Basic features of E×B 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 the most existing models is that we deal with a linearly unstable system such as the ion-temperate-gradient (ITG) 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 details. 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.
In this paper,the effect of finite Larmor radius (FLR) on high n ballooning modes is studied on the basis of FLR magnetohydrodynamic (FLR-MHD) theory.A linear FLR ballooning mode equation is derived in an 's α' type equilibrium of circular-flux-surfaces,which is reduced to the ideal ballooning mode equation when the FLR effect is neglected.The present model reproduces some basic features of FLR effects on ballooning mode obtained previously by kinetic ballooning mode theories.That is,the FLR introduces a real frequency into ballooning mode and has a stabilising effect on ballooning modes (e.g.,in the case of high magnetic shear s ≥ 0.8).In particular,some new properties of FLR effects on ballooning mode are discovered in the present research.Here it is found that in a high magnetic shear region (s ≥ 0.8) the critical pressure gradient (α c,FLR) of ballooning mode is larger than the ideal one (α c,IMHD) and becomes larger and larger with the increase of FLR parameter b 0.However,in a low magnetic shear region,the FLR ballooning mode is more unstable than the ideal one,and the α c,FLR is much lower than the α c,IMHD.Moreover,the present results indicate that there exist some new weaker instabilities near the second stability boundary (obtained from ideal MHD theory),which means that the second stable region becomes narrow.
