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icc2011_obryan_proceedings.pdf2011-08-26 14:45:36John O'Bryan
icc2011_obryan_poster.pdf2011-08-26 14:45:06John O'Bryan

Numerical Simulation of Non-Inductive Startup in the Pegasus Toroidal Experiment

Author: John B. O'Bryan
Requested Type: Poster Only
Submitted: 2011-06-10 14:35:39

Co-authors: C.R.Sovinec, T.M.Bird

Contact Info:
University of Wisconsin-Madison
1500 Engineering Dr.
Madison, WI   53706

Abstract Text:
Nonlinear numerical computation is used to investigate the relaxation of non-axisymmetric current channels of DC helicity injection from washer-gun plasma sources into "tokamak-like" plasmas in the Pegasus Toroidal Experiment (Univ. of Wisconsin). Resistive MHD simulations with the NIMROD code (nimrodteam.org) utilize fully three-dimensional, anisotropic, temperature-dependent thermal conductivity corrected for regions of low-magnetization [Braginskii, Reviews of Plasma Physics, 1965], temperature-dependent resistivity, and ohmic heating. The impact non-MHD effects have on current channel evolution and relaxation are investigated through computations which evolve separate ion and electron fluid temperatures with temperature-dependent thermal equilibration between species.
Our modeling of injection has been improved by implementing a new toroidal shape function for the current drive source. A gaussian source distribution has a more rapidly converging Fourier expansion than the original half-sine-wave shape, while retaining a similar current profile. The thermal boundary conditions have also been modified to better reproduce experimental current return paths. For current driven along open magnetic field lines, the original boundary conditions produce a steep thermal gradient at the domain boundary, which due to temperature-dependent resistivity, causes resistivity to be larger at the boundary than through the plasma. As a result, current drive along open field lines forms return paths through the plasma rather than crossing the domain boundary along the magnetic field lines. By combining insulating thermal boundary conditions with a uniform thermal decay along the surface, the temperature at the ends of current channels remains comparable to peak current channel temperatures while being much lower everywhere else on the boundary. This eliminates the undesirable current return through the bulk plasma.

Work supported by the US Dept. of Energy

Characterization: A7

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

ICC 2011