Improved Point Placement for XGC Meshes

Gyrokinetic codes typically employ field-aligned coordinates to exploit the anisotropic nature of plasma turbulence, characterized by k_⊥ρ_s ~1 and k_‖ << 1, allowing reduced resolution and lower-order numerical methods in the parallel direction. The XGC code takes a similar approach by placing mesh vertices on magnetic flux surfaces and aligning them with the equilibrium magnetic field, enabling coarser resolution in the toroidal direction. However, strict field alignment can lead to sharp resolution transitions, elongated elements, and conflicts with desired poloidal resolution. These challenges can be especially pronounced in complex geometries such as stellarators.

In this work, we explore improved point placement strategies that relax strict field-following to better balance field alignment, mesh quality, and poloidal resolution. We assess the impact of these methods on mesh characteristics and simulation fidelity. Finally, we report progress and plans for deploying these strategies in the Tokamak Modeling and Meshing Software [1] (TOMMS), developed at RPI.

[1] Riaz et al., Comp. Phys. Commun. 295 (2024), 108982. https://doi.org/10.1016/j.cpc.2023.108982