Aspect-ratio effects in the driven, flux-core spheromak
- Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94551 (United States)
Resistive magnetohydrodynamic simulations are used to evaluate the effects of the aspect ratio A (length to radius ratio) in a spheromak driven by coaxial helicity injection. The simulations are benchmarked against the Sustained Spheromak Physics Experiment (SSPX) [R. D. Wood et al., Nucl. Fusion 45, 1582 (2005)]. Amplification of the bias ('gun') poloidal flux is fitted well by a linear dependence (insensitive to A) on the ratio of gun current and bias flux above a threshold dependent on A. For low flux amplifications in the simulations, the n=1 mode is coherent and the mean-field geometry looks like a tilted spheromak. Because the mode has relatively large amplitude the field lines are open everywhere, allowing helicity penetration. Strongly driven helicity injection at A{<=}1.4 in simulations generates reconnection events which generate cathode-voltage spikes, relaxation of the symmetry-breaking modes, and open, stochastic magnetic field lines; this state is characteristic of SSPX. The time sequences of these events suggest that they are representative of a chaotic process. Near the spheromak tilt-mode limit, A{approx_equal}1.67 for a cylindrical flux conserver, the tilt approaches 90 deg.; reconnection events are not generated up to the strongest drives simulated. Implications for spheromak experiments are discussed.
- OSTI ID:
- 21277089
- Journal Information:
- Physics of Plasmas, Vol. 16, Issue 5; Other Information: DOI: 10.1063/1.3134064; (c) 2009 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 1070-664X
- Country of Publication:
- United States
- Language:
- English
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