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Abstract

Some important neoclassical phenomena are studied with drift-kinetic simulations of the ions: the current which drives neoclassical tearing modes (bootstrap and polarization current) and the bootstrap current at a steep gradient. The δ_f code HAGIS is used with pitch angle collisions modelled by a Monte Carlo procedure. An equilibrium with circular cross section and (for the island modelling) a single helical perturbation is considered, ITER- and AUG-like parameter are used. We find that the bootstrap current in small islands does not vanish, the full current is observed if the island width is smaller than the orbit width. The results can explain the scaling of the normalized beta at the onset of the NTMs in ASDEX Upgrade. The polarization current due to the island rotation is studied as well and is found to be strongly reduced at small island size. The transition to the collisional limit is only observed at high values of ν/εω. In the banana regime the bootstrap current at a steep density profile is reduced and depends non-locally on the gradient if the orbit width is not small compared to the the gradient length.