Laser-induced breakdown spectroscopy for Wendelstein 7-X stellarator limiter tile analysis

Laser-induced breakdown spectroscopy (LIBS) is a well-established elemental composition analysis method as well as one of the most promising candidates for in situ first wall diagnosis of fusion devices. In this work, limiter graphite tiles, which were exposed in the initial operational phase (OP1.1) of the Wendelstein 7-X stellarator to He and H plasma, are analyzed ex situ by LIBS employing a picosecond pulsed laser for the first time and compared with post mortem analysis techniques. Depth profiles of each element and 2D profile of the ratio of H and C atoms on the surface are investigated. Both H content and retention depth on the deposition dominated zone are higher than on the erosion dominated zone due to the formation of C-H co-deposition layer. The results from LIBS are in agreement with those from the cross-sectional scanning electron microscopic image and electron dispersive x-ray spectroscopy.