Turbulent transport associated with GAM dynamics near critical gradient regime; GAM intermittency

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

We found a new type of intermittent dynamics of transport near critical gradient associated with the emission and propagation of geodesic acoustic mode (GAM) coupled with zonal flow based on our global gyro-fluid ITG turbulence simulations. We may refer the prominent dynamics to GAM intermittency. The intermittent bursts are triggered by the onset of spatially propagating GAM when the turbulent energy exceeds a critical value. The GAMs suffer a collisionless damping during the propagation and nonlocally transfer local turbulence energy to wide radial region as that of zonal flow. The stationary component of zonal flows gradually increases over many periods of quasi-periodic intermittent bursts and eventually quenches the turbulence, leading to a nonlinear up-shift of the linear critical gradient, i.e., the Dimits shift. The gradual increase of zonal flow results from the accumulation of non-damped residual part during the propagation of GAMs in each cycle of bursts. The period of burst becomes shorter as the temperature gradient increases. An extended minimum model including dynamical GAM damping which qualitatively describes the intermittent dynamics obtained in our simulation is presented.