Two-dimensional numerical simulation of drift waves and trapped ion modes

Drift waves (DW) and trapped ion modes (TIM) are concurrently simulated by a two-dimensional pseudo-spectral nonlinear code with a dense wavenumber grid. This extends the authors' previous work which treated DW and TIM turbulence separately. The model includes curvature effects and collisionality. Two fluids are used to represent the dynamics throughout wavenumber (k)-space. Physically, the DW and TIM equations apply to disjoint regions of k-space characterized by ω much-gt ωbi and ω much-lt ωbi, respectively, where ωbi is the ion bounce frequency. Here, the boundary is modeled by a single, appropriate kyρsi. The DW and TIM fields can only interact via the E x B convective nonlinearity. The effect on the net diffusion is of interest. The DW turbulence when considered separately has a spectrum peaked at wavenumbers below the most linearly unstable ones; this implies an inverse cascade mechanism. The TIM are of long wavelength and therefore might contribute significantly to the diffusion if they were driven to large amplitudes either by the linear growth or by the DW turbulence. The preliminary results show that the net diffusion is not significantly affected by replacing the low ky DW dynamics by TIM equations. The spectrum is suppressed inside the TIM region compared to having no TIM (DW extending to ky = 0) in part because the lowest ky TIM are collisionally damped