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Self-consistent simulations of nonlinear MHD and profile evolution in stellarator configurations

Author: Mark G Schlutt
Requested Type: Consider for Invited
Submitted: 2012-12-07 13:18:31

Co-authors: C.C.Hegna, C.R.Sovinec

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

Abstract Text:
Self-consistent MHD equilibrium and nonlinear stability of 3-D magnetic configurations are investigated using the extended MHD code NIMROD. In these calculations, initial conditions are given by 3-D vacuum solutions with robust magnetic surfaces. We examine two classes of problems: those with current-driven instabilities and those with pressure-driven instabilities.

Ohmic discharges in the Compact Toroidal Hybrid (CTH) are simulated [1]. The vacuum magnetic field of CTH is initialized and current is driven by specifying a toroidal electric field at the vessel boundary. The driven current penetrates toward the core and raises the rotational transform profile. Island formation is observed that is linked to the n=5 periodicity of the device. A prominent feature of these simulations is the coalescence of n/m=5/10 islands to n/m=1/2 islands when the rotational transform exceeds 0.5. At high levels of current drive, complete flux surface destruction is observed. Comparison with CTH data show favorable agreement.

Finite beta discharges in a straight stellarator are simulated [2]. Vacuum magnetic fields are applied to produce stellarator-like rotational transform profiles with iota(0)≤0.5 and iota(0)≥0.5. The vacuum magnetic fields are either helically symmetric or spoiled by the presence of magnetic harmonics of incommensurate helicity. As heat is added to the system, pressure-driven instabilities are excited when a critical β is exceeded. These instabilities may cause abrupt loss of stored energy, or they may saturate nonlinearly as the equilibrium evolves. In all of these studies, anisotropic heat conduction is allowed with kpar/kperp = 104–107. Due to the finite parallel heat conduction, in some cases an equilibrium state persists that has a stochastic edge region which supports a pressure gradient.

[1] M.G. Schlutt, et al., 2012, Nuclear Fusion, 52, 103023.
[2] M.G. Schlutt, et al., submitted to Phys. Plasmas, 2012.
Research supported by U.S. DOE grant no. DE-FG02-99ER54546.

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