Influence of 3 T magnetic field on RF plasma self-bias and erosion pattern for the ITER wide angle viewing system

Abstract The ITER Wide Angle Viewing System (WAVS) relies on in-vessel metallic first mirrors (FM) to transport optical signals from the plasma to out-of-vessel diagnostics. Under the conditions of ITER, energetic plasma particles induce sputtering of plasma-facing components, and the resulting sputtered material can re-deposit on these mirrors, resulting in reflectivity degradation. In-situ plasma removal using radio-frequency (RF) capacitively coupled plasma discharges is one of the proposed methods to mitigate this issue. Experiments were conducted in a 3 T homogeneous magnetic field using a medical magnetic resonance imaging (MRI) system at the University Hospital of Basel. The WAVS First Mirror Unit consists of two mirror electrodes (M1 and M2), one or both of which can be powered with RF, while the remaining surfaces act as grounded areas. Systematic measurements of the self-bias voltage were performed for varying discharge conditions, including gas species (argon and helium), pressure (0.1–10 Pa) and mirror capacitance, as well as the angle θ between the mirror surface and magnetic field. The results revealed a strong dependence of the self-bias on the grounded-to-powered area ratio <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>A</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">g</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:msub> <mml:mi>A</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">p</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , which is affected by the magnetic confinement. Erosion and deposition patterns were examined in cases where one or both mirrors were powered. Erosion occurred primarily where a powered mirror faces a grounded surface, whereas overlapping discharges in the dual-powered case resulted in low ion kinetic energy and preferential net deposition at the overlapping area. These observations were supported by one-dimensional simulations of plasma potential evolution under representative conditions. These results provide critical insight into RF plasma behaviour in fully magnetised, geometrically asymmetric discharges and support the development of robust in-situ cleaning strategies for ITER optical diagnostics.