Dynamics of turbulent transport dominated by the geodesic acoustic mode near the critical gradient regime

Miki, Kazuhiro*; Kishimoto, Yasuaki; Li, J.*; Miyato, Naoaki

Effects of geodesic acoustic modes (GAMs) on toroidal ion temperature gradient turbulence and associated transport near the critical gradient regime in tokamak plasma are investigated based on global Landau-fluid simulations and extended predator-prey modeling analyses. A new type of intermittent dynamics of transport accompanied with emission and propagation of the GAMs has been found. The GAMs transfer turbulence energy to wide radial region nonlocally. Stationary zonal flows gradually grow over many periods of quasi-periodic intermittent bursts and quench the turbulence, leading to a nonlinear up-shift of the linear critical gradient, the Dimits shift. This process is categorized as a new class of transient dynamics, growing intermittency. An extended minimal predator-prey model is proposed, which qualitatively reproduces the main features of the growing intermittency and approximately predicts its various time scales observed in the simulations.