Inspection of W 7-X plasma-facing components after the operation phase OP1.2b: observations and first assessments

Abstract

Due to its twisted magnetic field configurations, the orientation of the plasma-facing components (PFCs) of the Wendelstein 7-X stellarator is changed from bean shape to triangular shape along the plasma axis within a module. The PFCs consist of graphite (test divertor units, baffles, head shield tiles, toroidal divertor closures) and stainless steel components (wall panels, pumping gap panels, poloidal closures). In order to document the status of these components as well as to get the first footprints of the plasma-wall interactions (PWI) that occurred during the plasma campaign, a detailed inspection of the whole plasma vessel after the plasma campaign is of vital importance. Such an inspection was carried out after the operation phase OP1.2b completed in October 2018. A number of observations were made, such as, molten stainless steel drops, melting of a long term probe, deposition on steel panels, arc traces, loosely bound particles and deposition stripes on the plasma vessel hidden behind the graphite tiles. In order to understand the possible causes, their implication and steps to avoid some of these problems for the next phase of machine operation, an assessment was made. The melting events should be avoided in the future, the others are due to usual PWI mechanisms and will be monitored after each campaign.