THE TRANSITION TO ION-TEMPERATURE-GRADIENT-DRIVEN PLASMA TURBULENCE

Recent advances in the understanding of the transition to collisionless ion-temperature-gradient-driven (ITG) plasma turbulence are described. A brief introduction to the physics of ITG modes is provided, then a model nonlinear system is motivated. The results of systematic bifurcation analysis of the model are reported, with emphasis on the unusual properties of the center manifold stemming from the existence of undamped zonal flows. The model is shown to exhibit a "Dimits shift" of the critical temperature gradient for the onset of ITG activity similar to that observed in large computer simulations and understood to be due to the excitation of zonal flows by a transient burst of ITG activity. For the model, the shift can be calculated from first principles.