Experimental studies of first mirror exposure and surface recovery on EAST

Abstract Systematic studies of first mirrors (FMs) contamination, deposition mitigation and cleaning have been performed in EAST and laboratory. It turned out that the auxiliary systems such as gas puffing, wall conditioning and limiters could lead to a serious deposition on FMs and result in at least 60% degradation of the reflectivity. Besides, the exposure environment and depth–diameter ratios (DDRs) of the protective ducts will influence the degree of deposition mitigation and homogeneity of deposition. The reflectivity of the FM protected with duct at the wavelength of 240–400 nm could sharply decrease at the beginning of the exposure, which differs from that in the wavelength range more than 400 nm. In addition, the total and specular reflectivity of FM cleaned by radio frequency magnetron sputtering plasma (RFMSP) could be recovered by up to 90% and 80 % respectively in the wavelength range of 300–800 nm.