Pilot experiments on RF plasma cleaning of Be deposits in QSPA facility

Scientific and technological program of ITER requires measurement of plasma parameters using variety of diagnostics, ~ 20 of which are the optical ones [1]. The plasma emitted light and/or laser beams will travel by in-vessel optical labyrinths between torus and analyzing equipment. The closest to plasma optical elements, so-called first mirrors (FMs) and windows (FWs), will operate in harsh environmental conditions such as high temperature, particle fluxes and deposition of eroded first wall materials (Be, W etc.) [3]. During periods of Mirror Conditioning States (MCS), the optical performance of FMs and FWs has to be restored by an appropriate cleaning system. The system based on the radio frequency capacitively coupled (RFCC) discharge with a mirror or window used as a RF electrode is currently considered as the main approach [4]. However, most research of the plasma cleaning were performed using Be proxy materials (Al, Au etc.) due to its high toxicity. The study of sputtering Be deposits are of high importance to prove the plasma cleaning efficiency. The report presents results of pilot experiments on deposition and in-situ sputtering of Be deposits in the QSPA-Be facility located in Bochvar institute. QSPA-Be facility was designed to simulate plasma heat loads related to ITER ELMs / disruptions [5]. The pure Be target evaporation by one pulse of QSPA-Be provided deposition of ~7 nm contaminant layer with ~70% of beryllium. The beryllium films were characterized using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The in-situ plasma cleaning was realized by exposing in 81 MHz helium discharge at pressure of 2 Pa. The cleaning process was controlled observing beryllium emission line in plasma column spectrum. The FW mock-up characterization was performed after 2 hours of the plasma exposure. Cyclic removal of Be-depositions in QSPA facility is planned to be done in 2020.