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

Journal Article · Fri May 15 04:00:00 EDT 2009 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.3134064· OSTI ID:21277089

Hooper, E B; Romero-Talamas, C A; LoDestro, L L; Wood, R D; McLean, H S ^[1]

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1. 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, Journal Name: Physics of Plasmas Journal Issue: 5 Vol. 16; ISSN PHPAEN; ISSN 1070-664X
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
ASPECT RATIO
HELICITY
INJECTION
MAGNETOHYDRODYNAMICS
PLASMA INSTABILITY
PLASMA SIMULATION
SPHEROMAK DEVICES