Fred J. Zutavern

Photoconductive semiconductor switches

Optically activated GaAs switches operated in their high-gain mode are being used or tested for pulsed power applications as diverse as low-impedance, high-current firing sets in munitions; high impedance, low-current Pockels cell or Q-switch drivers for lasers; high-voltage drivers for laser diode arrays; high-voltage, high-current, compact accelerators; and pulsers for ground penetrating radar. This paper will describe the properties of high-gain photoconductive semiconductor switches (PCSS), and how they are used in a variety of pulsed power applications. For firing sets, we have switched up to 7 kA in a very compact package. For driving Q switches, the load is the small (30 pF) capacitance of the Q switch which is charged to 6 kV. We have demonstrated that we can modulate a laser beam with a subnanosecond rise time. Using PCSS, we have demonstrated gain switching a series-connected laser diode array, obtaining an optical output with a peak power of 50 kW and a pulse duration of 100 ps. For accelerators, we are using PCSS to switch a 260 kV, 60 kA Blumlein. A pulser suitable for use in ground-penetrating radar has been demonstrated at 100 kV, 1.3 kA. This paper will describe the specific project requirements and switch parameters in all of these applications, and emphasize the switch research and development that is being pursued to address the important issues.

Photoconductive semiconductor switch experiments for pulsed power applications

Experiments have been performed to develop photoconductive semiconductor switches (PCSSs) for pulsed power applications that cannot be implemented with traditional high-power switching technology. A scalable lateral PCSS configuration has been tested and has demonstrated a potential for faster risetimes, less jitter, lower inductance, faster recovery, and optical triggering for new pulse power projects. These switches have been used as both closing and toggling switches at repetition rates up to 40 MHz. A high-field, gain mechanism (lock-on) was explored and tested which may eliminate the major disadvantage of this type of switch, its requirement for large optical trigger energies. >

High Gain Photoconductive Semiconductor Switching

Switching properties are reported for high gain photoconductive semiconductor switches (PCSS). A 200 ps pulse width laser was used in tests to examine the relations between etectric field, rise time, delay, and minimum optical trigger energy for switches which reached 80 kV in a 50 /spl Omega/ transmission line with rise times as short as 600 ps. Infrared photoluminescence was imaged during high gain switching providing direct evidence for current filamentation. Implications of these measurements for the theoretical understanding and practical development of these switches are discussed.

Measurement of the velocity of current filaments in optically triggered, high gain GaAs switches

Pictures are presented of the time evolution of current filaments during optically triggered, high gain switching in GaAs. Two filaments are triggered with two laser diode arrays and the time delay between them is varied. When the filament that is triggered first crosses the switch the voltage drops and the other filament ceases to grow. By varying the delay between the lasers, the tip velocity is measured to be 2±1×109 cm/s, 100 times larger than the peak drift velocity of carriers in GaAs. This observation supports switching models that rely on carrier generation at the tip of the filament.

Triggering GaAs lock-on switches with laser diode arrays

Progress toward the triggering of high-power photoconductive semiconductor switches (PCSSs) with laser diode arrays, is reported. An 850-W optical pulse from a laser diode array was used to trigger a 1.5-cm-long switch that delivered 8.5 MW to a 38.3- Omega load. Using 166-W arrays, it was possible to trigger a 2.5-mm-long switch delivering 1.2 MW with 600-ps rise-times at pulse repetition frequencies of 1 kHz. These 2.5-mm-long switches survived 10/sup 5/ pulses at 1.0 MW levels. In single-pulse operation, up to 600 A was switched with laser diode arrays. The goal is to switch up to 5 kA in a single-shot mode and up to 100 MW repetitively at up to 10 kHz. At electric fields below 3 kV/cm GaAs switches are activated by creation of one electron-hole pair per photon. This linear mode demands high laser power and, after the light pulse, the carriers recombine in nanoseconds. At higher electric fields GaAs acts as a light-activated Zener diode. The laser light generates carriers as before, but the field induces gain such that the amount of light required to trigger the switch is reduced by a factor of up to 500. The gain continues until the field across the sample drops to a material-dependent lock-on field. The gain in the switch allows for the use of laser diodes. >

Recovery of high-field GaAs photoconductive semiconductor switches

The authors discuss the recovery of GaAs PCSS (photoconductive semiconductor switches) after they are triggered into a high gain switching mode called lock-on. Fast recovery of GaAs switches after high field switching is of particular interest for high repetition rate applications where it is difficult to provide the large optical trigger energy required for switches operating at low fields. Three categories of circuits for inducing fast recovery after lock-on by temporarily reducing the field across the switch are examined. Measurements of recovery times from 35-80 ns, multiple monopolar and bipolar bursts at 5-40 MHz, and hold-off fields ranging from 5-44 kV/cm (corresponding to 15-66 kV across individual switches) are presented. >

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.

High current photoconductive semiconductor switches

It is shown that Si photoconductive semiconductor switches (PCSSs) can be used to switch high voltages (up to 123 kV), high fields (up to 82 kV/cm) and high currents (2.8 kA). The ability of the samples to withstand this type of high-voltage, high-current switching depends on the way in which the current penetrates the semiconductor. The appropriate use of water or contacts greatly improves the switching capability. It is also shown that the wafers can support large currents (4.0 kA for GaAs and 2.8 kA for Si) and large linear current densities (3.2 kA/cm for GaAs and 1.4 kA/cm for Si). For GaAs, this linear current density corresponds to about 1 MA/cm/sup 2/, given a penetration depth of about 10/sup -3/ cm. It is determined that the lock-on phenomenon can be triggered with light of varying photon energy to reach a lock-on field that is both impurity-concentration and sample-temperature dependent.<<ETX>>

Properties of high gain GaAs switches for pulsed power applications

High gain GaAs photoconductive semiconductor switches (PCSS) are being used in a variety of electrical and optical short pulse applications. The highest power application, which we are developing, is a compact, repetitive, short pulse linear induction accelerator. The array of PCSS, which drive the accelerator, will switch 75 kA and 250 kV in 30 ns long pulses at 50 Hz. The accelerator will produce a 700 kV, 7kA electron beam for industrial and military applications. In the low power regime, these switches are being used to switch 400 A and 5 kV to drive laser diode arrays which produce 100 ps optical pulses. These short optical pulses are for military and commercial applications in optical and electrical range sensing, 3D laser radar, and high speed imaging. Both types of these applications demand a better understanding of the switch properties to increase switch lifetime, reduce jitter, optimize optical triggering, and improve overall switch performance. These applications and experiments on the fundamental behavior of high gain GaAs switches is discussed. Open shutter, infra-red images and time-resolved Schlieren images of the current filaments, which form during high gain switching, are presented. Results from optical triggering experiments to produce multiple, diffuse filaments for high current repetitive switching are described.

Optically activated switches for low jitter pulsed power applications

Optically activated high voltage switches are commonly used in pulsed power systems for reliable low jitter, multichannel and multiswitch (low inductance) applications. In addition to low jitter switching, optical activation provides a high degree of electrical isolation between the triggering and switching power systems simplifying pulsed power design. The disadvantages of optical triggering for large-gap gas switches are the optical energy, line-of-sight optics, and system maintenance required to obtain reliable operation. This paper describes two technologies which can reduce or eliminate these disadvantages and provide more flexible optically activated switches for pulsed power systems. One approach is to reduce the optical trigger energy requirement for gas gap switches. Shorter optical pulses require less energy to initiate a plasma discharge. An experiment is being assembled to trigger a 50-100 kV gas gap switch with 120 fs wide optical pulses. Lower trigger energy has also been demonstrated by the introduction of metallic aerosols into a gas gap W. Frey (1997). The apparatus will be added to this experiment to reproduce these results and determine the optical energy and power density requirements over a range of wavelengths and pulse widths. The status of this experiment will be discussed. A second approach uses solid state switching in two configurations: (1) as the main switch and (2) as a trigger. High-gain photoconductive semiconductor switches (PCSS) are practical for some direct pulsed power switching applications. We have demonstrated switching up to 220 kV and 8 kA. Higher power optically activated switching can be obtained by combining solid state and gas gap switching technologies. Multimegavolt (MMV) switches can be triggered with fiber-optically triggered, remotely located PCSS. By placing the compact PCSS trigger extremely close to the trigger point, reliable, low-jitter, high power switching is achieved with low energy fiber-optic trigger systems that can easily be controlled and adjusted from a remote control center. Power for the trigger system can be derived from the electrical fields near the trigger, so all electrical cables to the trigger system are eliminated and replaced with 100 micron diameter fibers that trigger and monitor the operation of the system. Results from experiments with PCSS triggered gas gap switches and the design for a PCSS triggered multimegavolt switch will be reported. PCSS switching properties including new picosecond pulse results and fabrication procedures for improved longevity will also be described. A 120 fs wide 780 nm laser pulse was used to radiate THz bandwidth pulses with a GaAs PCSS operating in the linear mode. New approaches for PCSS contact fabrication are being developed and tested to simplify the growth procedure, increase the current per filament capability, and improve device longevity. Progress continues to make PCSS a more useful component for pulsed power applications.