Cleaning of two mirrors in the first mirror unit using radiofrequency capacitively coupled plasma

First mirror (FM) cleaning, using radio frequency (RF) plasma, has been proposed to recover FM reflectivity in nuclear fusion reactors such as the International Thermonuclear Experimental Reactor (ITER). To investigate the influence of simultaneous cleaning of two mirrors on mirror cleaning efficiency and uniformity, experiments involving single-mirror cleaning and dual-mirror cleaning were conducted using RF capacitively coupled plasma in the laboratory. For the test and simultaneous cleaning of two mirrors, the FM and second mirror (SM), both measuring 110 mm × 80 mm, were placed inside the first mirror unit (FMU). They were composed of 16 mirror samples, each with a dimension of 27.5 mm × 20 mm. These mirror samples consist of a titanium–zirconium–molybdenum alloy substrate, a 500 nm Mo intermediate layer and a 30 nm Al₂O₃ surface coating as a proxy for Be impurities. The cleaning of a single first mirror (SFM) and the simultaneous cleaning of the FM and SM (DFM and DSM) lasted for 9 h using Ar plasma at a pressure of 1 Pa. The total reflectivity of mirror samples on the DSM did not fully recover and varied with location, with a self-bias of −140 V. With a self-bias of −300 V, the total reflectivity of mirror samples on the SFM and DFM was fully recovered. The energy dispersive spectrometer results demonstrated that the Al₂O₃ coating had been completely removed from these mirror samples. However, the mass loss of each mirror sample on the SFM and DFM before and after cleaning varied depending on its location, with higher mass loss observed for mirror samples located in the corners and lower loss for those in the center. Compared with SM cleaning, the simultaneous cleaning of two mirrors reduced the difference between the highest and lowest mass loss. Furthermore, this mass loss for the mirror samples of the DFM facing the DSM was increased. This indicated that mirror samples cleaned face to face in the FMU simultaneously could influence each other, highlighting the need for special attention in future studies.