Abstract

E-beam-pumped KrF lasers are very promising drivers for Inertial Confinement Fusion due to their high efficiency, short radiation wavelength and a large bandwidth, as well as to principle feasibility to operate with repetition rate of a few Hz at multi-kilojoule energy level¹. One of the problems to be developed for reliable rep-rate KrF laser operation is a stability of output laser windows. In addition to intense UV laser light they would be exposed to fluorine etching (it is contained in a working gas mixture), and to irradiation by bremsstrahlung X-rays and energetic scattered electrons. A number of optical materials such as fused silica (SiC₂), calcium fluoride (CaF₂), magnesium fluoride (MgF₂), and synthetic leucosapphire (Al₂O₃) have been comparatively examined in regard of chemical resistance to fluorine, nonlinear and ionizing-radiation-induced absorption at KrF laser radiation wavelength of 248 nm. These experiments being carried out in extremely severe conditions with e-beam current density up to 200 A/cm², e-beam power density 20 MW/cm², and laser intensity 250 MW/cm² at EL-1 KrF installation² revealed that SiO₂ would be the best challenge for laser windows provided AR coating will prevent its etching by fluorine. Meantime Al₂O₃ and MgF₂ would be the promising coating materials. CaF₂ windows representing the lowest nonlinear absorption and absolute resistance to fluorine, are relatively cheap and available of a large size (~1-m diameter). They might be the only alternative to SiO₂ ones. But the data concerning CaF₂ induced absorption were rather inconsistent by today.

© 2000 IEEE