Speaker
Description
The island divertor concept was proposed for power and particle exhaust in low-shear stellarators, such as Wendelstein 7-X. So far low downstream density has been experimentally measured. Stronger scaling of the downstream density is necessary to ensure the reactor-relevance of the island divertor in terms of particle exhaust. Numerical studies of the island divertor SOL require 3D codes, among which EMC3-Eirene is the state of the art. In this work, a two-point model is used to investigate the recycling regimes in simplified island divertor geometries to interpret the results of 3D simulations.
The two-point model used for tokamaks has been adapted into a stellarator two-point model (STPM). This contribution extends the STPM to include volumetric parameters: a convected power fraction, a target-localized dissipated power fraction, and a more general parametrization of the momentum loss factor. The extended STPM is validated against EMC3-Eirene simulations, using an island divertor geometry that is limited to a few flux surfaces of the powercarrying layer (PCL) in order to neglect flux-surface-perpendicular transport. The importance of the volumetric parameters is demonstrated and, when extracted from the 3D simulations, a reasonable agreement is found between both models. This work also shows that the STPM predicts a detrimental ”diffusionlimited” transport regime which suppresses high recycling. The PCL geometry retains typical 3D features of the island divertor such as the non-axisymmetry of the divertor targets and the presence of a target-shadowed region (TSR). The impact of these 3D features on parallel and perpendicular profiles is presented. Finally, the extended STPM provides the ability to do two-point-model formatting which allows to interpret 3D simulations results. Open and closed island divertor geometries are compared using such a formatting.