Intermittent bursts in ion-temperature-gradient turbulence

Abstract

Linear and nonlinear evolution of ion-temperature-gradient (ITG) modes is numerically investigated by using a Landau-fluid model in a sheared slab geometry. Dependence of the linear growth rates of ITG modes with different radial mode numbers on various equilibrium parameters is discussed in detail. It is found that the ITG mode with a higher radial mode number is the dominantly unstable one in a narrow current sheet system with a low magnetic shear. In nonlinear simulations with different magnetic shears, two types of nonlinear states, specifically, oscillatory relaxation to a steady state and an intermittent bursting state, are observed in the weak turbulence regime. Similar phenomena of both nonlinear states are also observed in the simulations with different temperature gradients. The basic physical processes of both nonlinear states are thoroughly analyzed by means of quasilinear simulations. It is found that the system relaxes to a steady state when the ITG mode is slightly unstable, whereas it enters an intermittent burst state when the ITG mode is more unstable. During each burst, the ITG mode with a higher radial mode number becomes the dominant one due to the energy cascade in kx space, which is induced by self-generated zonal flows.