Control of the Process of Plasma Cleaning of Input Mirrors of Optical Diagnostics of ITER in Glow Discharge Controlled by Direct and Mid-Frequency Pulsed Direct Current

The intravacuum mirrors used in the systems of optical plasma diagnostics on fusion installations are subject to contamination and require periodic cleaning. It is proposed to clean the mirrors by ion sputtering of the impurity from the reflective surface in a gas discharge. In this work, results are presented from experimental studies of the possibility of controlling the mirror cleaning process in the glow discharge localized inside a grid hollow cathode and controlled by direct current (DC) or mid-frequency pulsed direct current (MDC). Three independent methods of control were studied: measurements of the voltage of the cleaning discharge at a fixed current, measurements of the frequency of the appearance of microarc couplings in the presence of dielectric contaminants, and emission spectroscopy studies of the cleaning discharge. Separately, to verify the cleaning effect, results were presented from measuring the reflection spectrum of the mirror. It was shown that all three methods can be used in the cleaning systems. The first of the discussed methods appears to be the most promising since it does not require additional detectors and uses only the data obtained from the discharge power supply unit, and it is applicable to cleaning mirrors from both current-conducting and thin dielectric contaminations. The control based on measuring the frequency of appearance of microarc couplings can be used when one alternates between the PDC and the DC regimes in the presence of layered metal–dielectric contaminations and cleaning the local dielectric contaminations resulting from emergencies. The spectroscopic control method provides the highest sensitivity and allows one to determine the composition of the contaminants, yet it also has higher requirements for the recording equipment. The considered methods can be used to automate the processes of removing the contamination in the systems for plasma cleaning of the input diagnostic mirrors of fusion installations and to control the finishing treatment of substrates before vacuum deposition of coatings.