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, $E_\textrm{r}$ 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 $E_\textrm{r}$ in the SOL to the sheath potential drop gradient at the target. Additionally, we investigate the effect of the edge $E_\textrm{r}$ 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 $E_\textrm{r}$ 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 ( $\lambda_\mathrm{q}$ ) resulting in a suitable proxy for the model-relevant $\lambda_\mathrm{T}$ , 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 $E_\textrm{r}$ in the SOL depends on parameters at the divertor, following a $E_\textrm{r} \propto T_\mathrm{e}/\lambda_\mathrm{q}$ qualitatively similar to that found in a tokamak. Then, from the analyzed plasmas, the observed $E_\textrm{r}$ shear at the edge is linked to a moderate local reduction of the amplitude of density fluctuations.

Keywords

• Doppler reflectometry
• stellarator
• SOL
• Wendelstein 7-X