Steam-Resistant Optical Materials for Use in Diagnostic Mirrors for ITER

The need for a steam ingress environmental experiment is very significant to understand the impact of accidental in-vessel coolant leaks at ITER and to study the exposure of optical diagnostics to steam and humid conditions. This could happen as a result of the damage to the cooling pipes due to runaway electrons generated during plasma disruptions in ITER. In order to know the scope of this potential impact, an assessment was carried out to simulate and to study the exposure of optical elements to strong and hostile moisture conditions. After test, different measurements on optical mirrors were performed to characterize the reflectance properties, observed both in the visible and infrared spectral ranges, as well as the analysis of wavefront error, coating adherence test, and X-ray spectroscopy. Modification of properties and fluctuations in the physical behavior of optical materials and components were observed. Substrates and coatings were affected at different levels due to corrosion and oxidative depositions that modify their optical performances. In general, there are large differences in the results obtained for the same material manufactured by different manufacturing processes. Steam and humidity affected, especially substrates and metal coatings. Substrates made of silicon carbide and stainless steel were the least affected by corrosion. Rhodium coating suffered less damage than the molybdenum coating.