Temperature effect on hydrocarbon deposition on molybdenum mirrors under ITER-relevant long-term plasma operation

Optical diagnostics in ITER will rely on mirrors near the plasma and the deterioration of the reflectivity is a concern. The effect of temperature on the deposition efficiency of hydrocarbons under long-term operation conditions similar to ITER was investigated in the linear plasma generator Pilot-PSI. Polycrystalline molybdenum mirrors were exposed for 30 min in the background of methane seeded argon plasma at 210, 340 and 490°C. The surface and the optical properties of the mirrors were characterized before and after exposure by reflectivity and polarization measurements and x-ray photoelectron spectroscopy. The 210°C mirror contained 10^-7 m thick deposits, which deteriorated the optical properties. Both the 340 and 490°C mirrors had negligible deposits. Where the 340°C mirror retained its original optical properties, the reflectivity of the 490°C mirror was decreased in the UV-visible and in the near-infrared wavelength region due to carbide formation.