Transport in island divertors: physics, 3D modelling and comparison to first experiments on W7-AS*

Abstract

Basic plasma transport properties in island divertors are compared to those of standard tokamak divertors. A realistic plasma transport modelling of highdensity discharges in island divertors has become possible by implementing a self-consistent treatment of impurity transport in the EMC3-EIRENE code. In contrast to standard tokamak divertors, the code predicts no high recycling prior to detachment, with the downstream density never exceeding the upstream density. This is mainly due to momentum losses arising from the cross-field transport associated with the specific island divertor geometry. This momentum loss is effective already at low densities, high temperatures and is responsible for the high upstream densities needed to achieve detachment. Numerical scans of carbon concentration for high-density plasma typically show first a smooth, then a sharp increase of the carbon radiation, the latter being accompanied by a sharp drop of the downstream temperature and density indicating detachment transition. The jumps of the radiation and temperature are due to a thermal instability associated with the form of the impurity cooling rate function and can be reproduced by a simple 1D radial energy model based on cross-field transport and impurity losses. This model is used as a guideline to illustrate and discuss the detachment physics in details, including detachment condition and thermal instability. Major EMC3-EIRENE code predictions have been verified by the first W7-AS divertor experiments. A comparison of calculations and measurements is presented for high-density, high-power W7-AS divertor discharges and the physics related to rollover and detachment is discussed in detail. The code has been recently extended to general SOL configurations with open islands and arbitrary ergodicity by using a new highly accurate fieldline mapping technique. The method correctly reproduces flux surfaces and islands over a high number of toroidal field periods, thus ensuring a clear distinction between parallel and radial transport. The technique has been tested * This paper is an extension of work originally presented at the 28th EPS Conf., Madeira.

First EMC3-Eirene simulations of the impact of the edge magnetic perturbations at ASDEX Upgrade compared with the experiment

“…Originally it was developed for the intrinsically three dimensional W7-AS [15] stellarator. During the last few years, however, the code also found application in the tokamak community in particular since magnetic perturbation coils are used, which break the toroidal symmetry of the tokamak.…”

“…the 'reversible field line mapping' technique described in Ref. [15]). The generalization of the field line reconstruction method for block-structured grids, which is required for poloidal divertor configurations, has been presented by Frerichs et al [20].…”

“…Since the perturbation fields have an n = 2 symmetry the toroidal extension of the grid was chosen as 180 • . The bilinear field-line interpolation method used in EMC3 requires convex cells [15]. Because of finite-shear effects this is possible only for field lines of limited lengths.…”

3D analysis of impurity transport and radiation for ITER limiter start-up configurations

“…For impurities (one species), the following mass and momentum transport model is implemented in the EMC3-EIRENE code for each ionization stage Z [12]: …”

Study of a Closed Divertor Configuration for the Three-Dimensionally Complicated Magnetic Structure in the LHD Plasma Periphery

“…EMC3-EIRENE code has been used widely for detailed analysis of the plasma and neutral particle transport in the peripheral plasma in helical/stellarator and tokamak devices [6]. This sophisticated analysis using a fully three-dimensional plasma fluid code coupled with the neutral particle transport code is too time consuming in our computational environment.…”