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The island divertor concept has successfully been realized at W7-AS. The divertor gives access to a new NBI-heated high density regime with densities up to 4×10²⁰m-3 and energy confinement well above customary scalings. This regime appears promising with respect to the requirements of both confinement and exhaust. Many features are reminiscent of a quiescent H-mode, but plasma particle transport behaves differently. Beyond a threshold density energy confinement approximately doubles whereas particle confinement dramatically decreases. The density profile flattens and the inward impurity pinch is reduced. This eases density control, prevents impurity accumulation, and allows for quasi-stationary conditions with radiation profiles peaked at the edge. At the highest densities partial detachment occurs with a radiated power fraction up to 90% at a tolerable expense of plasma energy. The sub-divertor pressure is sufficient for pumping. Major experimental results, such as the lack of a high recycling phase preceding detachment, are also predicted by the EMC3/EIRENE code.

W7-AS operates with an island divertor, which utilises the natural edge islands of the low-shear stellarator configuration to divert the plasma. High densities (up to 4×1020m-3) and partly detached divertor conditions have been attained. The details of the divertor operation will be described along with the 3D-efforts to model the divertor observations. At a density beyond 1.5×1020m-3 another confinement bifurcation appears, which allows steady state operation at good energy and low impurity confinement. The transition is possible from a state of normal confinement or from ELMy or quiescent H-modes. Starting with normal confinement, bifurcation is initiated by a broadening of the density profile. This regime can easily be heated above cut-off by ECRH using electron-bernstein-wave mode conversion. The highest beta-values (>3%) are achieved at high density. The relevance of the W7-AS data for Wendelstein 7-X and the Helias-reactor will be discussed.

In the scrape-off layer and outermost edge of the Wendelstein 7-AS (W7-AS) stellarator, fluctuations of, mostly, ion saturation current Isat and floating potentialΦfl have been measured with high poloidal, radial (both 2–3mm) and temporal (0.5 μs) resolution. Parallel to the magnetic field, measurements with two probe tips on the same magnetic field line were performed. The basic spatio-temporal structure of the fluctuations is presented in terms of correlation functions and of wavenumber–frequency (kf) spectra. They are characterized by the parameters of a fit function to the correlation function, which describe the spatio-temporal structure. The behaviour of these fluctuation parameters under variations of the discharge parameters is investigated. A high correlation of the fluctuations parallel to the magnetic field is documented, and we analyse, which frequency components contribute to this high correlation. In our radially and poloidally resolving measurements, we find that the fluctuation structures are inclined in the poloidal–radial plane at our position of measurement. The consequences of such inclined structures for the interpretation of measurements with one-dimensional arrays are discussed, with an emphasis on the risk of an erroneous interpretation of purely radially resolving measurements. There is strong evidence that this inclination is due to the local magnetic shear in W7-AS in conjunction with the high correlation of the fluctuations parallel to the magnetic field. A further contribution to the inclination may originate from the radial shear of the poloidal velocity of the fluctuations in the plasma edge. The different consequences of magnetic shear and velocity shear for an inclination of the fluctuations in the poloidal–radial plane are discussed.

Particle transport was studied in ASDEZ with modulated puffing of the discharge gas and of impurities. The energy transport is investigated by numerical simulation of the heat pulse after the swatooth crash. Small scale density fluctuations are investigated in the confinement region with far infrared scattering and reflectometry and in the edge plasma with langmuir probes and Ha diagnostic. In addition to a diffuse component of the particle transport, a strong inward drift is observed in all discharges. In ohmic discharges the transport coefficients decrease and saturate like 1/TE with increasing density. They are smaller in deuterium that in hydrogen. In the improved ohmic confinement (IOC)regime mainly D in the outer region is reduced. D increases proportionally to the heating power in L-mode discharges. The improvement of particle confinement in the H-mode is explained by a increase of the inward drift at the edge rather than a decrease of D. The impurity diffusion coefficient is independent of the impurity mass and charge. In ohmic discharges, it varies with ne like the bulk diffusion coefficient, is independent of B or increases weakly with B and increases with Ip. In L-mode discharges, Dimp increases linearly with the heating power. The electron thermal condustivity determined by heat pulse propagation exceeds the stationary value by a factor of 3-4, assuming merely diffusive heat transport. Convection does not significantly reduce this factor. however, non-diagonal terms.