Dispersion of ideal particles in a two-dimensional model of electrostatic turbulence

The dispersion of ideal test particles in electrostatic drift-wave turbulen ce is investigated numerically. A self-consistent model with an internal in stability drive is used to obtain the turbulent two-dimensional (2D) flow-f ield. It is shown that nonlinear couplings lead to the formation of coheren t vortical structures in the flow. The dispersion of the particles is found to be anisotropic, with the weakest dispersion in the direction of the den sity gradient. By distinguishing between particles trapped in structures an d free particles, it is demonstrated that the trapping and subsequent displ acement of particles by nonlinear vortex structures enhances the particle d iffusion in the direction of the background density gradient. Conditional d iffusion coefficients are obtained showing that particles trapped by the vo rtex structures are convected by the structures. The time a particle on the average stays trapped in the structure is closely related to the lifetime of the vortical structures. The relation between the diffusion coefficient obtained from the test particle dispersion and an effective diffusion coeff icient obtained from the cross-field turbulent flux is discussed. (C) 1999 American Institute of Physics. [S1070-664X(99)02112-6].