Plasma Cleaning of KU-1 Optical Quartz from Aluminum Films

The intensity of light emitted by plasma and passing through diagnostic quartz windows in tokamaks and other fusion power devices may decrease due to first-wall materials (Be in the International Thermonuclear Experimental Reactor (ITER)) sputtered and re-deposited onto the window surface facing the plasma. In the experiments, aluminum films (analog of Be) are removed from the surface of fused silica KU-1 in plasma of a RF discharge in a H2–N2 mixture prototyping one of the in-situ cleaning techniques. The admixture of nitrogen to hydrogen plasma increases the sputtering rate of aluminum films. Using X-ray photoelectron spectroscopy (XPS) and atomic-force microscopy (AFM), we demonstrate that cleaning is accompanied by the weak reduction of quartz, nitriding with the addition of nitrogen, and the formation of a “honeycomb” structure with increased roughness. These processes are more pronounced with the repeated deposition of Al and subsequent cleaning. Light transmittance in the range 400–1000 nm does not change within 1% in the case of a partial change in the stoichiometry of the surface layers and an increase in the roughness from 1.3 to 4.5 nm. Numerical estimates of the forward and backscattering of light by a rough quartz surface show that the reduction in transmission due to scattering does not exceed a few tenths of a percent, which is in agreement with the measurement data. Thus, the demonstrated stability of the quartz transmittance during the deposition of aluminum and subsequent long exposure in the plasma of a RF discharge makes it possible to consider the cleaning technique under consideration promising for use in ITER.