Dynamics of enhanced neoclassical particle transport of tracer impurity ions in ion temperature gradient driven turbulence

Abstract

Tracer impurity transport in ion temperature gradient-driven (ITG) turbulence is investigated using a global full-f gyrokinetic simulation including kinetic electrons, bulk ions, and low to medium Z tracer impurities, where Z is the charge number. It is found that in addition to turbulent particle transport, enhanced neoclassical particle transport due to a new synergy effect between turbulent and neoclassical transports makes a significant contribution to tracer impurity transport. Bursty excitation of the ITG mode generates non-ambipolar turbulent particle fluxes of electrons and bulk ions, leading to a fast growth of the radial electric field following the ambipolar condition. The divergence of E × B flows compresses up-down asymmetric density perturbations, which are subject to transport induced by the magnetic drift. The enhanced neoclassical particle transport depends on the ion mass because the magnitude of up-down asymmetric density perturbation is determined by a competition between the E × B compression effect and the return current given by the parallel streaming motion. This mechanism does not work for the temperature and, thus, selectively enhances only particle transport.