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Almost-invariant surfaces for magnetic field-line flows

Published online by Cambridge University Press:  13 March 2009

S. R. Hudson  and
R. L. Dewar
S. R. Hudson
Affiliation:
Department of Theoretical Physics and Plasma Research Laboratory, Research School of Physical Sciences and Engineering, The Australian National University, Canberra ACT 0200, Australia†
R. L. Dewar
Affiliation:
Department of Theoretical Physics and Plasma Research Laboratory, Research School of Physical Sciences and Engineering, The Australian National University, Canberra ACT 0200, Australia†
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Abstract

Two approaches to defining almost-invariant surfaces for magnetic fields with imperfect magnetic surfaces are compared. Both methods are based on treating magnetic field-line flow as a 1½-dimensional Hamiltonian (or Lagrangian) dynamical system. In the quadratic-flux minimizing surface approach, the integral of the square of the action gradient over the toroidal and poloidal angles is minimized, while in the ghost surface approach a gradient flow between a minimax and an action-minimizing orbit is used. In both cases the almost-invariant surface is constructed as a family of periodic pseudo-orbits, and consequently it has a rational rotational transform. The construction of quadratic-flux minimizing surfaces is simple, and easily implemented using a new magnetic field-line tracing method. The construction of ghost surfaces requires the representation of a pseudo field line as an (in principle) infinite-dimensional vector and also is inherently slow for systems near integrability. As a test problem the magnetic field-line Hamiltonian is constructed analytically for a topologically toroidal, non-integrable ABC-flow model, and both types of almost-invariant surface are constructed numerically.

Type
Research Article
Information
Journal of Plasma Physics , Volume 56 , Issue 2 , October 1996 , pp. 361 - 382
Copyright
Copyright © Cambridge University Press 1996

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