Abstract
The use of dielectric mirrors has been considered over the past few decades for both magnetic and inertial confinement systems. These mirrors are comprised of multiple thin bi-layers of high and low refractive index materials deposited onto a substrate. For magnetic fusion systems, dielectric materials are being primarily considered for use in plasma diagnostic systems. For the High Average Power Laser (HAPL) inertial fusion power program, dielectric mirrors are being considered for reflecting the laser beam through the reactor structure. This paper discusses the neutron exposure expected in the HAPL dielectric mirrors and an experimental program comprised of fabrication of advanced dielectric mirrors and testing of these mirrors exposed to prototypical irradiation environment. The specific purpose of the fabrication program was to screen mirror types for their irradiation performance. Specifically, three dielectric mirror types were fabricated to reflect at the KrF laser wavelength of 248 nm and these mirrors irradiated at ~ 175�C in the dose range of 0.001 to 0.1 x 1025 n/m2 (E>0.1 MeV.) The neutron dose range spans the HAPL fluences expected based on calculations from with a recently developed 3-D Monte Carlo code. Mirror reflectivity was measured on as-irradiated and on 300 and 400�C vacuum annealed mirrors. The best performing mirror overall, the alumina/silica multilayer mirror, did not appear to have degraded reflectivity in the as-irradiated or the as-irradiated and annealed conditions. For the highest dose, annealed condition degradation was observed in the hafnia silica mirror. Additionally, laser induced damage threshold (LIDT) was measured on the best performing mirror (the alumina/silica mirror) with a resulting threshold of > 1 J/cm2, and little effect of the neutron irradiation. These findings are somewhat in contradiction to earlier work on dielectric mirrors, which suggested poor performance of dielectric mirrors at an order of magnitude lower neutron dose. In conclusion, the current findings, while preliminary, suggest the possibility for using dielectric mirrors to much high dose levels.
