Basic Features of ExB Convection Nonlinearity in Tokamak Plasmas

Basic features of convection nonlinearity in tokamak plasmas, especially, large-scale coherent structures, are studied on the basis of both the model of three coupled modes and the model of four coupled modes. The difference of our models with most existing models is that we deal with a linearly unstable system such as the ion-temperate-gradient driven turbulence in tokamaks. Two types of coherent structure are identified with spatio-temporal characteristics called a zonal flow (ZF), and an oscillating shearing flow (OSF), respectively. At the same time, the anomalous heat fluxes in the system are analyzed in some detail. Results show that the two types of coherent structure play different roles in both the plasma turbulent fluctuations and the related anomalous transports. Moreover, only the large-scale coherent structure with zero frequency, namely, the zonal flow, can suppress the turbulent fluctuations effectively and hence benefits tokamak plasma confinements.