Speaker
Description
Particle drifts play a crucial role in the understanding of the edge physics of current and future fusion devices. While tokamak simulations routinely include drift effects, they have been largely neglected in stellarator simulations until now. The low field line pitch in a stellarator amplifies the relative importance of drifts compared to the parallel transport. Experiments in Wendelstein 7-X show observations which we can explain by the inclusion of drifts and currents. In this contribution, we present a mean-field Braginskii model that is applicable to stellarator geometries. The model is built on the BOUT++ framework and is a derivative of the Hermes model. It includes a self-consistent description of the electrostatic potential as well as drift effects. We will present first-of-its-kind simulations of the scrape-off layer of Wendelstein 7-X and use the simulations to interpret and address multiple experimental observations.
Strong poloidal ExB velocities on the order of $km\,s^{-1}$ and multiple shear regions are measured by Langmuir probes and the gas puff imaging diagnostic [Killer 2025]. The simulations reproduce ExB velocities of similar magnitude and indicate a strong increase in flow velocities in areas of densely packed flux surfaces. A region of strong ExB shear forms inside the scrape-off layer, which is qualitatively consistent with the measurements.
Coherence imaging diagnostics measure a dependence of the stagnation point on field direction for low density plasmas [Perseo 2021,Kriete 2023]. The same phenomena occur in the simulations due to parallel momentum transport by poloidal ExB flows.
Detailed analysis of the simulations shows that the electrostatic potential in the scrape-off layer mainly arises from the sheath potential drop, which gets modulated in the parallel direction by the parallel temperature and pressure gradients. Shallow gradients between the X-points compared to stronger gradients between the X-points and the divertor targets lead to potential isocontours that are predominantly aligned with the LCFS of the confined plasma.
The inclusion of drifts in the boundary modeling tools now allows for an advanced quantitative understanding of observations and should provide a more robust foundation for extrapolations to future reactors.