Heat Transport Scaling in the W7-X Island Divertor

The stellarator line of magnetic confinement fusion devices offers a promising pathway toward a scalable, carbon-free energy source. In the Wendelstein 7-X (W7-X) advanced stellarator experiment, magnetic islands at the plasma edge form the basis for an island divertor exhaust concept. Evaluating power exhaust performance in this inherently three-dimensional (3D) scrape-off layer (SOL) presents unique challenges. Building on approaches from tokamak studies, we present a novel framework for characterizing heat transport in the W7-X island divertor, inspired by the heat flux parameterization presented by Eich et al [1]. The framework decomposes the divertor heat flux pattern into three transport channels associated with distinct topological regions of the island SOL. Each channel is characterized by a representative length scale (width), which reflects the interplay between parallel and cross-field transport. Notably, the average power-channel width, Λ_𝑊, acts as a stellarator analogue of the tokamak SOL heat flux width 𝜆_𝑞. Empirical scaling of SOL transport widths in W7-X is presented. Finally, the model is used to discuss design considerations for a 3D closed island divertor geometry in W7-X, along with estimations of the heat load deposition.

[1] T. Eich et al, Phys. Rev. Lett. 107 (2011) 215001