ON THE CHOICE OF MATERIALS FOR THE FIRST MIRRORS OF PLASMA DIAGNOSTICS IN A FUSION-REACTOR

The reflectivity coefficient, R, of mirrors fabricated from different metals (Al, Cu, Mo, Ta, W) in the wavelength range 0.25-0.65 mu m was investigated under long-term sputtering with ions of ECR plasma of hyd rogen or deuterium, and R values versus thickness of sputtered layer w ere measured. For copper mirrors the influence of ion energy and ion e nergy spectrum on optical reflectivity was investigated in detail. Bec ause a strong effect was found, the comparative test of mirrors made o f different metals was carried out using ions with a wide energy distr ibution (0.1-1.5 keV) to have a more adequate simulation of fusion rea ctor environment conditions. The results obtained show that, with the exception of Al, the decrease of reflectivity coefficient with thickne ss of sputtered layer has approximately similar rate for metal tested. Thus, mirrors made of materials having lower sputtering coefficient, Y, will withstand a higher charge exchange atom fluence, in qualitativ e agreement with R/Y criterion suggested by the authors earlier. (C) 1 998 Elsevier Science B.V. All rights reserved.