Substrate-dependent deposition efficiency on mirrors exposed in the TCV divertor

Given the planned extensive use of metallic mirrors in the optical diagnostics systems of ITER, the study of reflectivity changes induced by erosion and/or redeposition of impurities on the mirror surfaces is of primary importance for the reliability of the diagnostics signals. This contribution will demonstrate that the mirror material choice can exert a significant influence on the relative importance of erosion/redeposition affecting the mirror reflectivity. A dedicated manipulator has been designed to allow exposure of mirror samples in the divertor region of the Tokamak à Configuration Variable (TCV) tokamak. Mirrors from different materials have been exposed both during short experimental campaigns and boronisation procedures. Before and after exposures the mirrors were characterized with different surface analysis techniques (XPS, SIMS, SEM, EDX, ellipsometry). Under identical exposure conditions, the mirror material can strongly influence the deposit thickness found on the sample: the carbon layer thickness on a Si sample is much higher than on a Mo sample. These results have potentially important consequences for the first mirror material choice in ITER.