GRILLIX as unified fluid turbulence code for tokamaks and stellarators

Abstract

The Flux-Coordinate Independent (FCI) approach in the edge fluid turbulence code GRILLIX has proven very successful for tokamaks in handling anisotropic turbulent structures in complex diverted geometries. We extend GRILLIX to non-axisymmetric geometries, maintaining a single, unified codebase for both tokamaks and stellarators. For this proof of principle, the 3D magnetic configurations are based on analytical expressions, that can be adjusted to experimental scenarios, and we demonstrate the code's ability to resolve stellarator geometries correctly via various numerical tests and verifications. We apply the global electromagnetic drift-reduced Braginskii fluid model with trans-collisional extensions and a three-moment fluid neutrals model to two distinct cases. In the first case, we simulate a configuration with imposed magnetic islands and observe profile flattening across the island on time scales consistent with analytic theory. In the second case, we perform a comprehensive turbulence simulation of the Wendelstein 7-AS (W7-AS) stellarator at realistic parameters, including segmented target plates modeled via the immersed boundary approach. Following an initial transient phase, the plasma settles into a self-consistent equilibrium state with Shafranov shift. We then observe small-scale turbulence, characterized by strong elongation along magnetic field lines, which drives turbulent cross-field transport of particles and heat, ultimately directed to target plates in the open field line region. Notably, a parallel mode reflecting the discrete toroidal symmetry of W7-AS is also present. Furthermore, we introduce a novel approach for visualizing data from locally aligned numerical frameworks.