Speaker
Description
The island divertor is the leading plasma exhaust concept in stellarators, and evaluating its reactor relevance is a key objective of the Wendelstein 7-X (W7-X) experiment [1]. From a power exhaust perspective, maintaining steady-state divertor heat loads below 10 MW/m² is essential for safe and sustained device operation. However, a robust framework for quantitatively characterizing the 3D transport and facilitating reliable extrapolation to reactor-relevant conditions has been lacking for stellarators.
In this contribution, we present SMoLID (Simple Model for Loads in Island Divertor) – a novel framework for interpreting target heat flux patterns and the underlying transport mechanisms [2]. The central idea is that the 3D transport can be decomposed into distinct channels, each associated with a specific topological region of the island scrape-off layer (SOL), namely, main SOL, private flux and target shadowed regions and characterized by a representative width ($\Lambda_W$, $\Lambda_D$ and $\Lambda_S$ respectively) reflecting the balance between parallel and cross-field transport processes. The heat transport in the main SOL is described by a 3D power-carrying layer, with its width $\Lambda_W$ (analogous to the $\lambda_q$ metric in tokamaks) inferred from target heat flux using an adapted Wagner-Eich formalism [3]. For the standard magnetic configuration of W7-X, $\Lambda_W$ values at the outboard midplane (bean-shaped cross-section) are typically of the order of 1 cm under attached divertor conditions.
Furthermore, we apply SMoLID to the Infrared thermography observations at W7-X [4, 5], to develop empirical scaling laws for the transport widths as functions of some key operational and plasma parameters. Notably, $\Lambda_W$ exhibits a weak dependence on the power entering the SOL (∼ P$_{SOL}^{-0.1}$). However, we observe that $\Lambda_W$ tends to exhibit a positive dependence on plasma density, with the scaling strengthening significantly above a line-integrated density threshold of approximately 7 × 10¹⁹ m⁻².
Finally, we demonstrate the utility of SMoLID for divertor design. In particular, the concept of transport channels is leveraged to obtain a heat load compatible closed island divertor geometry for W7-X. Overall, the SMoLID framework opens new avenues for advancing the power exhaust research in island diverted stellarators.
[1] M. Endler et al, Fusion Engineering and Design 167 (2021) 112381
[2] A. Kharwandikar, PhD Thesis. University of Greifswald 2025
[3] T. Eich et al, Phys. Rev. Lett. 107 (2011) 215001
[4] Y. Gao et al, Nuclear Fusion 59.6 (2019) 066007
[5] S. Thiede et al, 2025 submitted to Rev. Sci. Instrum.