High Gain GaAs Switches For Impulse Sources; Measurement Of The Speed Of Current Filaments

Abstract

A high peak power impulse pulser that is controlled with high gain, optically triggered GaAs Photoconductive Semiconductor Switches (PCSS) has been constructed and tested. The system has a short 50 ohm line that is charged to 100 kV and discharged through the switch when the switch is triggered with as little as 90 nJ of laser energy. The authors have demonstrated that the GaAs switches can be used to produce either a monocycle or a monopulse with a period of total duration of about 3 ns. For the monopulse, the voltage switched was above 100 kV, producing a peak power of about 48 MW to the 30 ohm load at a burst repetition rate of 1 kHz. The laser that is used is a small laser diode array whose output is delivered through a fiber to the switch. The current in the system has rise times of 430 ps and a pulse width of 1.4 ns when two laser diode arrays are used to trigger the switch. The small trigger energy and switch jitter are due to a high gain switching mechanism in GaAs. This experiment also shows a relationship between the rise time of the voltage across the switchmore » and the required trigger energy and switch jitter. The time evolution of the current filaments in an optically triggered, high gain GaAs switch was studied by recording the infrared photoluminescence from the filaments. When the system is triggered with two laser diode arrays that are activated within 1 ns of each other, two current filaments are observed. By delaying one laser with respect to the other, the evolution of the filament was recorded in a time resolved fashion. 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 trigger lasers, the tip velocity is measured to be up to 5.9 {+-} 1 {times} 10{sup 9} cm/s.« less