Aspect-ratio effects in the driven, flux-core spheromak





Citation of Original Publication

E. B. Hooper, C. A. Romero-Talamás, L. L. LoDestro, R. D. Wood, H. S. McLean; Aspect-ratio effects in the driven, flux-core spheromak. Phys. Plasmas 1 May 2009; 16 (5): 052506.


This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Resistive magnetohydrodynamic simulations are used to evaluate the effects of the aspect ratio (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 ⁠) 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 ≈1.67 for a cylindrical flux conserver, the tilt approaches 90°; reconnection events are not generated up to the strongest drives simulated. Implications for spheromak experiments are discussed.