Short-pulse KrF amplifier using spatially tunable x-ray preionization

The small saturation energy density of excimers requires amplifiers of large cross sections for amplification of short pulses of already medium power. Homogeneous excitation of large volumes of fluorine-based gas mixtures by discharge pumping is a critical interplay of the properties of both pumping and preionization, generally necessitating an intense spatially and temporally controlled x-ray preionization. In the present realization, the stringent intensity requirements of preionization are fulfilled by reducing the pulse duration of the x-ray flash to ∼16 ns and by positioning the x-ray source in the near vicinity of the active volume. It is proven both theoretically and experimentally that by proper choice of the positions of two cylindrical x-ray guns, the spatial distribution of preionization can be tuned to (and around) the optimum distribution. In this way, the spatial distribution of the discharge can also be controlled, giving a practical method to compensate for eventual inhomogenities of the E-field of excitation and to tune the discharge to the desired geometry. In this paper, design considerations and experimental realization of a KrF excimer amplifier of ∼5 × 4 cm² cross section and a spatially tunable x-ray preionization are presented.