Speaker
Description
Wendelstein 7-X (W7-X) operation successfully demonstrated power and particle exhaust with the island divertor. It is compatible with high performance operation using steady-state pellet fueling and homogenous detachment up to the current maximum heating power of 8 MW. However, the current open divertor geometry suffers from an unfavorable, approximately linear scaling of divertor density and neutral pressures with upstream density, which challenges the extrapolation of power and particle exhaust to reactors. The limitations seem to be the combined effect of the stronger prominence of cross-field transport due to lower fieldline pitch, the open divertor geometry and stellarator-specific geometry aspects, such as the target-shadow region. Luckily, W7-X provides a large flexibility in operational and magnetic configuration parameters. Modeling and experiment shows that this enables W7-X to operate in different boundary regimes that are dominated by different physics. This allows investigating the role of the complex geometry and relevant, leading-order physics.
Deducing physics purely from experimental measurements with limited spatial resolution is challenging in W7-X due to strong poloidal and toroidal asymmetries, but the fast advancement of imaging diagnostics allows to address this challenge. We observed clear signatures of the complicated 3D structure of the island divertor plasma, such as strongly toroidally localized radiation features or details of the target heat loads. In addition, first reactor-relevant scaling approaches for heat transport, the impact of drifts, and radiation capabilities are emerging.
New mean field and turbulence codes for the drift-reduced Braginskii model in 3D-geometries can now capture and allow understanding experimental observations such as the drift-driven evolution of counter-streaming flows towards uni-directional patterns, strong poloidal ExB drift flows along the last closed flux surface and particular turbulent features in the island.
Pushing reactor relevance of stellarators, the island geometry became part of the magnetic optimization. A more refined insight into relevant geometric island parameters (shear, pitch, size) to tailor their properties emerges. Divertor target optimization (closed vs open) addresses the observed limitation of the density-build up in W7-X. Both aspects show promising results for a consistent path towards a reactor-relevant exhaust performance.
The large variability of the magnetic configuration space of island divertors and other concepts, such as chaotic boundaries, make it a challenge to focus the research for fast progress. However, it also provides opportunities for innovative geometries to further improve the stellarator exhaust. Initial results show the potential for additional heat flux broadening, radiation control or more ‘tailored’ transport properties.