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NIMROD Extended MHD Simulations of Plasma Relaxation Dynamics

Author: Joshua P. Sauppe
Requested Type: Poster Only
Submitted: 2014-05-29 15:35:03

Co-authors: C. R. Sovinec

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

Abstract Text:
We report on numerical simulations of plasma relaxation that include two-fluid physics using the NIMROD code. At modest Lundquist numbers (S=20,000 and S=80,000) the time-scales of relaxation and drive are sufficiently well-separated to allow comparisons between the simulated driven-damped system and various relaxation theories. The global magnetic helicity is observed to be a more robust invariant than the magnetic energy, and the relaxation dynamics tend to flatten the plasma current profile as predicted by the theory.

Plasma flows are observed to be significant in the Madison Symmetric Torus RFP and understanding the coupling of flows and current relaxation is of interest. Within the single-fluid MHD framework, relaxation theories couple plasma flow to current relaxation through the cross-helicity, an invariant in ideal incompressible MHD. However, cross-helicity is not well-conserved relative to magnetic energy in our simulations. Two-fluid relaxation theories include species' flows in generalized helicities which include terms similar to the cross-helicity. In our simulations, these generalized helicities are well-conserved relative to the magnetic energy. However, the difference between electron and ion helicities, which excludes the dominant magnetic helicity term, is not well-conserved.

Single-fluid and two-fluid relaxation theories predict a relaxed state with plasma flow parallel to the magnetic field with a flat profile, similar to the relaxed current density. Our numerical simulations utilizing a single-fluid Ohm's law display little change in the plasma flow over the relaxation event. However, with a two-fluid Ohm's law, there is substantial change in plasma flow over the event, although this is not in general consistent with the flattening predicted by some theories. Simulations with an equilibrium parallel flow profile display either a steepening of the flow or a flattening, depending on the orientation of current density and magnetic field.

Characterization: 4.0

Comments:

Workshop on Exploratory Topics in Plasma and Fusion Research (EPR) and US-Japan Compact Torus (CT) Workshop
August 5-8, 2014
Madison, Wisconsin

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