Dynamics Of Enhanced Neoclassical Particle Transport Of Tracer Impurity Ions In Ion Temperature Gradient Driven Turbulence

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 ExB 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 ExB 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.