Comparative investigations of stainless steel and molybdenum first mirrors cleaning using radio frequency plasma

Highlights: • Radio frequency plasma cleaning method was effectively applied to SS and Mo mirrors. • The diffuse reflectivity of cleaned SS mirror was lower than 5%. • Mo mirror showed different behaviors from SS mirror. • The carbide was formed at the interface of Mo mirror, but SS did not. - Abstract: First mirrors (FMs) used in optical diagnostics in ITER are confronted with a serious threat of the re-deposition of impurities eroded from plasma facing components. The radio frequency plasma cleaning method was applied to remove deposits on stainless steel (SS) and polycrystalline molybdenum (Mo) FMs for recovering their high reflectivity. After cleaning process, the amorphous carbon film deposited on SS FM was successfully removed. The total reflectivity was recovered up to 95% and the diffuse reflectivity was lower than 5%. The roughness of the cleaned SS mirror was almost not changed compared with the initial value. For Mo mirror, the total reflectivity was restored to 85% in the visible wavelength, while the diffuse reflectivity was increased to 17% due to higher roughness. The XPS analysis on Mo mirror surface showed that Mo carbide was formed at the interface between the substrate and film, which made a contribution to the difference of cleaning results from SS mirror.