Edge Transport Barrier And Edge Turbulence During H-Mode Operation In The W7-As Stellarator

H-mode operation in W7-AS is obtained in narrow windows of the edge rotational transform which correspond to minima of the calculated poloidal rotation viscosity. The quiescent H-mode occurs through an ELMy phase in which the time intervals between the ELMs show fluctuation amplitudes and spectra identical to the fully developed quiescent H-mode. The ELMs are used as a spontaneous perturbation to study breakdown and re-establishment of the edge transport barrier. A strong poloidal propagation of density turbulence towards the electron diamagnetic drift direction is found even during the ELM. The changes in fluctuation spectra and macroscopic edge profile gradients associated with an ELM occur on the same timescale of about 100 mu s.