Plasma sources sputtering nanoscale contaminants with low-energy ion flux on front-end mirrors in ITER optical diagnostics

Abstract New plasma sources are proposed to clean optical mirrors in diagnostic instruments of the ITER experiment considering RF discharges operating at low pressures (1–10 Pa) in inert gases. There are nearly twenty optical diagnostics where the front-end optical mirrors may require plasma cleaning. The mirrors vary in size and would need up to 400 W in the discharge to form ion fluxes capable of removing Be- and W-containing contaminants to restore the optical performance with tolerable damage to the mirror. The plasma sources suggested to clean contaminants include a vacuum matching circuit placed close to the mirror and a quarter wavelength band stop notch filter when mirror water cooling is needed. Long-term operation stability and cleaning homogeneity may employ a driving frequency variation to tune the circuit and a phase shift for RF voltages to clean two mirrors simultaneously. In this paper, the plasma sources based on 40 MHz RF discharge to clean first mirrors are studied experimentally for two ITER optical instruments: the Edge Thomson Scattering and the Visible Spectroscopy Reference System. Frequency tuning and phase shifting are studied in realistic configurations prototyping diagnostic port plug geometries.