Self-interaction of turbulent eddies in tokamaks with low magnetic shear

TL;DR

This paper uses gyrokinetic simulations to show that in low magnetic shear tokamaks, turbulent eddies can become extremely elongated along magnetic field lines, significantly affecting transport and confinement.

Contribution

It reveals the existence of ultra long eddies in low shear conditions, their impact on transport, and introduces the concept of eddy squeezing as a confinement improvement mechanism.

Findings

Eddies extend hundreds of poloidal turns in low shear

Field line topology influences turbulence and transport

Eddy squeezing reduces perpendicular eddy size

Abstract

Using local nonlinear gyrokinetic simulations, we demonstrate that turbulent eddies can extend along magnetic field lines for hundreds of poloidal turns in tokamaks with weak or zero magnetic shear $\hat{s}$. We observe that this parallel eddy length scales inversely with magnetic shear and at $\hat{s}=0$ is limited by the thermal speed of electrons $v_{th,e}$. We examine the consequences of these "ultra long" eddies on turbulent transport, in particular, how field line topology mediates strong parallel self-interaction. Our investigation reveals that, through this process, field line topology can strongly affect transport. It can cause transitions between different turbulent instabilities and in some cases triple the logarithmic gradient needed to drive a given amount of heat flux. We also identify a novel "eddy squeezing" effect, which reduces the perpendicular size of eddies and their ability to transport energy, thus representing a novel approach to improve confinement. Finally, we investigate the triggering mechanism of Internal Transport Barriers (ITBs) using low magnetic shear simulations, shedding light on why ITBs are often easier to trigger where the safety factor has a low-order rational value.