Light Transmission and Surface Topography of KU-1 Optical Quartz After Sputtering and Cleaning from Al Films in RF Discharge of H2(D2)–Ne Mixtures

In plasma devices, in which plasma–wall interaction and material migration are significant, the diagnostic-window transmission may decrease due to the contamination of plasma-cleaning components by sputtered materials. We discuss the cleaning of KU-1 fused silica, which simulates the diagnostic window, from aluminum films in RF plasma generated in pure H2(D2) and Ne and in H2(D2)–0.23Ne mixtures. Aluminum is used as a chemical analogue of Be, which is the main material of the International Thermonuclear Experimental Reactor (ITER) first wall. The morphology of the plasma-treated surface is investigated by atomic force microscopy, the chemical composition is analyzed by X-ray photoelectron spectroscopy, and the transmission spectra in the range 400−1000 nm shows that plasma cleaning is accompanied by the slight reduction of quartz to suboxides and a simultaneous decrease in roughness Rq from 1.3 up to 1.0 nm. After plasma treatment of the quartz surface, a decrease in light transmission by 1.5–2% in the wavelength range 400–750 nm is observed. Further sputtering of the purified quartz surface with the removal of a layer with a thickness of more than 300 nm is accompanied by gradual smoothing of the surface and a decrease in Rq to 1 nm, but with the retention of reduced light transmission. All investigated gases, hydrogen isotopes, neon and D2(H2)–Ne mixtures, are suitable for removing Al films from the quartz surface at a RF power of several W/cm2 and temperatures of 20–100°C.