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Overview on Recent W7-AS Results

After ten years of operation many experimental goals of W7-AS have been reached and many problems solved as far as possible. Among the set of criteria used to design the further optimized W7-X the feasibility of modular coils, the reduction of the Pfirsch-Schlueter currents and the resulting reduction of the Shafranov shift were demonstrated on the partly optimized W7-AS. The neoclassical transport in the long mean free path regime and the anomalous transport are still being investigated. High average beta values of 2% were obtained, the beta limit will be studied further after increasing the neutral beam power. ICRH experiment were continued with a new double strap antenna. In addition, the experiments on W7-AS point out the necessity of a divertor. A first test of the island divertor concept foreseen for W7-X is one the major remaining goals. After installation of control coils to change the size and properties of boundary islands their influence on the plasma/ limiter interaction and on the transport was investigated. In a second step, divertor moduls will be mounted in the torus vessel, and the necessary divertor diagnostics added. 1. NEOCLASSICAL TRANSPORT Neoclassical transport is an important subject in optimized stellarators because the threedimensional magnetic field configuration is unavoidably connected with magnetic mirrors where particles can be trapped. Thus, neoclassical transport as the transport enhanced by trapped particle effects can be rather large in the long mean free path collisionality regime and needs to be optimized. The tool to calculate the neoclassical transport in 3D geometry is the DKES code [1]. Consequently, the investigation of the neoclassical transport in the partly optimized W7-AS was a major objective. The essential result is that the heat transport is well described by the DKES code if the important radial electric field effects are included. Thus its predictions for the further optimized W7-X should be reliable. Radial electric fields usually reduce the neoclassical heat transport losses [2]. Thus, in the case of the ion root (negative Er(r)) high ion temperatures could be obtained in W7-AS, and the electron root feature (positive Er(r)) leads to very high central electron temperatures [3]. Statements on particle transport are usually much more difficult. Neoclassical particle fluxes are assumed in the DKES code to determine selfconsistently the radial electric field for each flux surface from the local ambipolarity condition Γe = Γi. They are roughly consistent with the experimental particle fluxes derived by integrating the NB particle deposition, and also the calculated Er(r) values agree quite well with measured ones. Only at outer radii, where recycling results in strong particle sources an additional anomalous particle transport mechanism is needed [2]. In electron root discharges an interesting particle transport phenomenon should occur because a neoclassical outward drift of impurities is predicted for the center where the strong positive Er(r) is found. Measurements by active CXRS on He and N lines show somewhat hollow profiles and confirm this prediction at least partly. An anomalous particle diffusion as typically found for low density ECRH discharges by Al laser blow off seems to reduce the effect as compared with a simplified neoclassical transport model [4]. Generally, particle transport on W7-AS as derived from impurity transport studies improves with increasing electron density and deteriorates with heating power as is typical for an anomalous transport scaling. Compared with neoclassical predictions impurity transport in low density ECRH discharges is found to be higher. In high confinement discharges (high density, but rather low heating power) diffusion coefficients seem to be even smaller than simplified neoclassical estimates predict [5]. In these discharges a stationary state in the bolometer radiation and in impurity line measurements is not reached within pulse lengths of up to about 2 s. 2. ANOMALOUS TRANSPORT There are many indications that basically the same transport mechanisms are responsible for the anomalous transport in tokamaks and stellarators, which also means that the anomalous transport is essentially not understood in both cases. On the other hand, the large flexibility of W7-AS can be used to obtain at least some additional hints and to study e.g. the role of magnetic shear and the influence of high order rational magnetic surfaces on the transport. In the meantime, the wellFig. 1: Dependence of the diamagnetic energy content and of the impurity radiation on the rotational transform at the plasma boundary. Discharge parameters are: Bo = 2.5T, 350 kW ECRH, constant average electron density of about 2⋅1013 cm-3 known dependence of the diamagnetic energy content Wdia on the rotational transform ι in W7-AS (Fig. 1) is at least partly understood, i.e. it is understood why in the standard mode of net-current free operation and at rather low beta optimum confinement is usually found only close to low order rational values of ι like 1/3 and 1/2 where high order rational values are almost absent. Evidently these high order rational ι values seem to enhance the local transport as long as the shear is not too large. Increasing the magnetic shear for such degraded situations e.g. by an ohmic current, continuously improves confinement back to the optimum. The radial region dominated by neoclassical transport continuously expands towards the boundary due to the increase of neoclassical transport with temperature and a simultaneous reduction of anomalous transport. Thus there seems to be no essential difference between optimum confinement at low shear and the confinement at high shear, and this result is independent on the sign of shear [6]. In this way the dependence of Wdia on ι in net current free discharges can roughly be modelled di am ag ne tic e ne rg y [k J] 0 0.30 0.40 (a) ι 0.50 0.60 2 4 6