Numerical study of turbulent eddy self-interaction in tokamaks with low magnetic shear. Part II: Nonlinear simulations

Using local nonlinear gyrokinetic simulations and building up on linear results from acompanion paper (Volcokas 2024 Submitted for publication), we demonstrate that turbulenteddies can extend along magnetic field lines for hundreds of poloidal turns in tokamaks withweak or zero magnetic shears. We observe that this parallel eddy length scales inversely withmagnetic shear and ats=0 is limited by the thermal speed of electronsv(th,e). We examine theconsequences of these 'ultra long' eddies on turbulent transport, in particular, how field linetopology mediates strong parallel self-interaction. Our investigation reveals that, through thisprocess, field line topology can strongly affect transport. It can cause transitions betweendifferent turbulent instabilities and in some cases triple the logarithmic gradient needed to drivea given amount of heat flux. We also extensively study a novel 'eddy squeezing' effectintroduced in Volcokas (2023Nucl. Fusion63014003), which reduces the perpendicular size ofeddies and their ability to transport energy, thus representing a novel approach to improveconfinement. Finally, we investigate the triggering mechanism of Internal Transport Barriers(ITBs) using low magnetic shear simulations, shedding light on why ITBs are often easier totrigger where the safety factor has a low-order rational value.