Temperature dependence of the reflectivity degradation in single and polycrystalline Mo mirrors under the irradiation with low-energy helium ions

Abstract The degradation of the optical properties of single and polycrystalline Mo mirrors for plasma diagnostics under irradiation with low-energy helium ions at room temperature and 400 °C has been studied by in situ reflectivity measurements, SEM and TEM observations, and spectroscopic ellipsometry. It is found that the degradation of the reflectivity of Mo irradiated at high temperature is comparatively small in comparison with that at room temperature even at an irradiation to 1 × 1023 ions/m2. The degradation mechanism of the optical properties such as the reflectivity, the extinction coefficient and refractive index, and the temperature dependence of the degradation have been discussed in terms of the damage formation and diffusion in the surface and sub-surface regions of the Mo mirrors within the laser penetration depth. The single crystal-Mo mirror exhibits good performance under irradiation with 1 keV He+ ions even at room temperature.