TCV mirrors cleaned by plasma

Highlights: • Plasma cleaning of mirrors exposed in TCV was carried out. • Reflectivity measurements with a spectrophotometer compared to the simulated reflectivity using a refractive index of 1.5 and a Cauchy model for the aC:D gives good confidence in the estimated film thickness. • Recovery of the reflectivity was not complete for the Au mirror mostly due to damage of the surface by tokamak exposure i.e. breakdown phenomena typical for electrical treeing. • A fractal dimension of 2 was calculated for these breakdown phenomena which is typical for random motion of arc spot. - Abstract: Metallic mirrors exposed in TCV tokamak were cleaned by plasma in laboratory. A gold (Au) mirror was deposited with 185–285 nm of amorphous carbon (aC:D) film coming from the carbon tiles of TCV. Another molybdenum (Mo) mirror had a thicker deposit due to a different location within the tokamak. The thickness measurements were carried out using ellipsometry and the reflectivity measurements performed by spectrophotometry revealed a decrease of the specular reflectivity in the entire range (250–2500 nm) for the Mo mirror and specifically in the visible spectrum for the Au. Comparison of the simulated reflectivity using a refractive index of 1.5 and a Cauchy model for the aC:D gives good confidence on the estimated film thickness. Plasma cleaning using radio frequency directly applied to a metallic plate where the mirrors were fixed demonstrated the ability to remove the carbon deposits. A mixture of 50% hydrogen and 50% helium was used with a −200 V self-bias. Due to the low sputtering yield of He and the low chemical erosion of hydrogen leading to volatile molecules, 20 h of cleaning were needed for Au mirror and more than 60 h for Mo mirror. Recovery of the reflectivity was not complete for the Au mirror most likely due to damage of the surface during tokamak exposure (breakdown phenomena).