Edge transport barrier formation and ELM phenomenology in the W7-AS stellarator

In NBI discharges with density ramps in W7-AS, the quiescent H-mode is rest ricted to the same ranges of the edge rotational transform as in ECRH disch arges and occurs above threshold densities greater than or equal to 10(20) m(-3) which increase with heating power. Higher power needs higher density for stabilization. The approach to the quiescent H-mode often occurs? with increasing density and decreasing power flow through the edge, from grassy through dithering states to bursts of ELMs and, in a few cases, quasi-perio dic ELMs. This goes parallel with increasing radial gradients of the plasma pressure and E-field at the edge. Higher heating power reduces in particul ar the T-i gradients and hence the E-field gradients, which effect can be c ompensated by higher density. The correlations found are fairly consistent when an E x B flow sheer decorrelation of the turbulent transport is assume d. (C) 2001 Elsevier Science B.V. All rights reserved.