Resolving Gyrokinetic Turbulence Using the Non-Twisting Flux Tube in the Gyrokinetic Code GX

GX is a recently developed, GPU-enabled, pseudo-spectral gyrokinetic code used for simulating microturbulence in fusion relevant plasmas [1]. It uses the radially local flux-tube approximation with a field aligned coordinate system that aims to resolve the underlying structure of the turbulence [2]. At the outboard midplane, the perpendicular domain is rectangular and the normal/binormal directions are typically several gyroradii in length. However, due to both global and local magnetic shear, the perpendicular cross section shears into a parallelogram away from the outboard midplane. This perpendicular box shearing might unphysically cut off transport-relevant long-wavelength turbulence, while often unnecessarily simulating transport-irrelevant ultra-fine-structure turbulence. In this work, we present the implementation and results of a non-twisting flux tube (NTFT) in GX, which retains a rectangular cross section at all poloidal locations. Building off previous work that implemented a NTFT in the gyrokinetic code GENE [3], this presentation will focus on nonlinear implementation of the NTFT as well as linear and nonlinear results. We will also discuss benefits of the NTFT in device applications such as tokamak pedestals and stellarators.

[1] N. R. Mandell, W. D. Dorland, I. Abel, R. Gaur, P. Kim, M. Martin, and T. Qian, arXiv:2209.06731 (2022).

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

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