Impact of spatially varying transport coefficients in EMC3-Eirene simulations of W7-X and assessment of drifts

TL;DR

This study investigates the effects of spatially varying transport coefficients on the modeling of the W7-X stellarator's scrape-off layer, highlighting the importance of drifts for improving agreement with experimental data.

Contribution

It extends previous simulations by incorporating spatially varying diffusion coefficients and assesses their impact on modeling accuracy compared to experimental observations.

Findings

Spatially varying coefficients improve model-data agreement.

Discrepancies remain despite adjustments, indicating other physics are involved.

Drifts are likely crucial for resolving remaining differences.

Abstract

Modelling the scrape-off layer of a stellarator is challenging due to the complex magnetic 3D geometry. The here presented study analyses simulations of the scrape-off layer (SOL) of the stellarator Wendelstein 7-X (W7-X) using spatially varying diffusion coefficients for the magnetic standard configuration, extending our previous study. Comparing the EMC3-Eirene simulations with experimental observations, an inconsistency between the strike-line width (SLW) and the upstream parameters was observed. While to match the experimental SLW a particle diffusion coefficient $D \approx 0.2$ is needed, $D \approx 1$ is needed to get experimental separatrix temperatures of 50\,eV at the given experimental heating power. We asses the impact of physically motivated spatially varying transport coeffients. Agreement with experimental data can be improved, but various differences remain. We show that drifts are expected to help overcome the discrepancies and, thus, the development of SOL transport models including drifts is a necessary next step to study the SOL transport of the W7-X stellarator.