Time- and space-resolved interferometer measurements of a laser-produced plasma waveguide

Laser-heated plasma that generates soft x-rays is avery attractive light source for x-rayprojection lithography because it is a point source with high brightness. This system can be also relatively compact compared with other light sources (synchrotrons, gas puff plasma pinch generators). To realize an x-ray source of high average power enough for industrial applications, we are developing a high-repetition-rate pulse Nd:YAG slab laser system for x-ray driver. Its final goal is to reach pulse energy of 1 J, duration time of 10 ns, up to few hundreds pulse repetition rate, and high average power more than 100 W with a tabletop size. The laser amplifier system is illustrated by Fig. 1. The optical bench size is 1.2 X 2.4 m. In this system, a zig-zag slab geometry, imaging relay optics, and a SBS phase conjugation mirror have been used to reduce thermal wavefront distortion and other thermal effects that are very serious at high average output power level.' Two slab amplifiers were used, each of which included a Brewster-angle cut Nd:YAG slab (6.4 X 25 X 183 mm) pumped by two Xe flash lamps (10F6). Ring-resonator like amplifying optical pass was successfully chosen in this system to accommodate all of the amplifier heads, four-pass angular multiplexing, and imaging relays. Saturation single-pass small signal gain of the amplifier has been measured to be 6. We usually operated each amplifier at gain 3 in our experiment. Fluorinert (FC-75) was filled in the SBS mirror cell and its maximum reflectivity reached more than 80%. With this laser system maximum pulse energy of 0.72 J has been obtained with an oscillator output energy of 1 mJ. Maximum pulse repetition rate we tested so far was 160 pps and beam divergence was 0.6 mrad in X and Y-directions at this rate. When the focused beam of high-repetitionrate laser pulses heats metal targets, damage on x-ray optical components resulting from the plasma debris would not be negligible. Therefore the use of cryogenic targets such as inert gases is of critical importance.2 To study the x-ray emission intensity from