Estimation of the Degradation Rate of the In-Vessel Mirrors for the H-Alpha and Visible Spectroscopy Diagnostics in ITER

Mirrors for light transmittance will be used in more than 30 optical and laser diagnostics installed in the ITER tokamak reactor. Their reliable performance depends largely on the degradation rate of the first mirror directly facing the plasma. The fluxes of neutral atoms, mostly D, T, and Be, from the plasma are the most dangerous for the first mirror. The interaction with the high-energy atoms can lead to sputtering of the mirror surface; redeposition of beryllium sputtered from the walls of the vacuum chamber can lead to the formation of a film on the mirror. This paper gives a conservative estimate of the degradation rate of in-vessel mirrors in the equatorial channels of the H-alpha and Visible Spectroscopy diagnostics. Calculations were performed for stationary ITER operation using the Zemax OpticStudio program. The results show that the working area of the first mirror will be sputtered at a rate of no more than 100 nm/year, which is an acceptable erosion load for mirrors made of single-crystal molybdenum. Beryllium film formation is expected only at the edges of the mirror (outside the working area). Possible contamination of the working part of the mirror as a result of accidents or plasma events can be eliminated using the mirror cleaning system installed into the first mirror unit. For the other in-vessel mirrors, the estimate predicts no significant degradation owing to their location at a long distance from the plasma.