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approvedcallen_p3_017.pdf2018-07-10 09:08:49James Callen


Author: James D. Callen
Requested Type: Poster
Submitted: 2018-03-01 16:49:24

Co-authors: M.T. Beidler, C.C. Hegna

Contact Info:
University of Wisconsin
1500 Engineering Drive
Madison, WI   53706-1

Abstract Text:
Small 3-D magnetic fields occur in tokamaks in addition to their dominant axisymmetric fields. Such fields can be externally applied or due to field errors. They cause reconnection of magnetic field lines in tokamak plasmas at q = m/n rational surfaces where the 3-D fields are resonant [1]. Resonant 3-D fields can produce bifurcated equilibrium states [1] in a rotating tokamak plasma. This research analyzes how transient MHD events such as edge localized modes (ELMs) and sawteeth can precipitate a transition of this metastable plasma from a high-slip, flow-screened state with little magnetic reconnection into a low-slip, locked-mode state where the background 3-D field penetrates and can produce a significant width magnetic island. The slab model analysis used here first solves a transient equation for the resonant magnetic field response to an abrupt MHD-induced magnetic field in a flowing plasma. The phase-shifted dissipative part of this field determines the electromagnetic (em) force it produces in the reconnecting singular layer at the rational surface. This em force perturbs the flow in the vicinity of the singular layer through its effect on the flow equation which is solved using a local Green function approach. Simultaneous solutions of these two equations provide analytic predictions for NIMROD calculations [2] of the resonant field and flow changes caused by MHD transients. They also predict the parameters under which transitions from a high-slip into a low-slip, locked-mode, magnetic island state can be triggered by MHD transients. In particular, they predict a minimum initial flow where MHD transients could cause suppression of ELMs in tokamak plasmas with large enough externally applied resonant magnetic perturbations (RMPs).
[1] R. Fitzpatrick, Phys. Plasmas 5, 3325 (1998).
[2] M.T. Beidler et al., “Nonlinear Mode Penetration Caused by Transient Magnetic Perturbations,” 2018 Sherwood presentation.
Grants DE-FG02-92ER54139, -86ER53218, ORISE.