Ex situ analysis of W7-X divertor plasma-facing components by picosecond laser diagnostics

Abstract

A key question of long-pulse operation in the stellarator Wendelstein 7-X (W7-X) is the net erosion of plasma-facing components (PFCs), which determines W7-X PFC lifetimes as well as the impact on the impurity influx to the plasma. In order to judge the erosion and deposition balance at the passively cooled graphite divertor, a set of exchangeable graphite tiles with marker layer stripes consisting of 5–10 μm C deposited on a 200 nm thick Mo interlayer were installed as an integral part of the divertor surface and removed at the end of the first operational phase for post-operational analysis. Laser-induced breakdown spectroscopy (LIBS) combined with laser-induced ablation-quadrupole mass spectrometry (LIA-QMS) were used for further analysis. The LIBS results show that the erosion-dominated regions are close to the location of the strike lines in standard magnetic configuration of W7-X optimised for the island divertor concept. Almost the complete C/Mo marker layer is eroded in these regions, providing an average erosion rate of 4.3 nm s−1 in the 2481 s plasma seconds of the campaign. In parallel, only a little deposition at the edge of the divertor tiles, where Mo is observed on the bulk graphite material was found. Furthermore, LIA-QMS showed a high hydrogen content on uncoated bulk graphite areas, while a lower hydrogen concentration was detected in the erosion-dominated zone, caused by the high surface temperature at the strike line during plasma operation.