Material erosion and deposition on the divertor of W7-X

The net erosion and deposition pattern of carbon from the Test Divertor Unit (TDU) of the stellarator W7-X was determined. Special target elements with marker layers consisting of about 300 nm molybdenum and 5-10 μm carbon on top were used during the operational phase OP 1.2a. The thicknesses of the marker layers were determined by elastic backscattering spectrometry (EBS) using 2.5 MeV protons before and after plasma exposure and laser-induced breakdown spectroscopy on selected target elements after exposure. Scanning electron microscopy was used for investigating the surface morphology before and after exposure. Massive erosion of up to 20 μm carbon was observed at the strike line, in total 48 ± 14 g carbon were eroded from the 10 TDUs. The erosion was laterally non-uniform on the micro-scale. Strongly eroded surfaces were considerably smoother as compared to the original material. Only very little deposition of carbon is observed on the TDU: This means that the TDU is a large net erosion source.