High Resolution Studies of Edge Relaxation Events in Tokamaks Carl Sovinec, U. Wisconsin

Large-scale numerical computation is applied to solve fluid-based plasma models of edge localized modes (ELMs). These ‘modes’ are MHD-like instabilities that nonlinearly release bursts of particles and energy across the flow-induced transport barrier that lies near the separatrix of closed and open magnetic flux in tokamaks. Understanding their dynamics is important for the large international ITER experiment, which is now under construction. Numerical computation is required, but extreme anisotropy and scale separation make it a formidable task. Here, we present some of the first nonlinear computations of ELMs to include two-fluid effects. This modeling captures the separate drifts of ions and electrons which linearly stabilizes the highest wavenumber modes and avoids a nonlinear ‘ultraviolet’ catastrophe without ad hoc dissipation. However, the two-fluid system supports high-frequency dispersive modes that are not present in MHD, and a new implicit leapfrog algorithm has been implemented in the NIMROD code (http://nimrodteam.org) to run these computations. We describe the algorithm, its parallel implementation, and computational requirements, in addition to the physical results on nonlinear structure formation illustrated by the accompanying figure.

Contours of constant temperature along a particle density isosurface from a nonlinear two-fluid simulation of ELM activity.