Observation of a drift-driven local transport regime in the island divertor of Wendelstein 7-X

TL;DR

This study observes a drift-driven local transport regime in the Wendelstein 7-X island divertor, revealing how electric fields and E×B drifts influence temperature profiles and transport regimes at different densities.

Contribution

It identifies a density-dependent transition in transport regimes and demonstrates the role of E×B drifts and non-diffusive transport channels in the island divertor.

Findings

Hollow Te profiles at low densities imply strong E×B drifts.

Above a threshold density, drifts diminish and profiles become monotonic.

Modeling shows non-diffusive transport is necessary to explain observed profiles.

Abstract

Measurements of the poloidal electron density and temperature distribution in the W7-X island divertor reveal the existence of two distinct scrape-off layer transport regimes. At low and intermediate densities, $T_e$ profiles across the magnetic island are hollow, with a minimum at the island O-point. Above a threshold density $\overline{n}_e \approx 5 \times 10^{19}$ m$^{-3}$, the local minimum vanishes and the $T_e$ profiles become monotonic. At the lowest studied densities, the hollow $T_e$ profiles imply an electrostatic field of up to $\textbf{E}_r \sim 6$ kV/m, which can induce $\textbf{E} \times \textbf{B}$ drifts around the island, with drift velocities of up to $\sim3$ km/s. These $\textbf{E} \times \textbf{B}$ drifts result in strong convective heat transport poloidally around the island, binormal to the magnetic field, which supports the hollow radial temperature profile and hence provides a positive feedback to the electric field. Above the threshold density, these drifts strongly diminish, presumably due to increased anomalous diffusion, which causes the profile hollowness and therefore also the radial electric field to strongly diminish. This argument is supported by comparison to EMC3-EIRENE modeling and an analytical transport analyses showing that an additional, non-diffusive transport channel is required to maintain the profile hollowness up to the densities seen in experiment, and that a poloidal drift transport channel supports such hollowness only up to certain densities.