Impurity sources and fluxes in W7-X: from the plasma-facing components to the edge layer

Wendelstein 7-X (W7-X) is a nearly full-carbon machine with graphite divertors, baffles and shields in Operation Phase 1.2b (OP 1.2b). Divertor spectrometer measurements showed that an amount of helium and oxygen impurities existed in the predominately hydrogen plasma, which resulted in a high carbon impurity level by enhanced physical and chemical sputtering by these impurities in comparison with the pure impinging proton yields. In order to improve the wall conditions, especially to reduce the oxygen content, boronizations were applied in OP1.2b. After the boronization, an oxygen decrease by more than an order of magnitude was observed. Helium disappeared in comparison with OP1.2a due to reduced application of helium wall conditioning after introduction of boronizations. The overall radiation normalized to line integrated density was reduced by a factor of six. In addition, local CH4 injection was applied in the divertor in order to quantify the chemical sputtering by hydrogen on divertor plates. The experimentally determined effective D/XB of the A–X band of CH resulting from CH4 was ${\left[\tfrac{{D}}{{X}{B}}\right]}_{{{A}}^{2}{\rm{\Delta }}\to {{X}}^{2}{\rm{\Pi }}}^{{\rm{C}}{{\rm{H}}}_{4}\to {\rm{C}}{\rm{H}}}=16$ at T e ≈ 20 eV and n e ≈ 5 × 1018 m−3. It was applied to determine the hydrocarbon fluxes and further to deduce the particle flux ratio ГCH4/ГH on divertor plates.