medium Dielectric sample submerged in LN2. Early flashovers are across center (middle). After conditioning, discharge occurs at outer edge (bottom). InstaSpec Camera Torr Vacuum Pump Photodiode Over Pressure Safety Liquid N2 Level Monitor Voltage 0.1 V/A 0.1 V/A 0.2 V/A Breakdown in Liquid NitrogenTexas Tech University

6. OptoElectronic III-V Switches: The “SIT”University of Southern California • The USC-SIT is a vertical GaAs FET • Advantageous mobility & band gap make it a candidate for high speed & high hold-off voltage switching • Can be fabricated in optically gated stacks to simplify triggering • Will also examine II-VI, and other III-V’s. Integrated OptoElectronic SIT

7. Size Comparison BLT 175 High Power Thyratron Model P (W) Wgt (gr) I (kA) Dia. (“) 1802 110 20 ­2 4 HY 5 190 50 5-10 4.5 HY 7 1660 400 40 7 BLT175 2 2 40 1.75 Standby Reservoir HOLLOW ANODE 3 mm electrode separation HOLLOW CATHODE FLASHLAMP for triggering Super-Emissive Cathode Switches“BLT” & “Pseudospark” University of Southern California • Lower required power & parts-count make BLT attractive for “portable’ app’s • Super-emissive cathode • 10,000 A/cm2, over ­1cm2 • Stand-off voltage higher than thyratron’s • Very high rate of current rise (>1011 A/sec) • 100-kV forward voltage, 25 to >100kA peak current, 1250-MW peak output power Comparison of Thyratrons to BLT

8. Paschen Curve BLT, thyratron X spark gap Mo Anode Back-lighted thyratron, Pseudospark Anode-cathode separation 3 mm for high hold-off (pressure x d) High Voltage Hold-off Mechanism Mo Cathode USC Pseudospark and BLT Switches:Comparison with Thyratron Low pressure (­0.1-0.5 torr) 10's of kV, ~2-100 kA Hydrogen Thyratron Anode-grid separation 3 mm for high hold-off Mo Anode Mo Grid Cathode (heated thermionic)

9. Delay time of explosion of cathodic micro-protrusions versus plasma density (tungsten, 10 kV). Extremely Fast Transition from Hollow Cathode Emission to Super-Emission Transition from “non-explosive” to “explosive” occurs nearly instantaneously, when ne satisfies --> Delay changes from seconds to nanoseconds when ne changes by ~ 2 For Tungsten --> "Model for explosive electron emission in a pseudospark superdense glow” A. Anders, S. Anders and M. A. Gundersen, Phys. Rev. Lett. 71 (3), 364 (1993). "On electron emission from pseudospark cathodes", A. Anders, S. Anders and M. A. Gundersen, J. Appl. Phys. (1984)

10. Pseudospark Pulse Generator Primary pulse • Used for corona assisted ignition • 70 kV peak amplitude • 1 Hz repetition rate • 50 ns pulse width • Long life 60 ns FWHM 30 kV Secondary pulse into load 53 kV 200 A Work in progress