Degradation of optical reflectivity of in-vessel mirror materials by helium bombardment

The effect of helium irradiation on in-vessel mirror materials, i.e. molybdenum and rhodium, are investigated experimentally. By the exposure to helium plasmas with low incident ion energy (∼50 eV) at different surface temperatures, the optical reflectivity of molybdenum and rhodium decreases significantly. From the surface analysis, it is shown that fiberlike nanostructure is formed on molybdenum surface when the surface temperature is high (at 1500 K), while rough surface is observed when the surface temperature is low (<1000 K). The decrease in the optical reflectivity is significant particularly for short wavelength ranges, typically, less than 300 nm. The results indicate that the helium irradiation should be taken into account for in-vessel mirror materials for the optical diagnostics in ITER.