Investigation of Molybdenum Mirror Degradation during Simulated Leakage in the Water Cooling System of ITER

Contamination of ITER in-vessel components in accidents associated with the ingress of water from the cooling system into the vacuum vessel poses a threat to subsequent operation of the entire facility and individual diagnostic systems. In the present study, the process of degradation of the optical properties of single-crystal Mo mirrors, which are used in optical diagnostics, in their interaction with liquid water and water vapor is examined. The dynamics of variation of the specular reflection and diffuse scattering coefficients with exposure time and interaction mode is studied experimentally. It is demonstrated that the worst-case scenario (“steam event” in the ITER documentation) is modeled accurately by mirror exposure to distilled water at a temperature of ≈97°C. Samples of Mo mirrors with simulated contamination obtained in these experiments are used to test the operation of the plasma mirror cleaning system based on a direct- or pulsed-current abnormal glow discharge in a grid hollow cathode. It is demonstrated that the optical quality of Mo mirrors contaminated in simulated emergency events with water entering the vacuum vessel may be restored to near-baseline levels if Ar is used as the plasma-forming gas.