Three-dimensional MHD Simulations Of Counter-helicity Spheromak Merging In The Swarthmore Spheromak Experiment
Authors: C.E. Myers, E.V. Belova, M.R. Brown, T. Gray, C.D. Cothran, and M.J. Schaffer
Abstract:
Recent counter-helicity spheromak merging experiments in the Swarthmore Spheromak Experiment (SSX)
have produced a novel compact torus (CT) with unusual features. These include a persistent antisymmet-
ric toroidal magnetic eld pro le and a slow, nonlinear emergence of the n = 1 tilt mode. Experimental
measurements are inconclusive as to whether this unique CT is a fully-merged eld-reversed con guration
(FRC) with strong toroidal eld or a partially-merged \doublet CT" con guration with both spheromak- and
FRC-like characteristics. In this paper, the SSX merging process is studied in detail using three-dimensional
resistive MHD simulations from the HYbrid Magnetohydrodynamics (HYM) code. These simulations show
that merging plasmas in the SSX parameter regime only partially reconnect, leaving behind a doublet CT
rather than an FRC. Through direct comparisons, we show that the magnetic structure in the simulations is
highly consistent with the SSX experimental observations. We also nd that the n = 1 tilt mode begins as
a fast growing linear mode that evolves into a slower-growing nonlinear mode before being detected experi-
mentally. A simulation parameter scan over resistivity, viscosity, and line-tying shows that these parameters
can strongly a ect the behavior of both the merging process and the tilt mode. In fact, merging in certain
parameter regimes is found to produce a toroidal-fi eld-free FRC rather than a doublet CT.
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Submitted to: Physics of Plasmas, (September 2011)
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