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

TL;DR

This study investigates the formation of the radial electric field in the scrape-off layer of Wendelstein 7-X stellarator, confirming tokamak-like models and linking edge shear to turbulence reduction.

Contribution

It demonstrates for the first time that $E_{r}$ formation in W7-X's SOL follows tokamak-like relations involving divertor parameters, and explores turbulence suppression by shear.

Findings

$E_{r}$ depends on divertor parameters, following $E_{r} propto T_{e}/\lambda_q$

Edge $E_{r}$ shear correlates with reduced density fluctuations

Tokamak-like models are validated in a stellarator context.

Abstract

The formation of the radial electric field, $E_{\rm r}$ in the 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 3D island divertor of simple models, typically developed in tokamaks, relating $E_{\rm r}$ to the sheath potential drop gradient at the target. Additionally, we investigate the effect of the edge $E_{\rm r}$ 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 $E_{\rm r}$ and density fluctuations with good spatial resolution. Three-dimensional measurements of divertor parameters has been carried out using infrared cameras, with $\lambda_{\rm q}$ resulting a suitable proxy for the model-relevant $\lambda_{\rm T}$. In the investigated attached regimes, it is shown for the first time that the formation of the $E_{\rm r}$ in the SOL depends on parameters at the divertor, following a $E_{\rm r} \propto T_{\rm e} /\lambda_q$ qualitatively similar to that found in a tokamak. Then, from the analyzed plasmas, the observed $E_{\rm r}$ shear at the edge is linked to a moderate local reduction of the amplitude of density fluctuations.