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

In the Experimental Advanced Superconducting Tokamak (EAST), the reflectivity of the charge exchange recombination spectroscopy (CXRS) first mirror (FM) was dramatically dropped down to 20% of the original value after the operation of two EAST experimental campaigns from 2014–2015, leading to degradation of the signal intensity of the CXRS diagnostic to an unacceptably low level. The radio frequency (RF) plasma cleaning of the CXRS FM with a dimension of 303 × 81 × 76 mm³ and a small curvature of 0.008 mm^–1 was performed to remove deposits to recover the reflectivity. After 168 h cleaning by RF plasma, the maximum specular reflectivity of the FM could reach 92% of the original value at 532 nm, making the cleaned 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 cleaning uniformity and possible damage to the mirror surface. The specular reflectivity did not show obvious dependence on locations along the surface with the same cleaning time. The measured surface roughness gradually increased with sputtering time. The reflectivity remained almost unchanged regardless of different sputtering times and locations, indicating negligible damage to the FM surface even after 100 h sputtering. The recontaminated CXRS FM in the 2016 EAST campaign was firstly cleaned for 81 h, and the least reflectivity recovery for areas with relatively thick deposits was only 40%. After continuing cleaning to 147 h, redeposition of the sputtered residual deposits on the FM surface was observed. In the future for in situ cleaning of the FMs in EAST and ITER, deposits should be removed timely when they are very thin taking into account a very long cleaning time and presumable redeposition of thick and nonuniform deposits.