Description
Understanding and predicting heat exhaust properties is one of the main challenges on the path towards a stellarator power plant. The island divertor [1] is a key feature in the Scrape-Off Layer (SOL) of most current stellarator reactor designs, and it is also experimentally investigated in the Wendelstein 7-X (W7-X) stellarator [2]. Such a concept leverages a magnetic island chain formed at the plasma edge, which is intersected by divertor plates to enable effective power exhaust. Numerical modeling of island divertors usually relies on transport codes. However, significant discrepancies between these predictions and experimental data highlight the need for models of higher fidelity [3].
We report on our results produced by the transcollisional full-f fluid turbulence code GRILLIX [4] for the edge and SOL of stellarators. GRILLIX has a global, three-dimensional, electromagnetic model, in which the fluid equations are solved up to the third moment for both the plasma and the neutrals. The code is applied to a simplified island divertor geometry, which involves a toroidal magnetic field with circular cross section and a superposed 5/5 magnetic island chain. These are intersected by discrete target plates aligned with the islands, mimicking the divertor topology of W7-X. Such model enables the investigation of general island divertor phenomena, including the structure of the plasma potential, and the presence of sheared poloidal flows in the islands [2]. We also determine the heat- and particle transport channels in the SOL, as well as the characteristics of the observed turbulence. Furthermore, we explore the dependencies of the above mentioned processes on the separatrix density, and on the size of the closed flux surface region around the O-point of the island.
Lastly, we present the first, full-size, proof-of-principle edge and SOL simulation of W7-X with a turbulence code in a setup that includes the island divertors.
[1] P. Grigull et. al. Plasma Physics and Controlled Fusion 43 A175 (2001).
[2] C. Killer et al. Nuclear Fusion, 65, 056026 (2025).
[3] D. Bold et. al. Nuclear Fusion, 62, 10601 (2022).
[4] W. Zholobenko et. al. Nuclear Fusion, 64, 106066 (2024).