Numerical simulation of drift waves and trapped ion modes

Abstract

Two-dimensional numerical simulations are used to study the interaction of trapped electron drift waves (DW) and trapped ion modes (TIM). Wave-number (k) space is divided into long and short wave regions at a poloidal wave number corresponding to the ion bounce frequency. Two field models are used to describe trapped electron drift wave dynamics at short waves and trapped ion mode dynamics for long waves. The standard case has curvature effects and collisionality. The nonlinearity that couples the two regions includes a trapped ion banana width effect analogous to finite Larmor radius (FLR) polarization drift. The principal result of this study is that the TIM do not contribute to the diffusion significantly, regardless of the model for the nonlinear coupling to the DW. This conclusion is supported by a more general four field model that includes pressure dynamics and which allows ion temperature gradient (ITG) driven drift modes. When the collisionality is varied, the diffusion deviates from the γ/k2x form and tends to vanish towards the collisionless limit even though the system is strongly unstable linearly.