On the interaction between the island divertor heat fluxes, the scrape-off layer radial electric field and the edge turbulence in Wendelstein 7-X plasmas

The formation of the radial electric field, E-r in the scrape-off layer (SOL) has been experimentally studied for attached divertor conditions in stellarator W7-X. The main objective of this study is to test the validity in a complex three-dimensional (3D) island divertor of simple models, typically developed in tokamaks, relating E-r in the SOL to the sheath potential drop gradient at the target. Additionally, we investigate the effect of the edge E-r shear on the reduction of density fluctuation amplitude, a well-established phenomenon according to the existing bibliography. The main diagnostic for measurements in the SOL is a V-band Doppler reflectometer that can provide the measurement of the E-r and density fluctuations with good spatial resolution. 3D measurements of divertor parameters have been carried out using infrared cameras, with the exponential decay length of the divertor heat flux (lambda(q)) resulting in a suitable proxy for the model-relevant lambda(T), the exponential decay length of the temperature at the divertor. In the investigated attached regimes, it is shown for the first time that the formation of the E-r in the SOL depends on parameters at the divertor, following a E-r proportional to T-e/lambda(q) qualitatively similar to that found in a tokamak. Then, from the analyzed plasmas, the observed E-r shear at the edge is linked to a moderate local reduction of the amplitude of density fluctuations.