On the proper treatment of magnetic fluctuations in full-f field-aligned turbulence codes
Abstract
Plasma turbulence in the edge of magnetic confinement devices is customarily treated as full-f due to large fluctuations. For computational efficiency, field-aligned coordinates are employed, separating the magnetic field into equilibrium B0 and delta-f perturbations which are handled by the magnetic flutter operators. Evolving the full-f pressure with delta-f magnetic perturbations can cause inconsistency since the latter contain background components such as the Shafranov shift, which are actually parts of the equilibrium magnetic field. Such background components (Bs) contained in the magnetic perturbations undermine the field-aligned numerics when treated as flutter: errors arise if Bs/B0 l/h is not satisfied, with the perpendicular turbulence scale l and the parallel grid distance h. We find that the commonly used removal of Bs by subtracting the toroidal average of magnetic perturbations intervenes in the Alfv\'en dynamics, causing spurious E× B transport. Instead, we propose an improved method to dynamically filter out the evolving background from the turbulent magnetic fluctuations in the time domain. The filter is verified in both low and high confinement tokamak conditions, confirming its capability to preserve the turbulence fidelity, provided sufficient filter width.
Turn this paper into a full lesson
ArcXiv compiles a staged curriculum from this paper: 8-12 lessons across beginner → advanced, synthesised section guides, visuals, flashcards, a quiz, exercises, and on-demand deep dives per section. Grounded in the abstract, never invented.