Pulsed radiofrequency plasma for cleaning ITER first mirrors with and without notch-filter and magnetic field

Abstract

In ITER, the first mirrors of optical diagnostics will be prone to deposition of the first wall materials which will result in the degradation of the mirror's optical parameters and the parameters of the diagnostic itself. To restore the mirror's reflectivity a cleaning system based on the gas discharge will be employed. The present paper discussed low-frequency power modulation of capacitively-coupled RF discharge utilized to decrease the mean RF power supplied through the in-vessel feeding line. The impact of power modulation on the incident ion flux, distribution of ion energy, and sputtering rates of the first wall materials were studied for the cases of DC-coupled and -decoupled mirrors with/without external magnetic field. The parameters of the plasma were measured depending on modulation frequency (1 Hz – 10 kHz) and duty cycle (10 – 90 %) for He plasma at a pressure of a few Pa. Based on the measurements of ion flux and ion energy the sputtered rates of Be and BeO were estimated. The estimations of the sputtering rate were followed by experiments on BeO sputtering in He plasma. Finally, the recommendations are given for selecting an appropriate RF power required to clean the mirror from Be/BeO deposits in ITER.