Degradation of reflectivity in stainless steel mirrors under irradiation with low-energy helium ions

To evaluate the reflectivity change of first mirrors for plasma diagnostics, in situ measurement of the reflectivity in stainless steel has been done under the bombardment of low-energy helium ions. The damage structure associated with the degradation of the reflectivity was also examined by in situ transmission electron microscopy. It was found that the degradation behavior of optical reflectivity showed two stages; a rapid reduction stage occurred at a relatively low fluence of up to 1 × 1021 He/m2, and the following one decreased monotonically without saturation. The eventual reduction in the reflectivity reached about 30% of the initial value by irradiation with 8 × 1022 He/m2 at room temperature. The degradation of the reflectivity depended on the irradiation energy of helium in the order of 1, 5 and 3 keV, and its deep correlation with the radiation damage in the penetration depth of the laser light near the surface was suggested.