Plasma cleaning of ITER first mirrors in magnetic field

TL;DR

This study demonstrates plasma cleaning of metallic mirrors in magnetic fields, effectively removing aluminium and aluminium oxide deposits, which is crucial for maintaining optical performance in ITER diagnostics.

Contribution

It provides experimental evidence of plasma cleaning effectiveness on large mirrors under magnetic fields, simulating conditions in ITER.

Findings

Successful removal of 260 nm aluminium/aluminium oxide film

50 nm aluminium oxide removed in 0.35 T magnetic field

Cleaning efficiency confirmed by reflectivity, SEM, and XPS measurements

Abstract

To avoid reflectivity losses in ITER optical diagnostic systems, plasma sputtering of metallic First Mirrors is foreseen in order to remove deposits coming from the main wall (mainly beryllium and tungsten). Therefore plasma cleaning has to work on large mirrors (up to a size of 200*300 mm) and under the influence of strong magnetic fields (several Tesla). This work presents the results of plasma cleaning of aluminium and aluminium oxide (used as beryllium proxy) deposited on molybdenum mirrors. Using radio frequency (13.56 MHz) argon plasma, the removal of a 260 nm mixed aluminium/aluminium oxide film deposited by magnetron sputtering on a mirror (98 mm diameter) was demonstrated. 50 nm of pure aluminium oxide were removed from test mirrors (25 mm diameter) in a magnetic field of 0.35 T for various angles between the field lines and the mirrors surfaces. The cleaning efficiency was evaluated by performing reflectivity measurements, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy.