Turbulence-Generated Stepped Safety Factor Profiles in Tokamaks with Low Magnetic Shear

TL;DR

This paper shows that turbulence in tokamaks can create stepped safety factor profiles in low magnetic shear conditions, which significantly reduce heat transport and may improve plasma confinement.

Contribution

The study demonstrates through simulations that turbulence-induced currents can form stable stepped safety factor profiles in low shear tokamaks, a novel insight into plasma behavior.

Findings

Stepped safety factor profiles reduce heat transport.

Turbulence-generated currents flatten the safety factor near rational surfaces.

This mechanism may trigger internal transport barriers.

Abstract

Nonlinear local and global gyrokinetic simulations of tokamak plasmas demonstrate that turbulence-generated currents flatten the safety factor profile near low-order rational surfaces when magnetic shear is low, even when the plasma $\beta$ is small. A large set of flux tube simulations with different safety factor profiles (e.g. linear and non-linear safety factor profiles) and global simulations with reversed magnetic shear profiles show that such stepped safety factor profiles dramatically reduce the heat transport and are a robust phenomenon. This mechanism may play a key role in the triggering of internal transport barriers (ITBs) and more generally reveal novel strategies for improving confinement in devices with low magnetic shear.