Mirror dual cleaning of ITER equatorial diagnostic Wide Angle Viewing System

Abstract

The metallic first mirrors (FMs) are expected to play a crucial role in most optical diagnostic systems in the International Thermonuclear Experimental Reactor (ITER). However, these mirrors will be subjected to deposition of the first-wall materials, which will comprise their optical properties. Thus, the FMs will require periodic cleaning to restore their optical properties, which is anticipated to be achieved using an in situ plasma cleaning technique employing radio-frequency (RF) discharges. The left tangential line of sight of the Wide Angle Viewing System (WAVS) for the ITER Equatorial Port 12 FM unit, designed by CEA Cadarache, was simplified and manufactured at the University of Basel for RF-cleaning tests in a realistic geometry. Plasma ignition on mirrors M1 and M2 was achieved at a frequency of 13.56 MHz. Both mirrors were successfully cleaned with a 20-nm aluminum oxide (Al₂O₃) film replicating a contaminant layer. However, powering M1 and M2 in a dual cleaning regime with argon atmosphere at 1 Pa with 100 W of RF power for 1h and 45min (with a −300 V self-bias) allowed for the simultaneous removal of the Al₂O₃ layer from the rhodium mirror insets on both mirrors. The temperature of the mineral insulating (MI) cables used for powering the first mirror remained well below the maximum rated temperature. A cleaning demonstration was also performed on a stratified mirror prototype developed by CEA. The effectiveness of the dual cleaning technique for the Equatorial Port 12 WAVS diagnostic was validated by powering both FMs in a direct-current (DC) coupled regime, as all reported diagnostics will use a DC decoupled mode.