Thermo-oxidation is controlled exposure in an oxygen-containing atmosphere at elevated temperature and is being considered as a technique for the de-tritiation of carbon-based codeposits in ITER. In addition, unplanned oxidation may also occur during accidental air ingress. The impact of thermo-oxidation on ITER diagnostic mirrors causes concerns. A dedicated study was performed in DIII-D, where molybdenum and copper mirrors were installed in the main chamber, in the divertor, and at a location remote from the plasma and exposed for [approximately]2 hours to a mixture containing 80% helium and 20% oxygen at a total pressure of 1.27 kPa. Mirrors in the main chamber and in the divertor were exposed at 350°C to 360°C whereas the temperature of mirrors in the remote area was [approximately]160°C.

Reflectivity of all mirrors was degraded after thermo-oxidation showing a decrease in the UV range from 60% to 10% for molybdenum mirrors and a 90% drop for copper mirrors at the wavelength 250 nm. The reflectivity of mirrors exposed at lower temperature was less degraded. Surface analyses revealed formation of oxides on all mirrors.

In ITER, shutters planned for mirror protection are ineffective against thermo-oxidation. Nevertheless, in-situ cleaning systems planned for ITER mirrors may efficiently remove oxide layers.