Feedback-stabilized Nd:YLF amplifier system for generation of picosecond pulse trains of an exactly rectangular envelope

We describe the amplifier chain of the photocathode laser of the TESLA Test Facility. This chain is optimized for amplification of trains of picosecond pulses. At its output, pulse trains with a rectangularly shaped envelope of 0.7% flatness and a long-term stability of the energy in the individual picosecond pulses in excess of 0.9% are obtained. This stability is achieved by using a computerized feedback control system, which we describe in detail together with the versatile feedback algorithm applied. The feedback loop performs a tight adjustment of both the shape and magnitude of the flashlamp pulses as well as the starting time of the pump with respect to the pulse train. High-current insulated gate bipolar transistors are used to directly manipulate the flashlamp current in a microsecond time scale.