Publication
This thesis investigates turbulence self-interaction and self-organization, with a particular focus on their role in triggering internal transport barriers (ITBs). ITBs, which are routinely observed in the core of tokamak experiments, represent an improved confinement regime that significantly reduces energy losses caused by plasma turbulence. However, the underlying mechanisms leading to ITB formation remain poorly understood. We use gyrokinetic simulations to explore the fundamental dynamics of turbulence in tokamaks, focusing on its behavior when the safety factor is close to low-order rational values and the magnetic shear is weak - conditions often associated with the triggering of ITBs. In such conditions, we observe "ultra-long eddies," turbulent structures that remain correlated for hundreds of poloidal turns along the magnetic field lines. Such long eddies can "bite their own tails" when the safety factor value is rational, leading to strong self-interaction and extreme sensitivity to magnetic field line topology (e.g., integer, rational, or irrational values of the safety factor). We find that self-interaction is predominantly stabilizing, leading to complete stabilization in some cases, and causes stationary corrugations in the profiles of temperature, density, flow, and electromagnetic fields. A novel mechanism to reduce turbulence termed "eddy squeezing" has also been discovered, wherein a single eddy can cover a substantial fraction of a flux surface and restrict its own size for particular values of the safety factor. We also identify a new symmetry-breaking mechanism in the gyrokinetic equation, arising from the parallel boundary condition when magnetic shear is zero, which may provide an explanation for intrinsic rotation observed during certain ITB experiments. Lastly, we examine how plasma profile corrugations coupled with electromagnetic effects lead to turbulence-generated electric currents that create stepped safety factor profiles, reducing turbulent transport by as much as a factor of four. In cases where the minimum of the reversed shear safety factor profile is close to low-order rational surfaces, turbulence-generated currents can "pull" the minimum toward these surfaces and flatten the safety factor profile, leading to a buildup of zonal flows and a significant reduction in transport. These findings are corroborated using multiple numerical tools, strengthening our confidence in their validity. The results presented in this thesis suggest that turbulence self-interaction and self-organization in the vicinity of low-order rational surfaces can play a critical role in ITB triggering.