Sputtering tests of single crystal molybdenum and rhodium mirrors at high ion fluence for in situ plasma cleaning of first mirrors in ITER

First mirrors (FMs) are foreseen to be employed for optical diagnostics to view the burning plasma in ITER. Considering the dramatic degradation of the reflectivity of FMs due to deposits on the mirror surface, plasma cleaning is viewed as a crucially important method to actively remove deposits for recovering reflectivity of FMs. Single crystal (SC) molybdenum (Mo) and rhodium (Rh) mirrors are considered as the most promising materials for the application as FMs in ITER due to the capability of preserving good reflectivity under sputtering conditions. With aim of seeking the possible limit of sputtering ion fluence for SC Mo and Rh mirrors under the same conditions as assumed for in situ plasma cleaning of ITER, they underwent the exposure to helium ions with a high fluence of 4 × 1021 cm−2 in linear plasma device PSI-2. It was found that both SC Mo and Rh mirrors showed a slight reduction of the specular reflectivity (less than 10%) and a negligible increase of diffuse reflectivity. The amount of removed material after exposure in the case of SC Mo mirrors was 900 nm and around 1500–1900 nm for SC Rh mirrors, corresponding to about 90–190 in situ cleaning cycles in ITER. Compared with the results at lower fluence of 2 × 1021 cm−2 in 2015, the removed materials of all SC mirrors showed reasonable linear dependence on the ion fluence. Regarding the specular reflectivity, the SC Rh mirrors exhibited nearly the same degradation. The fact that the SC Mo mirrors had less degradation of the specular reflectivity than that at lower fluence, is presumably correlated with crystal orientation. The ability to preserve the reflectivity and resist plasma sputtering for SC Mo mirrors as a function of the crystal orientation will be investigated in the near future.