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The metallic First Mirrors (FMs) will play a crucial role in most optical diagnostic systems in ITER. As the initial elements in the optical path of diagnostic systems, the FMs will be subjected to deposition of the first tungsten wall materials and to regular boronization, compromising their optical properties. The FMs would thus need periodic cleaning to restore their optical properties, which is expected to be achieved using an in-situ plasma-cleaning technique based on radio-frequency (RF) discharges [1].
Boron and tungsten mixed films were deposited using magnetron sputtering with argon, argon/oxygen, and argon/deuterium atmospheres on a platinum mirror. Films of boron oxide, boron deuterium and boron tungsten were deposited and their chemical composition was measured in vacuo by X-ray Photoelectron Spectroscopy (XPS).
Removal of these films was performed using radio-frequency plasma cleaning with argon, helium, and deuterium gas at 250 and 350V, for 13.56MHz. Argon can erode all contaminant films, but it has a high erosion rate on the substrate. Helium and deuterium sputtered boron, but were ineffective on tungsten. On a platinum mirror, 15nm of boron was sputtered and reflectivity was recovered.
Argon and deuterium mixture also eroded EAST boronized films contaminant, with a partial restoration of the optical properties of the molybdenum mirror.
[1] A P. Shigin et al., Fusion Eng. Des. 64 (2021) 112162