A non-twisting flux tube for local gyrokinetic simulations

TL;DR

This paper introduces a non-twisting flux tube for local gyrokinetic simulations that maintains a rectangular shape, improving computational efficiency and accuracy especially in high magnetic shear scenarios.

Contribution

It derives and implements a novel non-twisting flux tube that remains rectangular, enhancing efficiency and accuracy over traditional twisted flux tubes in certain plasma simulation conditions.

Findings

30 times less computationally expensive in tests

More accurate modeling of inboard midplane with high magnetic shear

Most advantageous in high shear and multi-region turbulent domains

Abstract

Local gyrokinetic simulations use a field-aligned domain that twists due to the magnetic shear of the background magnetic equilibrium. However, if the magnetic shear is strong and/or the domain is long, the twist can become so extreme that it fails to properly resolve the turbulence. In this work, we derive and implement the "non-twisting flux tube," a local simulation domain that remains rectangular at all parallel locations. Convergence and runtime tests indicate that it can calculate the heat flux more efficiently than the conventional flux tube. For one test case, it was 30 times less computationally expensive and we found no case for which it was more expensive. It is most advantageous when the magnetic shear is high and the domain includes at least two regions of turbulent drive (e.g. stellarator simulations, pedestal simulations, tokamak simulations with several poloidal turns). Additionally, it more accurately models the inboard midplane when the magnetic shear is large. Lastly, we show how the non-twisting flux tube can be generalized to allow further optimization and control of the simulation domain.