Boundary layer and H-mode studies in W7-AS

The natural 5/m islands of the W7-AS high iota configuration shape the radial and poloidal distribution of plasma parameters in the SOL. A numerical basis for 3D edge modelling with island divertor is provided. Experimentally, it is shown that plasma fluxes are diverted by the island X-points. The particle diffusion coefficient in the SOL scales inversely with nea for both the limiter and the island separatrix case. Small poloidal electric field components give rise to convective flux contributions. At slash-l(a) = 0.53, the singular slash-l value where low power good H-mode operation is possible, the space potential conditions seem to be favourable for H-mode development. Experiments highlight the role of the ion temperature in the H-mode transition physics. (author). 7 refs, 4 figs