IN-SITU MEASUREMENT OF EROSION DEPOSITION IN THE DIII-D DIVERTOR BY COLORIMETRY/

Colorimetry was introduced into the DIII-D tokamak to measure in situ the growth and erosion of transparent wall coatings (a-C:H) on the div ertor. The colorimetric measurement system consisting of a halogen lig ht source, a set of three filters and a black/white camera is describe d together with a first erosion measurement. An insertable graphite sa mple with a diameter of 4.7 cm was precoated with a 300 nm thick amorp hous carbon film and was exposed in the divertor for several discharge s with its surface coplanar to the surrounding graphite tiles. For eac h of the discharges the plasma strike point was moved onto the sample for 1 s to erode the coating. Between the discharges a camera signal w ith each filter was recorded and the film thickness was evaluated alon g a radial line across the DIMES sample. Thus it has been possible for the first time to measure erosion and deposition of divertor material in situ and shot-by-shot. The avenge peak heat flux with the strike p oint on DIMES was about 110 W cm(-2). The measurement shows a strong d ecrease in the film thickness almost over the entire sample with an av erage erosion rate of similar to 9 nm s(-1).