A SUMMARY ON H-MODE STUDIES IN W7-AS

We will give a summary on the status of H-mode studies on W7-AS stella rator. The major H-mode characteristics compare well with those known from the tokamak H-mode. All major characteristics of the H-mode are r eproduced: The transition is spontaneous above a power and density thr eshold; particle and energy confinement improve simultaneously; a tran sport barrier at the edge develops with steep pressure gradients and E LMs appear; small scale fluctuations are strongly reduced and the deve lopment of a radial electric field is indicated by increased perpendic ular impurity flow velocity. The temporal development of the transitio n seems to be distinctively slower than in tokamaks. The H-mode can be initiated by ECRH or NBI, respectively. The power threshold can be sm aller than that of tokamaks. With ECRH, the density threshold is found to increase with heating power. The H-mode develops in small windows of the accessible iota range. These operational islands are characteri sed by a negative electric field already prior to the H-mode and a dis tinct maximum in space potential at the separatrix.