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

Abstract

The formation of the radial electric field, Er 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 Er in the SOL to the sheath potential drop gradient at the target. Additionally, we investigate the effect of the edge Er 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 Er 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 (λq) resulting in a suitable proxy for the model-relevant λT, 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 Er in the SOL depends on parameters at the divertor, following a Er ∝ Te/λq qualitatively similar to that found in a tokamak. Then, from the analyzed plasmas, the observed Er shear at the edge is linked to a moderate local reduction of the amplitude of density fluctuations.