Removing W-contaminants in helium and neon RF plasma to maintain the optical performance of the ITER UWAVS first mirror

Abstract

First metallic mirrors used in optical diagnostics such as the Upper Wide Angle Viewing System (UWAVS) in ITER may undergo contamination with beryllium and tungsten. Contamination levels of 10 nm can significantly degrade mirror performance. A UWAVS first mirror cleaning system prototype uses radio-frequency (RF) gas discharge to remove contaminants. Although tungsten is not expected to be a main contaminant, removing it with ion sputtering may become challenging due to the high atomic mass. Cleaning tests were focused on 10–20 nm tungsten layers removal from various substrates using a tailored RF circuit in He and Ne. W-coatings were also produced on an existing single-crystal (SC) Mo-mirror 105 mm diameter. He and Ne were chosen for many tests due to their effectiveness and suitability for ITER operations. Sputtering conditions were determined through ion energy measurements. Ion energies of 100–150 eV and currents of 0.5–0.8 A/m2 were found to be optimum for 30–40 MHz RF discharges. The cleaning rate in He was 0.3–0.5 nm/hour for W-deposits. Etching rate of 1 nm/hour was observed for the Mo-mirror. No surface roughness degradation was determined with SEM and optical interferometry after mirror material was removed. Overall exposure time of the SC Mo-mirror was 100 h.