Stable reflectivity of bulk metallic glass mirrors for ITER optical diagnostic through an irradiation-induced self-recovery mechanism

Reflection mirrors or first mirrors for ITER diagnostic are subjected to intense irradiations that deteriorate the reflectivity of the diagnostic mirrors. In the present study, the reflective optical properties of bulk metallic glasses Co61.2B26.2Si7.8Ta4.8, Zr65Al7.5Ni10Cu17.5 and Zr63.7Al9.0Ni27.3 after being exposed to hydrogen sputtering and helium erosion were compared to those of polycrystalline Mo and W. Surface structure and specular reflectivity of the mirrors were characterized before and after the exposures. Compared to polycrystalline Mo, W and Zr-based bulk amorphous alloys Zr65Al7.5Ni10Cu17.5 and Zr63.7Al9.0Ni27.3, no significant changes in specular reflectivity were observed for the mirror made from the Co–B-based bulk amorphous alloys Co61.2B26.2Si7.8Ta4.8 after the exposures. The initial surface crystallization of the bulk metallic glasses after a short influx of ions is shown to reverse to amorphization upon further bombardments, unveiling an erosion-induced self-recovery amorphization mechanism that guarantees stable surface structures and henceforth stable reflectivities.