Fluid simulations of plasma filaments in stellarator geometries with BSTING

Abstract

Here we present first results simulating plasma filaments in non-axisymmetric geometries, using a fluid turbulence extension of the BOUT++ framework. This is made possible by the implementation of the Flux Coordinate Independent scheme for parallel derivatives, an extension of the metric tensor components which allows them to vary in three dimensions, and development of grid generation.Tests have been performed to confirm that the extension to three dimensional metric tensors does not compromise the accuracy and stability of the associated numerical operators. Recent changes to the FCI grid generator in BOUT++, including a curvilinear grid system which allows for potentially more efficient computation, are also presented. Initial simulations of seeded plasma filaments in a non-axisymmetric geometry are reported. We characterize filaments propagating in the closedfield-line region of a low-field-period, rotating ellipse equilibrium as inertially-limited by examining the velocity scaling and currents associated with the filament propagation. Finally, it is shown that filaments in a non-axisymmetric rotating ellipse equilibrium propagate in a toroidally nonuniform fashion, and it is determined that the long connection lengths in the scrape-off-layer enable parallel gradients to establish, which has consequences for interpretation of experimental data.

The effects of non-uniform drive on plasma filaments

“…Additionally, these simulations can be extended to geometries with finite connection lengths, thereby providing relevance for edge and scrape-off-layer plasmas. Following the development of the BSTING [13] project, simulations of full stellarator edge and scrape-off-layer geometries is also possible. Therefore, verification of the work shown here in a broader geometry is foreseeable.…”

Global turbulence simulations of the tokamak edge region with GRILLIX

“…The values on map points are obtained from a 3rd order bi-polynomial interpolation within the poloidal planes ϕ k and ϕ k+1 . We note that the magnetic field is assumed axisymmetric in GRILLIX, which is however not a general constraint for the FCI approach [33,34]. In the same spirit operators are established that map quantities between the grid and the staggered dual grid and vice versa.…”

Global fluid simulation of plasma turbulence in a stellarator with an island divertor

“…Similarly to the experimental investigations, the simulation of plasma dynamics in the stellarator boundary is still at a very early stage. The fluid code BOUT++ simulated seeded plasma filaments in a slab geometry that emulates the radially varying connection lengths of W7-X [20] and it was recently extended to simulate non-axisymmetric magnetic fields such as a lowfield-period rotating ellipse [21].…”