The nonlinear evolution of collisionless drift instabilities in a shearFree magnetic field has been studied by means of gyrokinetic particle simulation as well as numerical integration of model mode-coupling equations. The purpose of the investigation is to identify relevant nonlinear mechanisms responsible for the steady-state drift wave fluctuations.It is found that the saturation of the instability is mainly caused by the nonlinear E * B convection of the resonant electrons and their associated velocity space nonlinearity. The latter also induces energy exchange between the competing modes, which, in turn, gives rise to enhanced diffusion. The nonlinear E x B convection of the ions, which contributes to the nonlinear frequency shift, is also an important ingredient for the saturation.

Ion temperature gradient drift instabilities have been investigated using gyrokinetic particle simulation techniques for the purpose of identifying the mechanisms responsible for their nonlinear saturation as well as the associated anomalous transport. For simplicity, the simulation has been carried out in a shear-free slab geometry, where the background pressure gradient is held fixed in time to represent quasistatic profiles typical of tokamak discharges. It is found that the nonlinearly generated zero-frequency responses for the ion parallel momentum and pressure are the dominant mechanisms giving riseto saturation. This is supported by the excellent agreement between the simulation results and those obtained from mode coupling calculations, which give t.ie saturation amplitude as |e*/T e | = (|u 8 + iir^|/si|)/(k x p s ) 2 . and the quasilinear thermal diffusivity as Xj = y i/ k j. z -where Wj and t^ are the linear frequency and growth rate, respectively, for the most unstable mode of the system. In the simulation, the time evolution of Xj after saturation Is characterized by its slow relaxation to a much lower level of thermal conduction. On the other hand, a small amount of electron-ion collisions, which has negligible effect on the linear staDNIty, can cause significant enhancement of Xj in the steady state.