Two-dimensional measurements of thin film erosion and deposition by colorimetry of interference colors and its application in fusion devices

Abstract

The paper describes time resolved observations of the erosion of a thin (≈ 105 nm) and transparent a-B:D film on a roof-like graphite test probe (60 mm × 63 mm) in the scrape-off layer plasma of the tokamak Textor-94. Steady state of the erosion pattern was almost achieved after 22 discharges (121.3 s total exposure time). After each of the pulses the interference color pattern in illuminating light of 3200 K color temperature was observed by means of conventional video technique and the color coordinates R, G, B measured. Since they can be related uniquely to the film thickness contour maps and profiles could be evaluated and erosion rates determined. Net deposition on parts of the surface is also observed. Most of the results can be described by erosion due to the hydrogen flux and the simultaneous deposition of carbon and other impurities from the background plasma. The deposition of impurities onto the a-B:D bulk seems to reduce the erosion rates which prolongs its lifetime. This interpretation is backed by AES and NRA analysis of the boron content and by EPMA.