April 23-25

Log in

Abstract Details

files Add files

status:file name:submitted:by:
approvedebrahimi_sherwood2018-1.pdf2018-03-01 08:47:51Fatima Ebrahimi


Author: Fatima Ebrahimi
Requested Type: Pre-Selected Invited
Submitted: 2018-03-01 08:46:31


Contact Info:
Princeton University/PPPL
C Site T 152 MS-28. Princeton
Princeton,   08543

Abstract Text:
The onset and nonlinear evolution of reconnecting current-carrying filaments are examined using global nonlinear three-dimensional resistive MHD simulations with NIMROD in a spherical tokamak. We show that physical current-sheets/layers develop near the tokamak edge under different circumstances, in particular as a peeling component of ELMs, 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. Our nonlinear simulations are the first to identify the time-evolving edge current sheets in ST configurations.[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. Secondly, we examine the stability and formation of reconnecting edge peeling-driven filaments during vertical displacement events (VDEs). In the simulations, VDEs are induced 1) by first driving large current in the open field region to form scrape-off layer currents (halo currents), 2) by turning off the voltage to allow plasma vertical displacement downward. We show that as the plasma is vertically displaced, edge halo current-sheet becomes MHD peeling-tearing unstable and form non-axisymmetric coherent edge current filamentary structures, which would eventually bleed into the walls. We also investigate the dependency of onset of peeling-driven filaments on Lundquist number. Similar to fast reconnection due to axisymmetric plasmoids, we find that the growth rate of these edge filamentary structures becomes independent of Lundquist number. Supported by DOE-SC0010565, DE-AC02-09CH11466.