Physics of island divertors as highlighted by the example of W7-AS

TLDR

The island divertor physics is presented within a theoretical framework, drawing on several years of theoretical and experimental research, and the paper outlines a self‑consistent physics picture of island divertor transport in W7‑AS. The study aims to describe the global scrape‑off layer transport associated with the island topology and to illustrate the elementary differences and similarities in divertor physics between a tokamak and a typical helical device. The authors examine key divertor functions—particle flux enhancement, neutral screening, impurity retention, thermal power removal via impurity line radiation and detachment—using simple models and estimations, with three‑dimensional modelling calculations providing self‑consistency checks and determining basic functional dependences. The analysis shows a correlation between the SOL and core plasma and demonstrates that the island divertor improves global plasma performance.

Abstract

Based on theoretical analysis, numerical simulations and experimental results, the paper outlines a self-consistent physics picture of the island divertor transport in W7-AS, as it emerges from the present understanding, documented over the past several years of theoretical and experimental research on the subject. Key function elements of a divertor, such as particle flux enhancement, neutral screening, impurity retention, thermal power removal via impurity line radiation and detachment, are examined for the island divertor and assessed with respect to tokamak divertors. The paper focuses on describing the global scrape-off layer (SOL) transport behaviour associated with the specific island topology and aims at illustrating the elementary differences and similarities in divertor physics between a tokamak and a typical helical device. Shown and analysed are also the correlation between the SOL and core plasma and the role of the island divertor for improving the global plasma performance. Discussion is mainly based on simple models and estimations, while three-dimensional modelling calculations serve only for control of self-consistency and for determining basic functional dependences not accessible otherwise. The island divertor physics is presented within a theoretical frame with most key issues, however, being related to experimental results.