Title : Onset and nonlinear relaxation of coherent current-carrying edge filaments during transient events in tokamaks
Abstract : The onset and nonlinear evolution of coherent current-carrying filaments are examined using global nonlinear three dimensional resistive MHD simulations in a spherical tokamak (ST). We show that physical current sheets/layers develop near the tokamak edge under different circumstances, in particular as a peeling component of ELMs (due to bootstrap currents), and during vertical displacement events (associated with the scrape-off layer currents). In all these cases, edge current sheets can become unstable to nonaxisymmetric 3-D current-sheet instabilities and nonlinearly form edge coherent current-carrying filaments. Time-evolving edge current sheets in ST configurations are identified in our nonlinear simulations. [F. Ebrahimi, Phys. Plasmas 23, 120705 (2016);24, 056119 (2017)] In the case of peeling-like edge localized modes, the longstanding problem of quasiperiodic ELM cycles is explained through the relaxation of the edge current source through direct numerical calculations of reconnecting local bi-directional fluctuation-induced electromotive force (emf) terms. Second, we examine the stability and formation of reconnecting edge peeling-driven filaments during induced vertical displacement events (VDEs) simulations. Similar to fast reconnection due to axisymmetric plasmoids, [F. Ebrahimi and R. Raman, Phys. Rev. Lett. 114, 205003 (2015)] 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 fi lament 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 the surrounding space.