Sniffer probe measurements in W7-AS

Wendelstein W7-AS is a modular Advanced Stellarator with a major radius R=2 m, effective plasma radius a≤0.2 m, fivefold symmetry of the configuration and B≤2.5 Tesla. The rotational transform z can be varied between 0.25 and 0.7, the vacuum shear being low. W7-AS allows an operation free of net plasma current using ECRH and/or NBI up to 3 seconds. Typical plasma parameters obtained with ECRH are Te≤3 keV, Ti≤0.7 keV with central densities neo<6x1019 m-3. The maximum electron density obtained with NBI heating was neo=3x1020 m-3, the energy confinement ranges between 5 and 30 ms. The edge plasma has a complex structure reflecting the 5/m symmetry of ''natural'' magnetic islands. A degradation of both energy and particle confinement is found at low order rational values of z(a) at the plasma edge, whereas optimum confinement can be established in narrow z-windows close to the resonances at z=1/3 and z=1/2. The poloidal cross-section of the plasma changes five times around the torus according to the fivefold periodicity from a vertical ellipse via a triangular shape to a vertical ellipse again. At low z the minor plasma radius is defined by two movable limiters whereas above z=1/2 the edge structure becomes separatrix dominated. The complex edge structure varying with z gives rise to inhomogeneous particle and energy flows to the wall and in-vessel installations which determine on the other hand the working gas recycling and the impurity generation affecting on their part the confinement properties of the plasma. The latter issues are essential topics of the experimental edge plasma programme of W7-AS. (author) 4 refs., 4 figs