The impact of thermo-oxidative wall cleaning in the DIII-D Tokamak on performance of mirrors for ITER diagnostics

Tritium retention in future fusion devices such as ITER is a critical issue for the machine availability. Thermo-oxidation is one of the promising techniques for tritium removal from carbon based co-deposits. To investigate possible collateral effects of thermo-oxidative wall cleaning on diagnostic components, metallic mirrors made from ITER-candidate materials were exposed in the DIII-D tokamak during an oxygen bake campaign. Molybdenum (Mo) and copper (Cu) mirrors were exposed for more than 2 hours in an oxygen-helium atmosphere (20% O2 + 80% He) at temperatures 160 C and 350 C and in-vessel pressure of 13.3 mbar. For exposed mirrors, strong surface oxidation and corresponding drop of the reflectivity in ultraviolet and visible wavelength ranges were observed. The decrease of the specular reflectivity of Mo mirror was as high as 50%, while it reached even 90% in case of Cu mirror. These results cause concerns about the safety of optical components in ITER. The applicability of thermo-oxidative wall cleaning for ITER are discussed in this contribution.