Pulsed power design for a small repetitively pulsed electron beam pumped KrF laser

Abstract

Electra is a 700 J, repetitively pulsed, electron beam pumped krypton fluoride (KrF) laser that is developing technologies to meet the inertial fusion energy (IFE) requirements for rep-rate, efficiency, durability and cost Sethian, JD et al., (2002). We have designed a pulsed power system for the preamplifier in the electra KrF laser system. This preamplifier is designed to produce 40 J of laser light in a 40 nsec pulse which will be used to provide the input to the main amplifier. The pulsed power for the front end will serve two roles. It will complete the laser system and it will serve as the demonstrator for the new advanced pulsed power topology that can meet the fusion energy requirements for durability, repetition rate, and cost. The pulsed power will first employ a gas-switched Marx, with anticipated maintenance intervals similar to that of the existing electra main amplifier Sethian, JD et al., (1999). Later the driver will be replaced (circa 2006) with a solid-state-switched Marx generator D. Weidenheimer et al., (2002). The output requirements for the pulsed power driver into counter-streaming electron beam diodes are 20/40/30 nsec (t/sub rise//flat-top/t/sub fall/), 150-175 kV, 60-80 kA per side and a 1.1 ohm nominal impedance. The pulser will operate in single-shot, burst, and continuous modes up to 5 pps, with 1 nsec (1 sigma) or less absolute timing jitter. Rather than build an individual driver for each e-beam diode as was done for the electra main amplifier Sethian, JD et al., (1999), a single pulsed power driver is coupled to the opposing electron guns via four liquid-filled transit time isolators (TTIs). These TTIs are necessarily compound (oil/water/oil) in order to balance their electrical lengths against unequal mechanical lengths. The Marx is gas-insulated and charges a 1.1-ohm water pulse forming line (pfl) in less than 100 nsec. An output magnetic switch with a saturated inductance of less than 14 nH discharges the pfl into the four parallel TTIs. A set of four (2 each side) Z-stack inverted bushings connect the TTIs to the diodes. This paper will present and discuss all major aspects of the electrical and mechanical design, as well as the anticipated performance from circuit simulations. The laser driver is scheduled to be delivered to and installed at the naval research laboratory's electra laboratory in late 2003.