Conference Agenda

As pulsed power systems scale toward large research platforms and commercial fusion energy applications, greater consistency and reliability under repeated high-energy discharges are demanded of spark gap switches. A single anomalous, premature self-breakdown can disrupt an entire machine comprising numerous switches, highlighting the urgent need for a deeper understanding of electrode behavior.

This work presents a comprehensive investigation of the effects of electrode surface evolution on spark-gap performance. An advanced apparatus was developed to deliver up to 80 kV across the gap and transfer approximately 0.1 C per discharge, while enabling automated imaging of all electrode surfaces between successive closures - from installation through >10,000 switching operations. These shot-to-shot observations uncover not only the spatial patterns of plasma channel formation and the progressive buildup of damage and debris, but also the evolving statistical behavior of spark attachment points - providing unprecedented insight into the mechanisms driving spark gap reliability.

Combining these imaging capabilities with surface profilometry and Energy-Dispersive X-ray Spectroscopy (EDS), we deliver a comprehensive evaluation of material erosion, selective alloy degradation, and the influence of midplane trigger electrodes on anomalous low-voltage closures. Drawing on data from several tens of thousands of discharges using brass, copper-tungsten, and stainless-steel electrodes, this presentation will highlight key findings and actionable insights to advance design strategies that improve spark-gap reliability in next-generation pulsed-power drivers.

This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia is managed by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

The authors would additionally like to acknowledge the contributions of Dr. Bo Zhao and the Texas Tech University College of Arts and Sciences Mircoscopy facility.

Semiconductor opening switches (SOS) are ultrafast, high-voltage switching devices employed in
pulsed power systems to abruptly interrupt large currents on nanosecond time scales. These devices leverage the reverse recovery behavior of semiconductors to produce high-voltage, high-current sub-nanosecond pulses, allowing for compact and fully solid-state pulsed power systems. Whilethe fundamental physical mechanisms governing SOS operation are reasonably well established, there remains a notable gap in practical, system-level modeling and design tools to support the integration and optimization of large-scale SOS-based pulsed power systems.

The SOS model presented here is based on empirical relationships between forward and reverse
charge during the pumping stages. These relationships are derived from experimental data obtained from a characterization testbed with capabilities of generating 17 kV pulses and multiple kA/cm current densities. This work presents an updated empirical SOS model developed for SPICE-based circuit analysis. Improvements include reduced error metrics relative to experimental data, a better understanding of the characteristics of SOS devices and the limitations inherent to empirical modeling approaches. This updated model also supports multiple diode stacks and shows consistent agreement with experimental data across a wide range of circuit configurations. Comparisons between measured and simulated data are shown.

We will report medium voltage (MV) vertical photoconductive semiconductor switches (PCSS) fabricated with manganese doped, semi-insulating gallium nitride (Mn-GaN). PCSS is a unique semiconductor switch that potentially replaces some of non solid-state MV and high voltage (HV) switches like spark gaps. It is a general trend of electronics to replace vacuum tube technologies like vacuum tubes, CRTs and light valves with solid-state technologies like transistors, LCD displays and LEDs because of longer lifetime of solid-state devices. To extend the service cycle and lifetime of MV/HV switches, PCSS is a potential candidate to replace some of the MV/HV gas switches for pulse applications. In addition, triggering the device with light enables very low jitter operation of multiple switches, making easier to construct high voltage/high current systems using multiple PCSS.

Despite the potential advantages, GaAs PCSS was not commonly used in the practical applications primarily because of degradation of devices. GaN, one of wide bandgap semiconductor materials, is much more robust than GaAs due to higher bond strength of atoms in the crystal lattice. However, it was quite challenging to grow bulk-shaped crystals of GaN. We have developed a near equilibrium ammonothermal (NEAT) method to grow low-defect bulk crystals of Mn-GaN. By taking advantage of thick (1~3 mm) crystals, we are developing MV vertical PCSS to achieve high peak current and long lifetime.

We will present results of vertical PCSS operated under DC bias of 15 kV with peak current of 74 A. The thickness of the crystal was 1.5 mm and the trigger laser was Nd:YAG pulse laser having wavelength of 532 nm. The laser peak intensity was 5.11 mJ and peak width was about 10 ns. The current flow upon laser irradiation continued to about 200 ns, indicating partially sustained carrier generation without light. This implies that lock-on operation of vertical Mn-GaN PCSS is possible with an appropriate device design and fabrication. We will also present 6-fold increase of peak current in one PCSS having vertical trenches in a window region. Exposure of sidewalls to light irradiation helped generating electrons deep in the device, allowing higher current flow. Availability of low-defect bulk crystal of Mn-GaN would enable us to develop highly reliable PCSS that can be used for practical applications in MV/HV pulse systems.

This work was supported by U.S. Department of Energy (DOE), Advanced Research Program Agency Energy (ARPA-E), OPEN 2021 program (DE-AR0001562), Program Manager: J. Snyder).

We have developed a Photoconductive Semiconductor Switch (PCSS) assembly with integrated light source. We utilize a lateral GaAs PCSS working in a high-gain mode which dramatically reduces the requirement for optical power to achieve conductivity. The PCSS units are triggered by an integrated laser diode array (VCSEL) which helps to achieve a unique package compactness and robustness. A compact VCSEL driver based on avalanche transistors was also developed to initiate PCSS triggering. The PCSS high-gain mode enables a small size factor of the switch unit which is hard to achieve with a conventional (linear-mode) PCSS when an external powerful optical source (usually a solid-state laser) is required for triggering. The PCSS is placed inside a compact Fluorinert (FC40) enclosure to increase the holdoff voltage up to 30 kV and the liquid circulation system is designed to maintain the surface cleanliness to increase the switch longevity. In addition, we utilize multi-filament current sharing to achieve high operating current up to 490 A per switch. The PCSS units are tested at several charging voltages up to 30 kV in a Pulse Forming Line (PFL) with Z=25-ohm and 50-ohm characteristic impedances and different pulse lengths up to 10 ns long. We achieved sub-nanosecond rise times. For different charging voltages we measured current rise times and avalanche delay times. At 30 kV we achieved 500 A/ns slew rate. Further increase of the operating current is possible with parallel stacking of multiple PCSS units.

In addition, we integrated a non-linear PCSS resistance model into REMCOM xFDTD solver. We compared numerical simulations of the PFL with experiments and determined model’s parameters (such as avalanche time). It allows us to simulate and predict PCSS behavior in more complicated circuits and devices. The PCSS switch units we designed can be utilized in applications where high-voltage, high-current pulses with sub-nanosecond rise time are required such as, laser diode array drivers, high-power microwave generators, q-switch drivers for solid-state lasers, plasma research, compact accelerators, and ground penetrating radar pulsers.