Using local nonlinear gyrokinetic simulations and building up on linear results from a companion paper (Volčokas 2024 Submitted for publication), we demonstrate that turbulent eddies can extend along magnetic field lines for hundreds of poloidal turns in tokamaks with weak or zero magnetic shear . We observe that this parallel eddy length scales inversely with magnetic shear and at is limited by the thermal speed of electrons . 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 extensively study a novel 'eddy squeezing' effect introduced in Volčokas (2023 Nucl. Fusion63 014003), 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.
This paper investigates how turbulent eddies behave in tokamaks with weak or zero magnetic shear. It shows that these 'ultra long' eddies can significantly impact transport by affecting field line topology. The study also reveals a novel 'eddy squeezing' effect that can improve confinement, and sheds light on the triggering mechanism of Internal Transport Barriers.