Ultra long turbulent eddies, magnetic topology, and the triggering of internal transport barriers in tokamaks

Local nonlinear gyrokinetic simulations of tokamak plasmas demonstrate that turbulent eddies can extend along magnetic field lines for hundreds of poloidal turns when the magnetic shear is very small. By accurately modeling different field line topologies (e.g. low-order rational, almost rational, or irrational value of the safety factor), we show that the self-interaction of such `ultra long' eddies can dramatically reduce heat transport. This extreme sensitivity of turbulent transport to the safety factor reveals novel strategies to improve confinement, constitutes experimentally testable predictions, and illuminates past observations of internal transport barriers.