The stellarator Wendelstein 7-X (W7-X) is presently under construction at Greifswald, Germany, and the start of operation is planned in 2006. W7-X is a large `advanced stellarator' of the HELIAS type (R = 5.5 m, a = 0.55 m, B0 = 3 T, five periods, moderate shear and variable rotational transform 5/6 ⩽ ι ⩽ 5/4 at the boundary) with the aims of demonstrating the reactor potential of this stellarator line in steady state operation close to fusion relevant parameters. The capability of stationary operation requires the realization of a superconducting magnet system consisting of 50 modular coils and 20 planar coils, the operation of a 140 GHz ECR CW heat source of 10 MW, the installation of a divertor to handle the power and particle flux, and to limit the impurity fraction to tolerable levels. Additional heating schemes, ICRF and NBI, will be provided for flexible experimentation.

The fundamental behavior of the W7-X island divertor under detached conditions, which has been theoretically predicted with the EMC3-Eirene code, is re-examined here under the experimental conditions achieved so far and compared with the first experimental results. Both simulations and experiments cover a range of divertor configurations and plasma parameters, and show the following common trends: (1) with rising impurity radiation, the target heat load decreases ‘uniformly’ over the entire target surface in the sense that both the peak and average heat loads can drop by an order of magnitude. Impurity radiation (mainly from intrinsic carbon) occurs primarily at the plasma edge and the resulting negative impact on the stored energy is less than 10%. (2) When the total radiation exceeds a critical level, the target particle flux (the recycling flux Γrecy) begins to fall and can drop by a factor of 3–5 at high radiation levels without an obvious indication of significant volume recombination. (3) While Γrecy decreases, the divertor neutral pressure continues to build up and reaches a maximum, at which point Γrecy has declined significantly. (4) During detachment, the electron temperature at the last closed flux surface falls in a way that is not quantitatively understandable from parallel classical heat conduction processes. This paper presents a physical explanation of the numerical/experimental results described above. Furthermore, using the EMC3-Eirene code as a diagnostic tool, we are able, apparently for the first time, to provide a full quantitative analysis of each transport channel in the island divertor, aiming to clarify how the island divertor plasma self-regulates to maintain particle, energy, and momentum balance under detached conditions.

Results from the investigation of neoclassical core transport and the role of the radial electric field profile (E r) in the first operational phase of the Wendelstein 7-X (W7-X) stellarator are presented. In stellarator plasmas, the details of the E r profile are expected to have a strong effect on both the particle and heat fluxes. Investigation of the radial electric field is important in understanding neoclassical transport and in validation of neoclassical calculations. The radial electric field is closely related to the perpendicular plasma flow (u ⊥) through the force balance equation. This allows the radial electric field to be inferred from measurements of the perpendicular flow velocity, which can be measured using the x-ray imaging crystal spectrometer (XICS) and correlation reflectometry diagnostics. Large changes in the perpendicular rotation, on the order of ∆u ⊥ ∼ 5 km/s (∆E r ∼ 12 kV /m), have been observed within a set of experiments where the heating power was stepped down from 2 M W to 0.6 M W. These experiments are examined in detail to explore the relationship between heating power, temperature and density profiles and the radial electric field. Finally the inferred E r profiles are compared to initial neoclassical calculations using measured plasma profiles. The results from several neoclassical codes, sfincs, fortec-3d and dkes, are compared both with each other and the measurements. These comparisons show good agreement, giving confidence in the applicability of the neoclassical calculations to the W7-X configuration.

The paper presents procedures which have been developed for a quantitative analysis of the divertor power deposition at Wendelstein 7-X. The development of these tools is motivated by the need to compare and verify scientific and engineering predictions with experimental measurements. The measurements have been performed by means of the thermographic diagnostic system, capable of exploring the divertor heat loads, with the aim to study the heat load symmetry, compare footprint patterns with theoretical expectations, but also investigate leading edges and divertor misalignment. In order to compare measurements and numerical calculations, an accurate mapping between the camera data, the divertor geometry and the 3D CAD models has been constructed. This mapping allows to find a correspondence between the data in different representations, simplifying data interpolation and visualization. This also provides a high resolution model of the target surface to compare numerical heat deposition calculations with experimental results from different cameras.