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Title: Aspect Ratio Effects in the Driven, Flux-Core Spheromak

Resistive magneto-hydrodynamic 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. Nucl. Fusion 45, 1582 (2005)]. Amplification of the bias ('gun') poloidal flux is fit 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 {le} 1.4 in simulations generates reconnection events which open the magnetic field lines; this state is characteristic of SSPX. Near the spheromak tilt-mode limit, A {approx} 1.67 for a cylindrical flux conserver, the tilt approaches 90{sup o}; reconnection events are not generated up to the strongest drives simulated. The time-sequence of these events suggests that they are representative of a chaotic process. Implications for spheromak experiments are discussed.
Authors:
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Publication Date:
OSTI Identifier:
958176
Report Number(s):
LLNL-JRNL-411044
Journal ID: ISSN 1070-664X; PHPAEN; TRN: US1000361
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas, vol. 16, no. 5, May 1, 2009, pp. 052506; Journal Volume: 16; Journal Issue: 5
Research Org:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION; AMPLIFICATION; AMPLITUDES; ASPECT RATIO; GEOMETRY; HELICITY; MAGNETIC FIELDS; PHYSICS