AI summary · arXiv: 2306.00423v1 · math.NA

Provably stable numerical method for the anisotropic diffusion equation in toroidally confined magnetic fields

Dean Muir, Kenneth Duru, Matthew Hole, Stuart Hudson
Original paper Submitted to arXiv on 01 Jun. 2023 33 pages, 8 figures License: CC BY 4.0

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The essentials

  • The authors present a novel numerical method for solving the anisotropic diffusion equation in toroidally confined magnetic fields.
  • The method is efficient, accurate and provably stable.
  • A discrete formulation is presented using operator splitting in time with the summation by parts finite difference approximation of spatial derivatives for the perpendicular diffusion operator.
  • Weak penalty procedures are derived for implementing both boundary conditions and parallel diffusion operators obtained by field line tracing.
  • The fully-discrete approximation is proven to be unconditionally stable and asymptotic preserving.
  • Convergence tests are shown for the perpendicular operator by itself, and a "NIMROD benchmark" problem is used as a manufactured solution to demonstrate that the full scheme converges even when perpendicular diffusion is zero.
  • An example magnetic field with chaotic regions and islands is presented to show that contours of the anisotropic diffusion equation reproduce key features in the field.
  • A Poincaré section provides context on this example magnetic field by highlighting three different field line trajectories; two stay close to "magnetic surfaces" while one lies in a chaotic region resulting in large excursions in ψ values; there is also a trajectory with distinct "holes" which may allow nearby field lines to pass through.
  • This paper provides an important contribution to numerical methods for solving anisotropic diffusion equations in toroidally confined magnetic fields, with applications to plasma physics research.

Key points written by AI: they may contain errors, check them against the paper.

AI-generated keywords: Anisotropic Diffusion Toroidal Magnetic Fields Operator Splitting Finite Difference Approximation Energy Estimates

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In this paper, the authors present a novel numerical method for solving the anisotropic diffusion equation in toroidally confined magnetic fields. The method is efficient, accurate and provably stable. The continuous problem is written in terms of a derivative operator for perpendicular transport and a linear operator obtained through field line tracing for parallel transport. Energy estimates of the solution of the continuous initial boundary value problem are derived. A discrete formulation is presented using operator splitting in time with the summation by parts finite difference approximation of spatial derivatives for the perpendicular diffusion operator. Weak penalty procedures are derived for implementing both boundary conditions and parallel diffusion operators obtained by field line tracing. The fully-discrete approximation is proven to be unconditionally stable and asymptotic preserving. Discrete energy estimates match the continuous energy estimate given the correct choice of penalty parameters. Convergence tests are shown for the perpendicular operator by itself, and a "NIMROD benchmark" problem is used as a manufactured solution to demonstrate that the full scheme converges even when perpendicular diffusion is zero. Additionally, an example magnetic field with chaotic regions and islands is presented to show that contours of the anisotropic diffusion equation reproduce key features in the field. Furthermore, a Poincaré section provides context on this example magnetic field by highlighting three different field line trajectories; two stay close to "magnetic surfaces" while one lies in a chaotic region resulting in large excursions in ψ values; there is also a trajectory with distinct "holes" which may allow nearby field lines to pass through. Overall, this paper provides an important contribution to numerical methods for solving anisotropic diffusion equations in toroidally confined magnetic fields, with applications to plasma physics research.

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