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

Abstract

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.

Authors:
; ; ; ;  [1]
  1. Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94551 (United States)
Publication Date:
OSTI Identifier:
21277089
Resource Type:
Journal Article
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 16; Journal Issue: 5; Other Information: DOI: 10.1063/1.3134064; (c) 2009 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 1070-664X
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ASPECT RATIO; HELICITY; INJECTION; MAGNETOHYDRODYNAMICS; PLASMA INSTABILITY; PLASMA SIMULATION; SPHEROMAK DEVICES

Citation Formats

Hooper, E B, Romero-Talamas, C A, LoDestro, L L, Wood, R D, and McLean, H S. Aspect-ratio effects in the driven, flux-core spheromak. United States: N. p., 2009. Web. doi:10.1063/1.3134064.
Hooper, E B, Romero-Talamas, C A, LoDestro, L L, Wood, R D, & McLean, H S. Aspect-ratio effects in the driven, flux-core spheromak. United States. https://doi.org/10.1063/1.3134064
Hooper, E B, Romero-Talamas, C A, LoDestro, L L, Wood, R D, and McLean, H S. 2009. "Aspect-ratio effects in the driven, flux-core spheromak". United States. https://doi.org/10.1063/1.3134064.
@article{osti_21277089,
title = {Aspect-ratio effects in the driven, flux-core spheromak},
author = {Hooper, E B and Romero-Talamas, C A and LoDestro, L L and Wood, R D and McLean, H S},
abstractNote = {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.},
doi = {10.1063/1.3134064},
url = {https://www.osti.gov/biblio/21277089}, journal = {Physics of Plasmas},
issn = {1070-664X},
number = 5,
volume = 16,
place = {United States},
year = {Fri May 15 00:00:00 EDT 2009},
month = {Fri May 15 00:00:00 EDT 2009}
}