Hot atoms with chaotically directed velocities are generated by charge-exchange with plasma ions of neutrals recycling from the vessel wall of a fusion reactor. Some of them flee into openings in the vessel made for ducts guiding to diagnostic installations; in particular, to first mirrors for optical observations. On the one hand, hot atoms, hitting the mirror directly, can erode its surface. On the other hand, impurity species, released from the walls of the vessel and of the diagnostic duct, migrate to the mirror and can be deposited there. Both the erosion of and impurity deposition on the mirror decline its reflection properties.Models elaborated to describe processes above are outlined, including a 2D kinetic description for neutral species in the vicinity of the duct opening, an assessment for the erosion of the duct walls and mirror surface by hot atoms, estimates for influxes of the wall material into the duct and a consideration of the migration of impurity atoms along the duct. Calculations are done for the conditions predicted for a fusion reactor like DEMO. The rates for erosion of and impurity deposition on first mirrors of Mo are assessed versus input parameters such as the duct radius, the distance from the opening to the mirror, the density ng of the working gas in the duct, the probabilities for impurity sticking to the duct wall and mirror surface. It is demonstrated that, by ng exceeding a level of m−3, the mirror sputtering can be reduced to the target level of 1 nm per full power year. Moreover, for long enough ducts the erosion rate of impurities deposited onto the mirror exceeds their deposition one and no formation of impurity precipitations on the mirror surface has to be expected.
Studies of reflectivity degradation of retroreflectors in LHD and mitigation of impurity deposition using shaped diagnostic ducts and protective windows