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Wendelstein 7-X (W7-X) operates with carbon-fiber composite (CFC) plasma-facing components (PFC) forming the island divertor used for particle and power exhaust. Erosion of PFCs leads to an influx of carbon (C) into the edge plasma where C radiation significantly cools down the divertor in the applied hydrogen plasmas. Rising the radiation in the divertor by hydrogen fueling and/or impurity seeding of low-Z species like N2 or Ne allows access to the “radiative detachment" regime with reduced target heat and particle loads [1,2] at high electron densities and moderate electron temperatures in the divertor of about 10 eV. However, carbon-based PFCs are not tolerable for a future fusion reactor due to unacceptable fuel retention. A promising alternative PFC material is tungsten (W), which shows in general low sputtering, low fuel retention, and good thermomechanical properties.
As a first systematic approach to study the impact of potential W PFCs in W7-X, we use EMC3-EIRENE as plasma boundary simulation code for 3D magnetic configurations. When C as instrinsic radiator is absent, impurity seeding is unavoidable for plasma operation at elevated power and particle fluxes [3]. We investigate detachment scenarios with the standard magnetic configuration of W7-X equipped with a W divertor in the current divertor design and analyze the dependence of detachment relevant parameters (e.g. target heat loads) on Ne impurity concentration and operation conditions (e.g. heating power). First results indicate a deeper penetration of recycled H neutrals into the confined region as a consequence of the increased reflection probability of H ions at the strike lines.
It is anticipated that low impact energies and cold divertor operation with electron temperatures below 5 eV are required to restrict W erosion caused by the seed impurities to avoid significant W accumulation in the core. At these temperatures, volume recombination processes become increasingly relevant and hence are also investiged since up to now, the research on volume recombination in the island divertor has been limited to the C divertor [4].
Both the hydrogen recycling and seed impurity concentration need to be adapted to get access to this reactor-relevant regime and eventually, a change of the island divertor geometry with higher baffeling might be unavoidable.
[1] Y. Feng et al 2021 Nucl. Fusion 61 086012, [2] F. Effenberg et al 2019 Nucl. Fusion 59 106020,
[3] S. Brezinsek et al 2019 Nucl. Fusion 59 096035, [4] Y. Feng et al 2025 Nucl. Fusion 65 066008