Irradiation Effects on Dielectric Mirrors for Fusion Power Reactor Application

Abstract

Dielectric mirrors have been considered for both magnetic and inertial confinement systems. Such mirrors are comprised of multiple thin bi-layers of high and low refractive index materials deposited onto a substrate. 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.) 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 was measured on the best performing mirror (the alumina/silica mirror) with a resulting threshold of > 1 J/cm², For this mirror, the damage threshold was not discernibly degraded by neutron irradiation. These findings are somewhat in contradiction to earlier work, 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 higher dose levels.