Study on plasma cleaning of the large-scalefirst mirror of the charge exchange recombination spectroscopy diagnostic on EAST

In the Experimental Advanced Superconducting Tokamak (EAST), the reflectivity of the chargeexchange recombination spectroscopy (CXRS) first mirror (FM) was dramatically dropped downto 20% of the original value after the operation of two EAST experimental campaigns from2014–2015, leading to degradation of the signal intensity of the CXRS diagnostic to anunacceptably low level. The radio frequency (RF) plasma cleaning of the CXRS FM with adimension of 303 × 81 × 76 mm³ and a small curvature of 0.008 mm^–1was performed toremove deposits to recover the reflectivity. After 168 h cleaning by RF plasma, the maximumspecular reflectivity of the FM could reach 92% of the original value at 532 nm, making thecleaned CXRS FM eligible to be reused for the CXRS diagnostic in the 2016 EAST campaign.Dedicated tests of sputtering polished mirror samples were performed to explore the cleaninguniformity and possible damage to the mirror surface. The specular reflectivity did not showobvious dependence on locations along the surface with the same cleaning time. The measuredsurface roughness gradually increased with sputtering time. The reflectivity remained almostunchanged regardless of different sputtering times and locations, indicating negligible damage tothe FM surface even after 100 h sputtering. The recontaminated CXRS FM in the 2016 EASTcampaign was firstly cleaned for 81 h, and the least reflectivity recovery for areas with relativelythick deposits was only 40%. After continuing cleaning to 147 h, redeposition of the sputteredresidual deposits on the FM surface was observed. In the future for in situ cleaning of the FMs inEAST and ITER, deposits should be removed timely when they are very thin taking into accounta very long cleaning time and presumable redeposition of thick and nonuniform deposits.