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A parametric study of extended MHD effects on interchange modes in spheromak equilibria.

Author: Eric c Howell
Requested Type: Poster Only
Submitted: 2014-05-30 14:38:18

Co-authors: C.R.Sovinec

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

Abstract Text:
A parametric study of extend MHD effects on linear interchange modes is performed using the NIMROD code [Sovinec et al JCP 2004]. We use a linear cylindrical equilibrium model adapted from [Jardin NF 1982] to allow for finite toroidal current at the edge. These current profiles are representative of SSPX discharges where currents are driven on the open field to maintain the safety factor above the 1/2 surface[McLean et al POP 2006]. The resulting spheromaks have weak magnetic shear, and interchange stability is important.

The Suydam parameter, D, is scaled in these calculations to study both resistive and ideal interchange modes. The calculated linear MHD growth rate increase exponentially with D. The resistive interchange scaling γ~η^(1/3) is observed for ideal stable equilibria, D < 0.25. Gyro-viscosity has a stabilizing effect, reducing the growth rate. The stabilization is significant for resistive and weakly ideal unstable modes D<0.5. Here the growth rate scales as γ~η^(2/5). The effect of the Hall term depends on the equilibrium temperature and density gradients. In isothermal equilibria, where the equilibrium pressure gradient is entirely due to a density gradient, the Hall term has a stabilizing effect on resistive interchange modes, but it destabilizes ideal interchange modes. In equilibria with uniform density, where the pressure gradient is due to a temperature gradient, the Hall term strongly destabilizes both ideal and resistive modes, in some cases doubling the linear growth rate at large D. Results that consider the effect of the cross-field diamagnetic heat flux and other extended MHD effects will be presented.



*Work supported by US DOE grant DE-FC02-08ER54975

Characterization: 1.1

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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