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The divertor and the pumping system of Wendelstein 7-X is designed for stationary operation at various modes of operation. Maximum fluxes up to 10 24 electron–ion-pairs/s are expected at the targets. Various cases of plasma–wall interaction have to be considered: at low densities convective power losses and proceeding to higher densities a high recycling mode, including detachment dominated by radiation. External fluxes (NBI, gas puffing and pellet injection) up to 10 22 particles/s have to be pumped. The geometry of the target plates, the installation of additional baffle plates and the position of the pumping gap were optimised on the basis of numerical studies with the 3D neutral particle code EIRENE. For particle exhaust turbomolecular pumps (TMPs) and cryo-panels will be installed. Taking in account the capabilities of the TMP in the magnetic stray field an effective pumping speed for H 2 of 4200 l/s at one divertor box will be reached. Including the cryo-panels integrated inside the divertor boxes a total pumping speed of up to 200 000 l/s will be provided.

The new island divertor in W7-AS enables quasi steady-state operation with NBI at very high density including scenarios with stable detachment from the targets. Experiments with reversed B-field indicate that the interaction zones on the targets are affected in first order by E×B drifts. Stable detachment is restricted to magnetic field configurations with sufficiently large separation between x-points and targets and not too small field line pitch inside the islands. It is always partial in the sense that it does not extend over the full target area. This inhomogeneity is ascribed to an in/out asymmetry of the electron temperature at the upstream separatrix position.

A promisingnew operational reg ime on the Wendelstein stellarator W7-AS has been discovered, fulfillingthe conditions of optimal core behavior in combination with edge parameters suitable for successful divertor scenarios. This regime, the high density H-mode (HDH), displays no systematically evident mode activity, and is edge localized mode (ELM)-free. It is extant above a power-dependent threshold density and characterized by flat density profiles, high energy- and low impurity-confinement times and edge-localized radiation. Impurity accumulation, normally as

Extensive parameter studies have been carried out with the EMC3-EIRENE code. Major code predictions, namely the absence of high recycling prior to detachment, additional momentum losses associated with the specific island divertor geometry and the jump of the radiation at detachment transition have been verified by the W7-AS divertor experiments. Measurements and simulations are compared for high density, high power W7-AS divertor discharges and the physics related to rollover and detachment is discussed in detail. Local comparisons with the W7-AS experiment have been started with a new code version accounting for the real open-island geometry. Specifically, the observed asymmetric power unloading of the target plates at detachment transition could now be reproduced and explained. Agreement with the experiment was also found for the unexpected spatial structure of particle deposition by including classical E×B drifts into the code.

In NBI discharges with density ramps in W7-AS, the quiescent H-mode is restricted to the same ranges of the edge rotational transform as in ECRH discharges and occurs above threshold densities ≥ 10 20 m 3 which increase with heating power. Higher power needs higher density for stabilization. The approach to the quiescent H-mode often occurs, with increasing density and decreasing power flow through the edge, from grassy through dithering states to bursts of ELMs and, in a few cases, quasi-periodic ELMs. This goes parallel with increasing radial gradients of the plasma pressure and E-field at the edge. Higher heating power reduces in particular the T i gradients and hence the E-field gradients, which effect can be compensated by higher density. The correlations found are fairly consistent when an E x B flow shear decorrelation of the turbulent transport is assumed.

An extended version of the 3D Monte Carlo edge plasma transport code EMC3 including more complex physics is presented. The balance equations for mass, momentum and energies are formulated in a general conservation form suited for direct application of the Monte Carlo solving algorithm. The new extended version is applied to the planned W7-AS island divertor. First results of the investigations, which are focused on neutral pump-out efficiency, power load distribution on the plates and high recycling performance, are presented.

Edge plasma scenarios in island divertor configurations (‘natural’ magnetic islands intersected by targets) are studied by comparing data from moderate to high density NBI discharges with 3D code (EMC3/EIRENE) results. The data strongly indicate that high recycling with significant particle flux enhancement was achieved in this geometry. But, plasma pressure losses towards the targets are relatively strong, and high recycling sets in only at n e > 10 20 m −3 . The respective density enhancement in front of the targets is moderate (up to a factor of about three relative to the upstream density). These scenarios are also in basic agreement with B2/EIRENE code predictions. At n e > 1.5 × 10 20 m −3 detachment seems to develop. Improvements are expected from additional coils controlling the field line pitch inside the islands, and from optimized targets which will better focus recycling neutrals into the islands. Both are in preparation.

Basic topological features of the island divertor concept for low shear stellarators are discussed with emphasis on the differences to tokamak divertors. Extensive measurements of the edge structures by two-dimensional plasma spectroscopy and by target calorimetry are in excellent agreement with predicted vacuum and equilibrium configurations, which are available up to central β values of ∼ 1%. For this β value the calculated field-line pitch inside the islands is twice that of the corresponding vacuum case. Video observations of the strike points indicate stability of the island structures for central β values up to ∼ 3.7%. The interpretation of the complex island divertor physics of W7-AS has become possible by the development of the three-dimensional plasma transport code EMC3 (Edge Monte Carlo 3D), which has been coupled self-consistently to the EIRENE neutral gas code. Analysis of high density NBI discharges gives strong indications of stable high recycling conditions for n e ≥ 10 20 m −3 . The observations are reproduced by the EMC3/EIRENE code and supported by calculations with the B2/EIRENE code adapted to W7-AS. Improvement of recycling, pumping and target load distribution is expected from the new optimized target plates and baffles to be installed in W7-AS.

A fully 3D self-consistent Monte Carlo code EMC3 (edge Monte Carlo 3D) for modelling the plasma transport in island divertors has been developed. In a first step, the code solves a simplified version of the 3D time-independent plasma fluid equations. Coupled to the neutral transport code EIRENE, the EMC3 code has been used to study the particle, energy and neutral transport in W7-AS island divertor configurations. First results are compared with data from different diagnostics (Langmuir probes, Hα cameras and thermography).

The properties of an island divertor for Wendelstein 7-AS were investigated by using the B2-Eirene package. Because B2 is a two dimensional code the calculation needs to be done on a toroidally averaged grid. Carbon influx resulting from physical and chemical sputtering was treated self-consistently. The model predicts a transition from low to high recycling at a separatrix density of 1 ∗ 10 19 m −3 . A detailed discussion of the momentum removal mechanism for this geometry will be presented.

To justify the design of the divertor plates in W7-X the magnetic fields of finite-β HELIAS equilibria for the so-called high-mirror case have been computed for various average β-values up to 〈 β 〉 = 0.04 with the NEMEC free-boundary equilibrium code [S.P. Hirshman, W.I. van Rij and W.I. Merkel, Comput. Phys. Commun. 43 (1986) 143] in combination with the newly developed MFBE (magnetic field solver for finite-beta equilibria) code. In a second study the unloading of the target plates by radiation was investigated. The B2 code [B.J. Braams, Ph.D. Thesis, Rijksuniversiteit Utrecht (1986)] was applied for the first time to stellarators to provide of a self-consistent modelling of the SOL including effects of neutrals and impurities.

At low ι (∼ 0.34), W7-AS is operated with limiter; smooth flux surfaces extend into the limiter shadow. At higher ι, the plasma surface is separatrix dominated. The resonant 5/m island chain gives rise to corrugated surfaces. This paper deals with two difficulties on the way to a complete edge transport modelling in this complex 3D geometry. (1) At low ι, strong deviations from exponential edge density decay (shoulders) are observed. Measurements with reversed B-field indicate that they are caused by superimposed, stationary Eθ × B particle drifts. (2) The open edge topology of the corrugated high ι case cannot be represented by standard magnetic coordinates. As a first step towards 3D modelling, the open structures were parametrized on the basis of the flux conserving, open magnetic surface concept. As a first application, wall recycling fluxes for a low density ECRH discharge have been derived by mapping Langmuir probe data onto the wall along the open surfaces and used to simulate the 3D neutral gas distribution with the EIRENE code.

In the modular advanced stellarator W7-AS, the plasma performance and the main characteristics of the plasma-wall interaction are strongly affected by the three-dimensional edge topology. Both limiter-and separatrix-dominated configurations are possible. TiC and bulk-boronized limiter materials have been used. The impurity behaviour and the accessible plasma parameter ranges are compared for different limiter and wall conditions. With limiters, optimum plasma performance in currentless ECRF-or NBI-heated discharges was achieved with bulk-boronized graphite limiter material and boronized walls. Solid target sputter boronization, however, was found to be ineffective in comparison with boronization by He/B 2 H 6 glow discharges. For separatrix-dominated discharges, conditioning by wall coating has short-term effects only. Enhanced, localized plasma outflow to the wall due to islands at the boundary quickly erodes the layers. The possibility to develop a divertor concept is discussed. Basic properties of the plasma edge edge as derived from Langmuir probes and limiter calorimetry are described. Modeling is complicated by three-dimensionality. In a first approach, a 1D edge transport model on the basis of distinct flux bundles is applied.