Onset and nonlinear relaxation of coherent edge current-carrying filaments during VDEs

The onset and nonlinear evolution of coherent current-carrying filaments are examined using global nonlinear three-dimensional resistive MHD NIMROD simulations in a spherical tokamak (ST). We show that time-evolving current sheets/layers develop near the tokamak edge under different circumstances during transient events. In particular, during induced vertical displacement event (VDE) simulations by driving large current in the open field region, we investigate the stability of halo current and the formation of reconnecting edge peeling-driven filaments. We show that as the plasma is vertically displaced, the edge halo current sheet becomes MHD peeling-tearing unstable and form non-axisymmetric coherent edge current filament structures. Similar to fast reconnection due to axisymmetric plasmoids, we find that the growth rate of these edge filamentary structures becomes independent of Lundquist number. As well as edge reconnection physics in tokamaks, the 3-D coherent current-carrying filament structures and their nonlinear dynamics due to the dynamo effect presented here are also relevant to flares, which also exhibit ejection of field-aligned filamentary structures into surrounding space. Supported by DOE grants DE-SC0010565, DE-AC02-09CH11466.