Nonlinear saturation mechanism of ion-temperature-gradient turbulence at finite normalized pressure is identified by analysis of the nonlinear entropy transfer in global gyrokinetic simulations of the turbulence. Turbulence at such finite normalized pressure is electromagnetic and often exhibits non-saturation due to a lack of zonal flows by the influence of magnetic fluctuations of the turbulence in local gyrokinetic simulations. The present study identifies a new saturation mechanism caused by global entropy transfer due to turbulent E × B flow convection in real space. The convection of the entropy associated with the turbulence in the radial direction produces global zonal flows at the both sides of the most active region of the turbulence to avoid the effect of the magnetic fluctuations, and then global zonal-flow excitation is not suppressed, leading to a steady state of the turbulence.
A model of the saturation of coupled electron and ion scale gyrokinetic turbulence