Avalanche Growth Calculations

INTRODUCTION A fast, efficient and reliable switch is the basic ingredient of a pulse power accelerator. -Two switches have been proposed so far: the solid state switch, and the vacuum photodiode switch. The solid state version has been tested to some extent, albeit at low (few kilovolts) level, with risetime around 10 ps in the radial line transformer configuration! The vacuum photodiode is being investigated by Fisher and Rao at Brookhaven National Laboratory.2 Common to both switches is the need of a short laser pulse; near infrared for the solid state switch, and ultraviolet for the vacuum photodiode switch. Another common feature is the poor energy gain of these switches: the gain being the ratio between the electrical energy switched and the laser energy needed to drive the switch. For the solid state switch, calculations. and experimental data’ show that the energy gain cannot exceed a value between 5 and 10. For the vacuum photodiode, the situation is somewhat similar, unless very high quantum efficiency, rugged photocathodes can be found. A closing switch also can be used to produce short pulses of RF at frequencies related to its closing time, using a well-known device called the frozen wave generator.3 For a risetime of the order of 30 ps, one could produce several Gigawatts of RF at Xband at very low cost.