The features of this invention allow construction and operation of a variety of high voltage, high repetition rated pulse generators of the Marx type that are switched with photon initiated semiconductor switches of the closing type. The photon initiated semiconductor switches can be constructed with bulk materials or in layered devices such as thyristors. Variations on the invention permit the formation of shaped high voltage pulses; particularly those that are nearly rectangular: with controlled rise and fall times, minimal or no overshoot, and minimal voltage ripple.