Trigger systems are becoming increasingly important in pulsed power systems with large numbers of switches 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 r-m-s jitter, scalability to high power (220 kV and 6 kA demonstrated), and device lifetimes up to 108 shots with 20 A 5 ns wide pulses. 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 switches are required for repetitive pulsed power applications.The key parameters that impact PCSS lifetime are voltage, current, and pulse width. Voltage affects the lifetime of a lateral switch when the electric field near the surface of the switch approaches the surface breakdown limit, which is approximately 100 kV/cm for pulse charged switches under transformer oil or FluorinertTM (a liquid dielectric). The current in high-gain GaAs PCSS always forms filaments, so this lifetime dependence can be considered in terms of the current per filament, and most of our lifetime testing is done with switches producing a single main filament. Since PCSS can have sub-nanosecond risetime and jitter, most of our interest in lifetime is for switches that produce 1-100 ns long pulses. These requirements lead us to testing high gain PCSS in high speed, 50 ohm transmission line, discharge circuits that can deliver up to 300 A. Control of the PCSS is achieved with a fiber-coupled laser directed between the PCSS metal contact pads resulting in randomly formed filaments (one primary).Devices demonstrating long lifetimes are operated at up to a few kilohertz, whereas higher current and longer pulse tests are operated at lower repetition rates. In all cases, rep-rates are below the limit where bubbles form in the liquid dielectric. New switches are always tested at 20 A for direct comparison to the lifetime data that we have accumulated over the last 20 years. Higher current tests are performed to predict switch lifetimes for specific applications that don't require such long device lifetimes. This paper will discuss testing procedures, circuits, and dramatic changes in PCSS component lifetime due to contact methods (e.g. soldering versus ribbon bonding).

Cost and complexity of pulsed power systems continue to increase as researchers strive for more precise control and flexibility. Systems with large numbers of switches or large numbers of switch trigger times result in the need for more cost effective high performance triggering systems. These needs can be addressed by photoconductive semiconductor switches (PCSS) that have demonstrated 300 ps jitter triggering of 300kV switches. This previous work has motivated continued research into the trigger system parameters that allow for reliable cost effect operation'.To support this research a test-bed has been developed to allow PCSS trigger testing with a variety of high voltage switches (HVS) and switch configurations. The multiple subsystems used in this test bed include: (1) the laser diode array (LDA) driver circuit, (2) the PCSS trigger circuit, and (3) the circuit that is switched by the HVS. Items of particular interest are: (1) the benefits and issues with pulse charging vs. DC charging the PCSS trigger system, (2) fiber coupled versus multi-filament laser diode PCSS triggering, (3) high bandwidth diagnostics to measure and determine the impact of sub-nanosecond rise time and jitter triggers for the HVS, and (4) the PCSS trigger circuits required to determine minimum voltage, current, and pulse width for optimal HVS triggering. Our testbed consists of a single module ("brick"), from a linear transformer driver (LTD)2 experiment, housing two 40 nF capacitors, the HVS being tested, and a load. A low inductance HVS that can handle +/-100 kVDC and 5OkA are being tested with this system. The laser diode array (LDA) that triggers the PCSS can be located with the PCSS and the HVS or in the screen room. The LDA is driven by a low energy avalanche trigger circuit that can be optically triggered and floated to HVS. This paper describes the requirements, methods, and the apparatus developed to determine the PCSS minimum electrooptical requirements for optimal HVS triggering. Specific current and voltage waveforms obtained from the sub-systems will be discussed along with the parameters adjusted to deliver a range of pulse widths, rise times, and energies to the HVS.