H-Mode studies in W7-AS

A summary on H-mode studies on W/-AS stellarator will be presented. The major H-mode characteristics compare well with those known from the tokamak H-mode. all major characteristics of the H-mode are reproduced: The transition is spontaneous above a power and density threshold; particle and energy confinement improve simultaneously; a transport barrier at the edge develops with steep pressure gradients and ELMs appear; small scale fluctuations are strongly reduced and the development of a radial electric field is indicated by increased perpendicular impurity flow velocity. The temporal development of the transition seems to be distinctively slower in tokamaks. The H-mode can be initiated by ECRH or NBI, respectively. The power threshold can be smaller than that of tokamaks. With ECRH, the density threshold is found to increase with heating power. This might highlight the role of the ion thermal equilibrium at the edge. The H-mode develops in small windows of the accessible iota range. These operational islands are characterised by a negative electric field already prior to the H- mode and a distinct maximum in space potential at the separatrix. 17 refs