Two-fluid studies of edge relaxation events in tokamaks

Large-scale numerical computation is applied to fluid-based plasma modeling of edge localized modes (ELMs). The modes are magnetohydrodynamic-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. Some of the first nonlinear computations of ELMs to model the separate drifts of ions and electrons, which linearly stabilize the highest wavenumber modes and avoid a nonlinear "ultraviolet catastrophe" without the use of ad hoc dissipation, are presented. The two-fluid system supports high-frequency dispersive modes that are not present in magnetohydrodynamics, and a new implicit leapfrog algorithm has been implemented to run these computations. We briefly describe the algorithm and computational requirements, in addition to physical results on nonlinear structure formation.