Abstract
An active beam-pointing stabilization system has been developed for a high-power KrF laser system to eliminate the long-term drift of the directional change of the beam in order to have a stable focusing to a high intensity. The control of the beam direction was achieved by a motor-driven mirror activated by an electric signal obtained by monitoring the position of the focus of the output beam. Instead of large sized UV-sensitive position sensitive detectors a simple arrangement with scatter plates and photodiodes are used to measure the directionality of the beam. After the beam stabilization the long-term residual deviation of the laser shots is ~14 μrad, which is comparable to the shot-to-shot variation of the beam (~12 μrad). This deviation is small enough to keep the focal spot size in a micrometer range when tightly focusing the beam using off-axis parabolic mirrors.
The present status of the development of a high-power KrF laser system, Ashura, is described. The main amplifier has generated 710 J (95 ns) at the pumping density of 1·lMW/cm3 with the wall plug efficiency of 2·0%. Maximum power of 9·0 GW (200 J/22 ns) per beam has been obtained from the beam lines of six-time pulse multiplexing. Power density of 1 × 1014 W/cm2 has been achieved on target with a 10−6 pre-pulse.
Fundamental parameters are studied for the detailed design of a picosecond high power KrF laser system. The saturation energy density of 2.3 mJ/cm2 and gain recovery time of 1.9 ns in an e-beam pumped KrF laser are observed. A 6 ps pulse is amplified to 1.9 J. The system at the Electrotechnical Laboratory is expected to deliver 10 ps pulses with total energy of several tens of joules in several beams.