First direct comparative test of single crystal rhodium and molybdenum mirrors for ITER diagnostics

All optical and laser diagnostics in ITER will use mirrors to observe the plasma radiation. In the ITER environment, mirrors may become contaminated with plasma impurities hampering the performance of corresponding diagnostics. An in-situ mirror cleaning is proposed, which relies on ion sputtering of the contaminants and the affected mirror material. Previous research demonstrated the advantages of single crystal (SC) molybdenum (Mo) under sputtering conditions over polycrystalline concepts. Recently, the first single crystal rhodium (Rh) mirrors became available and tests have been started at the Forschungszentrum julich. In a direct test, SC Rh and SC Mo mirrors were exposed under identical conditions in steady-state helium plasmas in the linear plasma device PSI 2. The energy of impinging ions was similar to 100 eV matching conditions expected in the in-situ cleaning system in ITER. During exposure, molybdenum mirrors lost 420-500 nm due to sputtering. Rhodium mirrors lost about 1 mu m. Exposure corresponded to 50-100 cleaning cycles in ITER. Nevertheless, rhodium mirrors have preserved their specular reflectivity, showing the maximum degradation of less than 7% at 250 nm. Molybdenum mirrors demonstrated a moderate decrease of specular reflectivity of 12-25%. Results open new perspectives for the use of single crystals in ITER diagnostics. (C) 2017 Elsevier B.V. All rights reserved.