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 degrees 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 degrees C mirror contained 10(-7) m thick deposits, which deteriorated the optical properties. Both the 340 and 490 degrees C mirrors had negligible deposits. Where the 340 degrees C mirror retained its original optical properties, the reflectivity of the 490 degrees C mirror was decreased in the UV-visible and in the near-infrared wavelength region due to carbide formation.