Effect of irradiation with low-energy helium ions on the spectral reflectance of single-crystal molybdenum mirrors

A comparative study of the effect of irradiation with helium ions on the optical properties of single-crystal molybdenum mirrors with crystallographic orientations $\langle$110$\rangle$ and $\langle$111$\rangle$ is presented. The irradiation mode corresponds to the conditions in the built-in systems for plasma cleaning of the input mirrors from contamination in the optical diagnostics of the ITER international thermonuclear reactor using helium as a working gas. Such irradiation results in a variation in the specular reflection and diffuse scattering of the mirror that is virtually independent of the initial structure of the surface layer of the mirror and the duration of the irradiation process. The authors explain the changes of the optical characteristics of the mirror using the formation of nanosized bubbles in the surface skin layer. The Rayleigh scattering of incident radiation by the bubbles leads to a decrease in the intensity of specular reflection and an increase in the diffuse scattering. A model of the formation and growth of nanosized bubbles and their effect on optical properties is proposed. The results must be taken into account in the analysis of experimental data in the ITER optical diagnostics after removing of contaminants using helium, when choosing a working gas for plasma cleaning systems for mirrors, and also for the formation of a nanoporous structure in a thin surface layer of metals.