Local gyrokinetic simulations of tokamaks with non-uniform magnetic shear

TL;DR

This paper extends local gyrokinetic simulations to include non-uniform magnetic shear by modifying the flux tube domain and deriving new model terms, enabling studies of complex safety factor profiles in tokamaks.

Contribution

It introduces a systematic method to incorporate ion gyroradius-scale variations in magnetic shear into gyrokinetic models, with successful implementation in simulation code.

Findings

New terms for parallel streaming along modified field lines

Flux tube simulations of non-monotonic safety factor profiles

Profile shearing effects are similar in positive and negative triangularity shapes

Abstract

In this work, we modify the standard flux tube simulation domain to include arbitrary ion gyroradius-scale variation in the radial profile of the safety factor. To determine how to appropriately include such a modification, we add a strong ion gyroradius-scale source (inspired by electron cyclotron current drive) to the Fokker-Planck equation, then perform a multi-scale analysis that distinguishes the fast electrons driven by the source from the slow bulk thermal electrons. This allows us to systematically derive the needed changes to the gyrokinetic model. We find new terms that adjust the ion and electron parallel streaming to be along the modified field lines. These terms have been successfully implemented in a gyrokinetic code (while retaining the typical Fourier representation), which enables flux tube studies of non-monotonic safety factor profiles and the associated profile shearing. As an illustrative example, we investigate tokamaks with positive versus negative triangularity plasma shaping and find that the importance of profile shearing is not significantly affected by the change in shape.