Plasma Cleaning of KU-1 Optical Quartz from Aluminum Films

Abstract

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.