Radiation resistance of transmissive optics for IFE reactor chamber and KrF laser driver

Complete text of publication follows. Laser driver of the Inertial Fusion Energy (IFE) power plant should operate at repetition rate of 5-10 Hz uninterruptedly for two years with pulse energy ∼ 1 MJ and overall efficiency ∼ 7%, producing totally ∼ 3 x 108 pulses. Among present candidates only DPSSL and e-beam-pumped KrF lasers can satisfy these severe conditions. High radiation stability is required for laser and reactor chamber optics suffering from different kinds of ionizing radiation. For instance, cumulative absorbed doses ∼ 1 MGy of hard X-rays in KrF laser or target chamber windows during operation run was shown to reduce their transmittance significantly. We performed comprehensive studies of transient and residual absorption induced in VUV, UV, and visible ranges in fused silica glasses Corning 7980, Russian KU-1 and KS-4V, crystals CaF2, MgF2, and Al2O3, being irradiated at several pulsed facilities by 280-keV e-beam, high-intensity UV laser light, and bremsstrahlung X-rays. Optics response to hard X-ray photons (hv ∼ 400 keV), which are usually generated during e-beam pumping of KrF amplifiers, was measured at a linac-based powerful quasi-CW X-ray source. It allowed X-ray dose rate ∼ 40 Gy/s and amassed does absorbed in samples volume ∼ 1 MGy. Another test bench rep-rate X-ray source based on high-voltage glow discharge produced average irradiation intensity 2-3 mW/cm2 in a 50 Hz train of ∼ 1 μs pulses with photon energy of hv = 6-20 keV. Being absorbed in a thin surface layer it provides dose rates up to 5 Gy/s. Just in the same soft X-ray range there is the most release of an imploding fusion contribution into X-ray spectrum illuminating KrF laser windows. The obtained results show that transient absorption at both 248-nm (KrF) and 353-nm (3σ DPSSL) wavelengths hardly affect on optics transmission, while residual absorption results in rather high losses especially for UV laser light. Annealing at elevated temperatures should be anticipated, which will reduce color centers formation and keep the initial optics transmission. Acknowledgements. This work was supported by Russian Foundation for Basic Research (grant No. 08-02-01330) and by KrF IFE program of the US Naval Research Laboratory.