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

Abstract

The first mirror (FM) cleaning, using radio frequency plasma, has been proposed to recover the FM reflectivity in nuclear fusion reactors such as 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 radio frequency (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 × 80 mm2, were placed inside the first mirror unit (FMU). They were composed of 16 mirror samples with a dimension of 27.5 × 20 mm2 each. These mirror samples consist of a titanium-zirconium-molybdenum (TZM) alloy substrate, a 500 nm molybdenum intermediate layer, and a 30 nm aluminum oxide (Al2O3) surface coating as a proxy for Be impurities. The cleaning of a single first mirror (SFM) and the simultaneous cleaning of FM and SM (DFM and DSM) were lasted for 9 h using argon (Ar) plasma at a pressure of 1 Pa. The total reflectivity of mirror samples on the DSM did not fully recover and varied with locations with a self-bias of −140 V. While with a self-bias of −300 V, the total reflectivity of mirror samples on SFM and DFM was fully recovered. The energy dispersive spectrometer (EDS) results demonstrated that the Al2O3 coating had been completely removed from these mirror samples. However, the mass loss of each mirror sample on SFM and DFM before and after cleaning varied depending on their locations, with higher mass loss observed for mirror samples located in the corners and lower for those in the center. Compared to the single mirror cleaning, the simultaneous cleaning of two mirrors reduced the difference of the mass loss between the highest and lowest. Furthermore, this mass loss for the mirror samples of DFM facing the DSM was increased. It indicated that the simultaneous cleaning of mirror samples face to face in the FMU could influence each other, highlighting the necessity for special attention in future studies.