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 hydrogen or deuterium, and R values versus thickness of sputtered layer were measured. For copper mirrors the influence of ion energy and ion energy spectrum on optical reflectivity was investigated in detail. Because a strong effect was found, the comparative test of mirrors made of different metals was carried out using ions with a wide energy distribution (0.1-1.5 keV) to have a more adequate simulation of fusion reactor environment conditions. The results obtained show that, with the exception of Al, the decrease of reflectivity coefficient with thickness 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 qualitative agreement with R/Y criterion suggested by the authors earlier. (C) 1998 Elsevier Science B.V. All rights reserved.