A BOUT++ transport model for island diverted stellarators including drifts

The inclusion of drifts has been shown to be critically important for the boundary physics of tokamaks and has a significant impact on the predictions of divertor heat loads and impurity transport. For stellarators, the decreased field line pitch increases the relative importance of drifts with respect to the parallel dynamics by about two orders of magnitude. Experiments in W7-X show numerous indications of drift effects. Such effects could significantly alter the heat loads on the divertors or redistribute the plasma profiles.

In this contribution, we present the development and verification of a 3D simulation for island diverted stellarators. The model is derived from the Hermes-2 physics model by parameterizing the divergence of the polarization drift with a set of anomalous diffusion coefficients. Additionally, the electrostatic potential equation is treated as a diffusion equation. The simulation tool allows for the analysis of potential and current distributions for stellarators.

We will show first of its kind results of W7-X simulations. The electrostatic potential in the confined region is dominantly governed by force balance, but the distribution in the scrape-off-layer shows a significant impact of the island divertor on the potential. In the islands, the current demonstrates double counter-streaming flows. The diffusion coefficients describing, the divergence of the polarization current, have substantial impact on the electrostatic potential.