On Viscous Damping and Rotation in Low-Shear Stellarators

one of the main results of the Wendelstein 7-AS stellarator (major radius 2 m, average plasma radius 0' l8 m, magnetic field 2.5 T' low shear) is the achievement of the H-mode confinement in ECRand NBl-heated plasmas. The features of the H-mode are similar to those of a tokamak: reduction of the Ho-emission, onset of poloidal rotation anda30vo improvement of the energy confinement time, which is acompanied by a reduction of the turbulence level. However, the critical dependence on the rotational transform is a particular feature of this low shear stellarator and in the following paper an attempt will be made to understand this phenomenon by investigating the specific influence of the magnetic geometry on the onset of poloidal shear flow. In the frame of a dissipative model of a steady-state plasma equilibrium with flow the poloidal and toroidal force balance on magnetic surfaces are investigated. The neoclassical poloidal viscosity has a strong dependence on the external rotational transform. The H-modes windows found in wendelstein 7-AS can be identified with regions of high order rational magnetic surfaces, where viscous damping is small. The paper discusses the conditions for convective plasma flow, which in conjunction with the inertial forces is providing the spin-up mechanism of poloidal shear flow.