Deuterium as a cleaning gas for ITER first mirrors: experimental study on beryllium deposits from laboratory and JET-ILW

Abstract

Cleaning techniques for metallic first mirrors are needed in more than 20 optical diagnostic systems from ITER to avoid reflectivity losses. Plasma sputtering is considered as one of the most promising techniques to remove deposits coming from the main wall (mainly beryllium and tungsten). Previous plasma cleaning studies were conducted on mirrors contaminated with beryllium and tungsten where argon and/or helium were employed as process gas, demonstrating removal of contamination and recovery of optical properties. Still, both above-mentioned process gases have a non-negligible sputtering yield on mirrors. In this work, we explored the possibility to use a sputter gas having a small impact on mirrors while being efficient on Be deposits, e.g. deuterium. Two sputtering regimes were studied, on laboratory deposits as well as on mirrors exposed in JET-ILW, namely physical sputtering (220 eV ion energy) and chemically assisted physical sputtering (60 eV ion energy) using capacitively coupled plasma with radio frequency. The removal of Be and mixed Be/W contaminants, as well as the recovery of reflectivity, was achieved when deuterium was employed at 220 eV while cleaning at 60 eV was only fully efficient on laboratory beryllium deposits. On mirrors exposed in JET-ILW, the situation is more complex due to the presence of tungsten in the contaminant film, leading to the formation of a tungsten enriched surface that is not easily sputtered, especially at 60 eV.

The mechanism study of low-pressure air plasma cleaning on large-aperture optical surface unraveled by experiment and reactive molecular dynamics simulation

“…Contaminants such as carbon, Sn and Si produced a reduction reaction with hydrogen plasma to generate CH x , SnH 4 and SiH in synchrotron beamline optics, EUV facilities. Inert gases (such as helium, argon, and neon) plasmas removed beryllium and tungsten particles from the surface of optical components in ITER by physical bombardment [14].…”

RF discharge mirror cleaning system development for ITER diagnostics

“…RF discharge based cleaning was and still is the subject of R&D in several laboratories around the world. Its efficiency has been demonstrated under ITER relevant constraints: (i) deposits composed of Be and W [5] and (ii) mirrors of relatively large size required by ITER diagnostics [6]. However, implementing RF plasma cleaning on ITER first mirrors, taking into account the specific features required of systems deployed on ITER, often implies constraints not encountered in a typical laboratory environment [7].…”

First mirror erosion–deposition studies in JET using an ITER-like mirror test assembly

“…As such, they reinforce the expectation that ITER FMs may be characterized by mixmaterial co-deposits (Be, C, Ni, W with fuel species). They may also partly contribute to the consideration of requirements for mirror cleaning methods [43][44][45].…”