Viscous damping of rotation in Wendelstein 7-AS

One of the main results of the Wendelstein 7-AS stellarator (major radius 2 m, average plasma radius 0.18 m, magnetic field 2.5 T, low shear) is the achievement of the H-mode confinement in ECR- and NBI-heated plasmas. There is a strong dependence on the external rotational transform; the H-mode confinement can be found only in a narrow window close to l = 1/2. Viscous damping and the interaction with a neutral background are the only mechanisms proposed so far as damping mechanism, which inhibits the poloidal shear flow. Viscous damping is computed in collisional approximation and in weakly collisionless approximation (plateau regime). Large values of the poloidal viscosity are found on rational magnetic surfaces, while in the neighborhood of low order rational the viscosity is very small. Islands provide a mechanism for enhanced momentum transport in radial direction, which leads to an effective shear viscosity. In Wendelstein 7-AS islands exist on the ''natural'' rational surfaces with l = 5/9, 5/10, 5/11.. Furthermore, a next generation of islands exist on l = 10/19, 10/21. Experimental results in Wendelstein 7-AS confirm the hypothesis that H-mode only can arise if none of these islands exists in the plasma. The regions of rotational transform predicted by this hypothesis roughly agree with those of the experiment. The extrapolation towards Wendelstein 7-X shows that there a similar case is expected. Numerical calculations of islands are made in vacuum fields; the evolution of the islands with rising plasma pressure is unknown. Finally a qualitative model of H-mode development is discussed. (orig.)