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main.pdf2011-08-26 13:34:11Eric Howell
icc.pdf2011-08-26 13:30:51Eric Howell

Extend MHD simulations of decaying spheromaks

Author: Eric C. Howell
Requested Type: Poster Only
Submitted: 2011-06-10 11:53:48

Co-authors: C.R.Sovinec

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

Abstract Text:
Nonlinear extended MHD simulations of decaying spheromaks are studied using the NIMROD code (JCP 195, 2004). Earlier work has shown good agreement between resistive MHD simulations and experimental measurements of the Sustained Spheromak Physics Experiment (SSPX), but simulations under predict the peak observed electron temperature by as much as 40%. This work investigates three extended models to see if they can explain the observed temperature discrepancy. This work uses the same pulse waveform as used by Sovinec, et al. (PRL 94, 2005). A formation pulse is ramped up to 400 kA at 0.08ms and maintained until 0.12ms. Then the guns are turned off and the spheromak is allowed to freely decay until 0.5ms. At 0.5ms a second 200 kA sustainment pulse is applied and maintained until 2.0ms. Resistive MHD is used to model the formation phase. Three different models are considered for the evolution starting at 0.148ms. The first model uses a resistive MHD ohm’s law but evolves the electron and ion temperatures separately. The electrons are heated Ohmicially and the ions are heated through collisions with the electrons. The second model uses a single fluid temperature evolution but includes ion gyroviscosity and a two-fluid Ohm’s law that includes the Hall term, electron pressure, and electron inertia. The third model combines the two previous models, using ion gyroviscosity, a two fluid Ohm’s law, and separate temperature evolution for the electrons and ions. All three models are compared against a single temperature resistive MHD model.
The combined model, using both the two fluid Ohm’s law and separate temperature evolutions, predicts the greatest electron temperature of 62eV at 0.5ms, about 20eV hotter than the other models. However, shortly after the sustainment pulse is applied, a large n=3 mode appears and rapidly cools the plasma. A similar instability is observed when the two-fluid Ohm’s law is used with a single temperature. Following the instability, the q-profile transitions from a reversed shear profile to a monotonically decreasing profile. After the transition the spheromaks electron temperature is limited to 65 eV. This n=3 instability is not observed when the resistive MHD Ohm’s is used, and a conventional spheromak profile is maintained throughout the entire simulation resulting in better stability. The separate temperature resistive MHD model behaves qualitatively similar to the single temperate MHD model, and has a peak electron temperature of 71 eV. The single temperature model has a peak temperature of 73 eV. Work on improving resistive MHD interchange stability will also be presented.

Work supported by US DOE through grant

Characterization: A7

Comments:

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011