A pulsed power system is an electric circuit that accumulates electric energy during a comparatively long time and then releases the stored energy in the form of a brief short pulse. Figure III‑1 displays an example of the pulse compression, the output energy equals to the input energy, the 1 kW during 1s input is compressed into a 1 GW, 1µs output. This system consists essentially of an energy-storage element such as a capacitor, inductor or transmission line charged by means of a high power source and a power switch. A basic PPS diagram is given in Figure 1. In this model, the temporal profile of generated voltage pulse directly relates to the action of the power switch.
Fig. 1 Principle of pulse compression (a) input waveform (b) output waveform
A mercury-wetted reed pulser was normally used to produce voltage pulses with sub-nanosecond rise time. However, this pulse generator is operated merely at low pulse repetition frequency and relative low voltage. Spark gaps are frequently used in high-voltage application thanks to its high breakdown voltage. For instance, high voltage pulses greater than 100 kV, a “Marx” generator based on spark gaps switches is commonly used. Whereas, its repetition rates is also limited. Power semiconductor switches are found to have wide applications for generation of HV pulses, providing triggerable repetition rates up to kHz range. Indeed, numerous semiconductor devices are served as switches within pulse generators including thyristors, metal-oxide-semiconductor field effect transistors (MOSFE), insulated gate bipolar transistors (IGBT), etc.
The thyristor is the first semiconductor switch which can be controlled by external triggering signal, it can sustain very high voltage at the expense of switching speed. The insulated-gate bipolar transistor (IGBT) was developed in the eighties and expanded rapidly in the 1990s, this component inherits the high power handling capability of the thyristor, however, it still has a relatively slow switching speed of 200 ns. Comparing to the IGBT and thyristor, the operating frequency of MOSFET is very high in low voltage applications (less than 200 V). None of the power semiconductor switchs meets well the requirements, i.e. fast switching less than 100ps and maximum output voltage up to 2 kV.
After the photoconductivity effect was firstly observed by Jayaraman and Lee in 1972, Auston (1975) demonstrated the switching, gating and sampling of voltage pulse in a microstrip line using Si photoconductors. The basic principle of this device is to increase the electrical conductivity of semiconductor by photoelectric effect. Photoconductive switch is a very suitable candidate for the high-power pulse application because they are simple, scalable, and optically controllable. Several advantages for pulsed-power application of the photoconductive switch result from the optical control. With the advent of femtosecond lasers, optoelectronic devices have subpicosecond temporal resolution. Triggering conventional power switch is generally made by driving circuits connected to the switch. The laser controlled photoconductive switch isolates the controlling signal from the power switch. This isolation makes the switch easily integrated with other electronic devices. An additional advantage of the photoconductive switch is that they can be scaled to high voltages and currents in a device without losing the switching speed.