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Nimrod Simulations of FRC Formation and Sustainment with Rotating Magnetic Field Current Drive

Author: Richard D. Milroy
Requested Type: Consider for Invited
Submitted: 2009-12-03 17:18:07

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

Contact Info:
PSI-Center
University of Washington
Seattle, WA   98195
USA

Abstract Text:
Three dimensional simulations of Field Reversed Configuration (FRC) formation and sustainment with Rotating Magnetic Field (RMF) current drive have been performed with the NIMROD code. The Hall term is a zeroth order effect with strong coupling between Fourier components, and recent enhancements to the NIMROD preconditioner allow much larger timesteps than was previously possible. A two-fluid option with a finite electron mass set equal to 1/100 of the ion mass is used. Boundary conditions to capture the effects of a finite length RMF antenna have been added, and simulations of FRC formation from a uniform background gas have been performed with parameters relevant to the TCSU experiment at the University of Washington.
The formation process is found to be turbulent, and numerical stability requires both the viscosity and resistivity be set to relatively large values during this phase of the calculation. However, the viscosity can be reduced as steady-state conditions are achieved. Most calculations only include the n=0 and n=1 modes, but calculations with n=0 through n=5 have also been performed, and these calculations show the configuration to be stable to disruptive instabilities. Boundary conditions have been implemented for both even-parity and odd-parity antennas, and it is found that while even-parity opens all of the field lines, some field lines do close for the odd-parity case. Simulations have been limited to a relatively resistivity, which limits the ratio of the n=0 averaged poloidal field to the RMF field to values less than about 4. This ratio is expected to increase if we can operate at a reduced resistivity, permitting more field lines to close. It is noted that while even-parity opens all of the field lines, relatively good particle confinement times are achieved.
It has also been found that the RMF effects extend considerably beyond the ends of the antenna. This means that an FRC that is longer than the antenna can be formed and sustained, however it is also found that a large n=0 Bθ can develop in the open-field line region. This has two consequences: A back-torque is produced opposing the RMF, and a kink instability can develop in the open-field-line region beyond the FRC ends. This effect is sensitive to resistivity and applied boundary conditions. Results will be compared with TCS-U experimental measurements.
In addition, we plan to investigate the possibility of forming an FRC with a high powered transient RMF, and to examine whether a weaker non-penetrating RMF can be employed to provide dynamic stabilization to an existing FRC.

Characterization: A1,E10

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