Study of lateral distribution of impurities on samples exposed in the ASDEX Upgrade using microbeam of 3He and 1H

Abstract

Abstract In this paper, we present the use of focused MeV ion beams to study the distribution of deuterium (D), boron (B) and nitrogen (N) on tungsten (W) samples exposed in the divertor region of the ASDEX Upgrade tokamak during 15N-seeded L-mode discharges in deuterium and during non-seeded H-mode discharges in helium. In both experiment samples of various surface roughness were exposed and analyzed: W coatings on milled or polished graphite substrates and bulk W samples, ranging from the roughest (milled) to the smoothest (bulk W), to study the effects of surface roughness on deposition profiles of D, B and N. In the case of samples from the 15N experiment, we found that D, N and B are distributed quite homogeneously over the sample on the micrometer scale with some small variation inside the analysed area. The surface densities show strong variations in the poloidal direction, with maximum values slightly above the strike point. The amounts of the retained (D, B, N) were strongly correlated with the surface roughness of the samples, being the highest on the rough samples. Samples originating from the He campaign showed inhomogeneous distribution of impurities with a distinct micro-scale structure, which is most pronounced on pre-damaged W samples, where a rough fuzz-like surface is created during the exposure in the GLADIS machine. We observe a distinct difference in behavior of deposited B profiles from both experiments with higher retention of B in the He experiment.