Transport of Impurity Ions in the Wendelstein 7-AS Stellarator Plasma

(W7-AS Team) Institut fur Plasmaforschung, Universitat Stuttgart, D-70569 Stuttgart, Germany (NBI- and ECRH Group) The impact of global plasma parameters on impurity transport in the stellarator W7-AS was investigated by laser blow-off technique. Both, density and heating power were identified to have a strong influence on impurity confinement I ~ ne1.2/PECRH0.8. In spite of stationary conditions at lower densities, an increasing trend for accumulation was observed at plasma densities beyond 5*1019 m-3 due to reduction of the diffusion coefficients. Up to densities of at least 9*1019 m-3, launching of electron cyclotron resonance heating (ECRH) power of 1.2 MW is able to counteract the impurity accumulation by deterioration of the impurity confinement with heating power according to the scaling law as given above. In neutral beam injection (NBI) heated plasmas at densities higher than 1*1020m-3, long confinement times were observed, often accompanied by loss of density control and degradation of plasma energy due to increasing radiation losses. The installation of island divertor allowed a general extension of the range of accessible densities up to 4*1020m-3: beyond a certain power-dependent threshold density (1.5-2.1*1020m-3), the plasma enters the High Density H-mode (HDH) regime and the impurity confinement time drops to values comparable to the energy confinement time. High density plasmas could be sustained quasi-stationary with a low level of impurity radiation. The favourable impurity behavior goes along with a reduction of the inward impurity convection in the core plasma and possible changes in the edge transport. For the characterization of the general impurity behavior in W7-AS plasmas the usual transport models for axisymmetric devices are not sufficient and additional stellarator specific processes have to be considered.