Fiber-optic Control Of Current Filaments In High Gain Photoconductive Semiconductor Switches

The discovery of current filaments in GaAs photoconductive semiconductor switches (PCSS) raised concerns about the location and density of the current distribution during high gain switching. This paper describes experiments using fiber-optic coupled laser diode arrays (LDA) to control the location and number of current filaments in GaAs PCSS. Infrared (IR) images of the recombination radiation, that is emitted from the surface of the PCSS, show precisely where the current is concentrated. These were recorded as the fiber-optic configuration and switch test parameters were varied. Up to twenty individually-coupled laser diodes and fibers were used to initiate multiple, simultaneous filaments. Fibers were configured in a row both parallel and perpendicular to the direction of current. Two types of experiments were performed: high current and high voltage. System impedances of 0.25 {Omega} and 50 {Omega} were used with switch currents up to 5.2 kA and initial charging voltages up to 100 kV, respectively. Switch sizes were 1{times}5 mm{sup 2} and 15{times}38 mm{sup 2} (parallel {times} perpendicular to the current). With the 0.25 {Omega} system charged to 4 kV (40 kV/cm across the PCSS), individual filaments were triggered with 150 nJ of optical trigger energy. In this case, filaments were triggered atmore » both ends near the contacts with pairs of 250 {mu}m fibers, each of which delivered 75 nJ in 140 ps. The use of multiple fiber pairs to distribute the current more uniformly across the switch improved switch performance. It allowed higher total currents to be switched with narrower switches. With the 50 {Omega} system charged to 100 kV (67 kV/cm across the PCSS), individual filaments were triggered with as little as 90 nJ of optical trigger energy. The trigger pulse was delivered in 2 ns through one fiber which was 400 {mu}m in diameter.« less