Michael E. Swalby

Fiber-Optically Controlled Pulsed Power Switches

The development and testing of fiber-optically controlled trigger generators (TGs) based on high gain photoconductive semiconductor switches (PCSSs), constructed from high resistivity GaAs, are described in this paper. The TGs are optimized to trigger the high voltage switches (HVSs) in pulsed power systems, where they control the timing synchronization and amplitude variation of multiple pulse forming lines that combine to produce the total system output. Future pulsed power systems are even more dependent on triggering, as they consist of many more HVS and, in some cases, produce shaped pulses by independent timing of the HVS. The goal of the PCSS TG is to improve timing precision and replace high voltage trigger cables or line-of-sight optics with fiber-optic trigger control. The PCSS trigger has independent EMP-free timing control via 200-mum-diameter optical fibers. This design is simpler than other TG because optical isolation allows PCSS triggers to be remotely located near the HVS at any voltage. PCSS can improve the performance of prime power HVS, diverters, and diagnostics by supplying trigger pulses with subnanosecond jitter and rise time that are more precise and easily adjusted than the conventional TG. For pulse-charged HVS, the PCSS TG can generally derive their trigger energy from the stray fields of the HVS. High gain PCSS capabilities for producing pulsed power TG have been demonstrated previously (not all simultaneously): 220 kV, 8 kA, 350-ps rise time, 100-ns pulsewidth, 50-ps rms jitter, and 10-kHz repetition rate. Furthermore, PCSS has previously triggered a 300-kV trigatron with 100-ps rms jitter.

Pulsed- and DC-Charged PCSS-Based Trigger Generators

Prior to this research, we have developed high-gain, GaAs, photoconductive semiconductor switches (PCSSs) to trigger 50–300 kV high voltage switches (HVSs). We have demonstrated that PCSSs can trigger a variety of pulsed power switches operating at 50–300kV by locating the trigger generator directly at the HVS. This was demonstrated for two types of DC-charged trigatrons and two types of field distortion mid-plane switches, including a ±100 kVDC switch produced by the High Current Electronics Institute (HCEI) used in the linear transformer driver. The lowest rms jitter obtained from triggering a HVS with a PCSS was 100 ps from a 300 kV pulse-charged trigatron. PCSSs are the key component in these independently timed, fiber-optically controlled, low jitter trigger generators (TGs) for HVSs. TGs are critical sub-systems for reliable, efficient pulsed power facilities because they control the timing synchronization and amplitude variation of multiple pulse forming lines that combine to produce the total system output. Future facility scale pulsed power systems are even more dependent on triggering, as they consist of many more triggered HVSs and produce shaped-pulses by independent timing of the HVSs. As pulsed power systems become more complex, the complexity of the associated trigger systems also increases. One means to reduce this complexity is to allow the trigger system to be charged directly from the voltage appearing across the HVS. However, for slow or DC charged pulsed power systems this can be particularly challenging as the DC hold off of the PCSS dramatically declines. This paper presents results seeking to address HVS performance requirements over large operating ranges by triggering using a pulsed charged PCSS based TG. Switch operating conditions as low as 45% of self break were achieved. A DC charged PCSS based TG is also introduced and demonstrated over a 39 kV – 61 kV operating range. DC charged PCSS allow the TG to be directly charged from slow or DC charged pulsed power systems. GaAs PCSSs and neutron irradiated GaAs (n-GaAs) PCSSs were used to investigate the DC charged operation.

DC-Charged GaAs PCSSs for Trigger Generators and Other High-Voltage Applications

The demand for greater flexibility and increased energy density in pulsed-power systems is moving highly interactive components closer together. The development of compact technologies for less complex and more robust system designs is critical. A key system component that can impact these goals is the trigger generator (TG). Inexpensive, compact, and fiber-optically controlled TGs that deliver trigger pulses with subnanosecond jitter have been created with photoconductive semiconductor switches (PCSSs). However, high-voltage (HV) GaAs PCSSs are typically pulsed charged for less than 100 s so that they can hold off 60-100 kV/cm without self-triggering into high-gain (lock-on) switching or initiating surface flashover. Since many new pulsed-power system designs are based on dc-charged HV switches, pulse charging the trigger system is an additional complication requiring space, HV switching components, and HV cables. A further improvement in PCSS-based TG is to move from pulsed to dc-charged PCSSs. This paper reports results from dc-charged GaAs PCSSs with 0.25-1.0 cm gaps, extending previously reported results on smaller devices at 3 kV to a new regime of 100 kV. To hold off high fields for longer periods and to extend GaAs PCSSs to dc applications, we have utilized neutron-irradiated GaAs (n-GaAs). Neutron irradiation in GaAs increases the defect density, shortens the carrier recombination time, and (for devices with large insulating regions) reduces the dark current, which improves the dc hold-off strength. PCSS contacts in this research were created using rapid thermal annealing (RTA) to produce high adhesion and low contact resistance. However, this can reduce the defect density near the contacts by annealing some of the n-induced defects. Hence, a range of RTA temperatures and neutron doses was studied to understand the tradeoff space for contact adhesion and dc hold-off. This paper presents results from I-V characterization and dc hold-off on irradiated and nonirradiated GaAs PCSSs. These PCSS devices were demonstrated to hold off fields of 39-61 kVDC/cm, respectively. Irradiation doses over a range of 3×1013 - 1×1015 (1 MeV Si equivalent) were explored in search of the optimal performance. Additionally, the impact of the fabrication processes on the benefits of irradiation is explored, and the observation of unusual low-frequency oscillations during GaAs I-V testing is discussed.

Pulsed and DC charged PCSS based trigger generators

Prior to this research, we have developed high-gain, GaAs, photoconductive semiconductor switches (PCSSs) to trigger 50–300 kV high voltage switches (HVSs). We have demonstrated that PCSSs can trigger a variety of pulsed power switches operating at 50–300kV by locating the trigger generator directly at the HVS. This was demonstrated for two types of DC-charged trigatrons and two types of field distortion mid-plane switches, including a ±100 kVDC switch produced by the High Current Electronics Institute (HCEI) used in the linear transformer driver. The lowest rms jitter obtained from triggering a HVS with a PCSS was 100 ps from a 300 kV pulse-charged trigatron. PCSSs are the key component in these independently timed, fiber-optically controlled, low jitter trigger generators (TGs) for HVSs. TGs are critical sub-systems for reliable, efficient pulsed power facilities because they control the timing synchronization and amplitude variation of multiple pulse forming lines that combine to produce the total system output. Future facility scale pulsed power systems are even more dependent on triggering, as they consist of many more triggered HVSs and produce shaped-pulses by independent timing of the HVSs. As pulsed power systems become more complex, the complexity of the associated trigger systems also increases. One means to reduce this complexity is to allow the trigger system to be charged directly from the voltage appearing across the HVS. However, for slow or DC charged pulsed power systems this can be particularly challenging as the DC hold off of the PCSS dramatically declines. This paper presents results seeking to address HVS performance requirements over large operating ranges by triggering using a pulsed charged PCSS based TG. Switch operating conditions as low as 45% of self break were achieved. A DC charged PCSS based TG is also introduced and demonstrated over a 39 kV – 61 kV operating range. DC charged PCSS allow the TG to be directly charged from slow or DC charged pulsed power systems. GaAs PCSSs and neutron irradiated GaAs (n-GaAs) PCSSs were used to investigate the DC charged operation.

High current, multi-filament photoconductive semiconductor switching

High current switching is the most critical challenge remaining for photoconductive semiconductor switch (PCSS) applications in Pulsed Power. Many authors have described the advantageous properties of high gain PCSS such as, low optical trigger energy and inductance, sub-nanosecond risetime and jitter, optical isolation and control, pulsed or DC charging, and long device lifetime, provided the current per filament is limited to 20–30A for short pulse (10–20ns) applications [1,2]. Low energy optical triggering, long device lifetime, and current filaments are related features of high gain PCSS that make high current switching a challenge. Since the location and number of current filaments can be controlled with parallel “lines” of optical pulses across the insulating gap, the problem of high current, multi-filament PCSS switching is essentially the problem of producing a reliable, efficient, multi-line, optical delivery system [3].

PCSS Lifetime Testing for Pulsed Power Applications

Trigger systems are becoming increasingly important in pulsed power systems with large numbers of high voltage switches (HVSs) or large numbers of different switching times. Performance can be critical with demands for fast rise-times, sub-nanosecond jitter, and long lifetimes. In particular component lifetimes affect maintenance costs and the available operational time of the system. High gain photoconductive semiconductor switches (PCSSs) deliver many of the desired properties including optical-isolation, 350 ps risetime, 100 ps rms jitter, scalability to high power (220 kV and 8 kA demonstrated), and device lifetimes up to 108 shots with 21 A per filament in 5 ns wide pulses [1], [2]. However, higher current and longer pulse applications can drastically reduce device lifetime. For typical single shot pulsed power applications, lifetimes of several thousand shots are required and much longer-lived HVSs are required for repetitive pulsed power applications.

PCSS Triggered Pulsed Power Switches

This paper describes results from a 3 year project to develop fiber-optically controlled trigger generators (TG) from high gain photoconductive semiconductor switches (PCSSs) for high voltage switches (HVSs) in pulsed power applications. Triggers for HVSs are critical components for reliable, efficient pulsed power systems because they control the timing synchronization and amplitude variation of multiple pulse forming lines that combine to produce the total output. Proposed pulse power systems are even more dependent on triggering as they consist of many triggered HVSs and produce shaped-pulses by independent timing of the HVSs.

GaAs PCSSs for DC applications

As the demand for greater flexibility and increased energy density moves highly interactive components closer together in pulsed power systems, the development of technologies for less complex and more robust system designs is critical. A key system component that impacts these goals is the trigger generator (TG). Inexpensive, compact, fiber-optically controlled TGs that deliver trigger pulses with sub-nanosecond jitter have been created with photoconductive semiconductor switches (PCSSs) [1]. A further simplification in pulsed power design is to move from pulsed to DC charged components. This paper reports results from DC charged GaAs PCSSs with 0.25–1.0 cm gaps extending previously reported results on smaller devices at 3 kV [2] to a new regime of 100 kV.

Fiber-Optic Controlled PCSS Triggers For High Voltage Pulsed Power Switches

This paper reports on progress in the development of fiber optically triggered photoconductive semiconductor switches (PCSS) for triggering high voltage pulsed power switches with improved precision. This technology can eliminate the need for large-diameter trigger cables and line-of-sight optics. It also has the potential of significantly reducing the cost of trigger generation systems. It has the potential to improve the performance of prime power switches, diverters, and diagnostics because of the low-jitter sub-nanosecond rise times. Test results will be presented that demonstrate sub-nanosecond jitter from a PCSS, less than 1.2 nanoseconds of jitter from a commercial trigatron switch triggered by PCSS, and the performance analysis of a 200 kV class switch triggered by a PCSS based triggering system. This work is a step toward the creation of a reliable self-contained PCSS-based trigger generator that can be physically located on a high voltage switch, allowing the triggering hardware to float. The only connection to the trigger system will be a low energy optical fiber, thereby eliminating the need for HV cables traversing high potential fields or for line-of-sight-optics