Speaker
Description
The island divertor is the leading exhaust concept in stellarators. Used in W7-X, it has proven to provide density control, stable detachment, and impurity screening [1]. Although a regime with enhanced recycling has been observed in W7-X, moderate neutral pressures of up to 0.18 Pa have been measured so far [2]. Higher recycling conditions are necessary to ensure the reactor-relevance of the island divertor in terms of pumping efficiency.
Simplified models are used in this work to understand the leading order physics and investigate the recycling behavior in simplified island divertor geometries. One of these simplified models, the two-point model, has been found to successfully represent the tokamak SOL [3]. This model has been extended into a stellarator two-point model (STPM) by Feng [4]. We show that the model predicts a detrimental "diffusion-limited" transport regime which suppresses high recycling. This contribution extends the STPM to include correction factors: a convected power fraction, a target-localized dissipated power fraction, and a more general parametrization of the momentum loss factor.
We validate the STPM against EMC3-Eirene simulations, concentrating on the island power-carrying-layer. We elucidate the importance of the correction factors and show that when extracted from the simulations, a reasonable agreement is found between both models. This provides the ability to do two-point-model formatting [3] which allows to gain significant physics insight. We show how the volumetric losses and the diffusion-limited regime interact to limit recycling and identify a threshold parameter to avoid this detrimental transport regime.
We also show the impact of typical island 3D features (target shadow region TSR) leading to non monotonic parallel teperature profiles and strongly localized presure variations in the divertor interaction region.
[1] M Jakubowski et al 2021 Nucl. Fusion 61 106003
[2] V Haak et al 2023 Plasma Phys. Control. Fusion 65 055024
[3] D Moulton et al 2017 Plasma Phys. Control. Fusion 59 065011
[4] Y Feng et al 2006 Nucl. Fusion 46 807