Influence of Erosion and Deposition on Metallic First Mirror in HT-7 Tokamak

The first mirror (FM) will be used in ITER for the diagnostic systems observing the optical signals of plasma radiation in the ultraviolet (UV), visible (VIS) and infrared range (IR). Mirrors have to operate in a harsh environment with intense electromagnetic radiation, high fluxes of energetic particles, neutron and gamma irradiation. Erosion, which is caused by fast neutrals from the plasma, and deposition of impurities may degrade the optical characteristics of the mirrors. Dedicated experiments in the HT-7 tokamak were performed to investigate the influence of erosion and deposition on the mirror samples. The first mirror samples made of polycrystalline (PC) stainless steel (SS), molybdenum (Mo) and tungsten (W) were fixed on a holder at an angle of 45° with respect to the horizontal plane and set at different locations with different connection lengths along the magnetic field. The optical reflectivity of the first mirror was measured by a spectrophotometer before and after plasma exposure. It was found that the surface morphology and specular reflectivity of the mirror samples after the exposure were different with respect to the different distances from the mirror surface to the last closed flux surface (LCFS) of the plasma in the tokamak. It was also found that shortening the connection length before the mirror surface would weaken the influence of the plasma erosion and impurity deposition on the mirror surface. In order to maintain the optical characteristics of the mirror surface, it is necessary to adopt the in-situ cleaning and mirror protection techniques.