Implementation of the Non-Twisting Flux Tube in the Gyrofluid Code GX

Simulating microturbulence in fusion plasmas involves gyrokinetic codes that average over the fast perpendicular gyromotion timescale. The computational expense is further reduced through a “flux tube” domain, which uses a field aligned coordinate system to match the structure of the underlying turbulence [1]. At the outboard midplane, the perpendicular domain is rectangular and the normal/binormal directions are several gyroradii in length. Due to magnetic shear, the perpendicular cross section twists into a parallelogram and can damp certain types of turbulence. In this work, we implement a non-twisting flux tube in the pseudo-spectral code GX [2]. We build off a previous study [3] that redefines the binormal spatial coordinate and subsequent normal wavenumber grid to maintain a rectangular cross section at all poloidal locations. We will review the code alterations and compare the findings and runtimes of conventional and non-twisting flux tubes for both linear and nonlinear benchmarks.

[1] M.A. Beer, S. C. Cowley, and G. W. Hammett, Phys. Plasmas 2, 2687 (1995).

[2] N. Mandell, W. Dorland, and M. Landreman, Journal of Plasma Physics, 84(1), 905840108 (2018).

[3] J. Ball and S. Brunner, Plasma Phys. Control. Fusion 63, 064008 (2021).