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It has been reported that the reflectivity of the retro-reflectors in LHD reduced drastically in a short period of time due to plasma-surface interaction. In the present work, it was tried to find mec...hanism for the reflectivity reduction by carrying out reflectivity measurements, surface observation and surface chemical analysis. The surface at the central part of the reflectors, which were used for the reflection of the laser, were covered by thick deposition composed of Fe, Cr, Ni and O, because deposition exceeds erosion by sputtering. Due to the inner corner structure like a funnel, impurity atoms were effectively transported toward the center by sputtering and this directional movement resulted in the very rough surface structure, which was the main reason for the reduction of the optical reflectivity. These results suggest effective measures against the reduction of reflectivity of the retro-reflectors in LHD.show more
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