Carbon erosion/deposition on the divertor of W7-X during the operational period OP 1.2b

Carbon net erosion and deposition at the test divertor unit (TDU) of Wendelstein 7-X (W7-X) were measured for the discharge period OP 1.2b in the year 2018 using 18 specially prepared target elements in all 10 TDUs. These had lengths between 30 and 60 cm and were coated with marker layers for erosion/deposition investigations of 5–10 μm carbon on top of about 300 nm molybdenum. The marker layer thicknesses were measured by elastic backscattering spectrometry (EBS) before and after plasma exposure using 2.5 MeV protons; the surface morphology was investigated using scanning electron microscopy (SEM) and focused ion beam cross-sectioning (FIB), the surface roughness was determined using a two-dimensional optical profiler. Plasma-exposed surfaces were considerably smoother than unexposed surfaces with decreased mean roughness and a shift of the inclination angle distribution towards lower values. The erosion on the 10 TDUs was unequal within a factor of about two. During the discharge period in total 20.4 ± 5.7 g carbon was eroded from the 10 TDUs. Adjacent to the strike line some deposition of carbon was observed. Compared to the discharge period OP 1.2a in the year 2017, the net carbon erosion rate dropped by a factor of 5–6 due to regular boronizations, which reduced the oxygen (and subsequently also the carbon) content in the plasma by 1–2 orders of magnitude. The significance of erosion/deposition processes for long-pulse discharges is discussed.