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

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.

Numerical analysis of particle recycling in the TEXTOR helical divertor

“…The former can be caused by direct losses from the plasma flow due to volume recombination. Other mechanisms to reduce f mom are charge exchange with recycling neutral particles, which is expected to drive the flux roll-over observed in poloidal divertor configurations, or cross-field particle diffusion and viscous transport acting on counter-flows, which has been concluded to be the relevant mechanism in stellarators like W7-AS [22] and the Tore Supra ergodic divertor [5]. The non-conductive heat flux corrections in f cond account for parallel heat convection along open field lines and, in particular in the TEXTOR helical divertor configuration, cross-field energy transport from the separatrix to the target.…”

3D simulations of gas puff effects on edge plasma and ICRF coupling in JET

“…These free parameters can be derived through comparisons of the simulated plasma parameters with the experimental ones. Besides its wide applications in stellarators [28,29], EMC3-EIRENE is also attracting extensive interest for its implementation in tokamaks where some 3D SOL physics aspects need to be accounted. For example, the code was applied to model ICRF antenna coupling experiments [6,8], the influences of the Resonant Magnetic Perturbation fields [30], the 3D edge plasma convection [20] and the plasma-wall interactions [20,31].…”

Parallel impurity dynamics in the TJ-II stellarator

“…However, the incomplete understanding of some of these operational regimes make their extrapolation to future devices and reactor conditions uncertain. Generally the observations of low impurity content in plasmas with negative radial electric field is attributed either to conditions of reduced impurity transport in the scrape-off layer region, resulting in lower influx into the confinement volume [6,5] or to anomalies in the radial transport of impurities in the confinement region [4,7].…”