E × B advection of trace ions in tokamak microturbulence

The E×B advection of trace ions in realistic tokamak microturbulence (as described by nonlinear gyrokinetics) is investigated. In order to understand the consequences of effects such as large gyroradii, fluctuation anisotropies, zonal flows, or poloidal drifts, they are first studied in the framework of a model that is based on self-created stochastic potentials. Direct numerical simulations are performed, and a semi-analytical model is presented that provides qualitative as well as quantitative insight into the nature of passive tracer transport. One finds that the resulting diffusivities may be larger than expected as long as the gyroradii do not exceed the turbulence correlation length(s) and the poloidal drift velocities are sufficiently small.