T. Tork (P): Plasma boundary modeling for island diverted stellarators

• Tobias Tork (IPP Bereich Greifswald)

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. Current state-of-the-art stellarator boundary simulations like EMC3-Eirene do not include drifts, but experiments in W7-X already 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 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 in stellarators. We will show first of its kind results of W7-X simulations. Stagnation points of the parallel ion flow demonstrate a significant poloidal shift which depends on the magnetic field direction. The electrostatic potential is mainly aligned with the separatrix of the last closed flux surface, which lead to a poloidal ExB flow. In the confined region, the radial electric field is proportional to the radial gradient of ion pressure, which can be explained via the balance of ExB and diamagnetic flows. The distribution in the scrape-off-layer shows a significant impact of the island divertor on the potential.

C.W. Rogge (P): Modelling the pellet-produced plasmoid drift in JOREK

• Carl Rogge (IPP Garching)

Pellet refuelling will be a key technique for sustaining plasma density in future fusion reactors based on the stellarator concept. Although this method is more mature in tokamak experiments, the departure from axisymmetry in stellarators introduces unique challenges and opportunities that remain poorly understood. Here, the tokamak version and stellarator extension of the 3D nonlinear MHD code JOREK are used to investigate the radial drift of a fully ionised pellet. Since this radial drift depends on the pressure evolution inside the pellet-produced plasmoid, the parallel expansion strongly influences the radial deposition of the new material. Therefore, special care must be taken to accurately model the heating of the plasmoid by hot background plasma particles, which drives the parallel expansion. The long electron mean free path expected in the core of future reactors means that non-local heating by nearly free-streaming electrons must be considered in our fluid model. Two non-local heating schemes are investigated to be verified against analytic theory.