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Turbulent transport of the W ions in tokamak plasmas: properties derived from a test particle approach

A detailed analysis of a special type of turbulent pinch mechanism for tungsten (W) ions is presented. It is based on the numerical simulation of ion trajectories in a complex test-particle model that contains realistic representations of the turbulence and of the confining magnetic field, plasma rotation, ion collisions, the polarization drift and the parallel stochastic acceleration. The origin of the pinch is a special type of order generated by an average poloidal velocity V p superposed on the E × B stochastic drift. It consists of a pair of radial symmetric drifts, the hidden drifts (HDs), that exactly compensate. The polarization drift and the parallel acceleration drive radial pinch velocities V x by destroying the perfect anti-symmetry of the HDs. These phenomena that were found in simplified models are theoretically validated here in a realistic model of W ion transport. The main conclusion of the paper is that this turbulent mechanism can be experimentally relevant for a large domain of parameters, which cover both existing plasmas and ITER conditions, and it can be responsible for an important amount of the W ion accumulation.