Use of isotopically-enriched carbon probes for erosion/deposition measurements in the ASDEX divertor

An isotopic marker technique has been developed and used to measure near-surface erosion and carbon and impurity deposition on graphite samples in the ASDEX divertor chamber. Papyex graphite strips were enriched to approx. 20% 13C over the first 1000 A by ion implantation. The implanted 13C can be clearly distinguished from 12C of the host by Rutherford backscattering using 2.7 MeV 4He. Carbon erosion and deposition were determined from changes in the depth distribution of the 13C, and impurity accumulation was also monitored in the backscattering spectrum. The 13C profiles remained sharp in spite of high heat fluxes to the samples (sufficient to melt stainless steel), and the results showed clear evidence of simultaneous erosion and deposition of carbon and impurities. During 2.4 MW neutral injection, net carbon erosion exceeded 250 A/s in the divertor throat. High carbon erosion was also observed during periods of substantial Fe and Ti impurity accumulation on the samples. The results demonstrate the applicability of the 13C technique for measuring erosion/deposition at high-heat-flux surfaces such as limiters or divertor plates