GARPUN laser experiments on e-beam transportation through Al-Be foil and optical window stability addressed to the NRL rep-rate-e-beam-pumped KrF facility

摘要

Two key engineering issues in the development of a KrF laser driver for Inertial Fusion Energy are high transmitting and long life e-beam window and optical laser windows. We have performed experiments with e-beam-pumped KrF laser installation GARPUN on the transport of relativistic e-beams through Al-Be and Ti foils and compared them with Monte Carlo numerical calculations. It was shown that both 50-mum thick Al-Be foils and 20-mum thick Ti foils had equal transmittance of about 75percent for approx300 keV, 50 A/cm~2, 100 ns e-beams. However in both cases the observed transmission was lower than calculated one. In contrast to Ti foil, whose surface was strongly etched by fluorine, no surface modification or fatal damages were observed for Al-Be foil after approx1000 laser shots and protracted fluorine exposure. It was shown that applied magnetic field of approx1KG significantly reduced electron scattering both across and along laser cell at typical pumping conditions with 1.5-atm pressure working gas. Without magnetic field irradiation of optical windows by scattered electrons resulted in additional transient and residual induced absorption of laser radiation. The analysis of different optical materials for KrF laser windows and coatings has also been done.