In detached divertor regimes, especially in future tokamak-reactors, plasma in the divertor is strongly collisional. When ion-ion and ion-neutral collision frequencies become comparable with the ion gyrofrequency, plasma transport is strongly affected by the motion of neutral particles. This paper demonstrates that in such partially ionized plasma, ion transport is regulated by the motion of neutrals which are dragged along the magnetic field and diffuse in the perpendicular direction due to the ion-neutral friction force. The motion of neutral particles and (due to the friction force) ions is to a large extent controlled by the viscosity of atoms in the presence of charge-exchange collisions. Analytic models are compared with the results of ITER simulations performed with the SOLPS-ITER plasma boundary transport code. Specific attention is paid to currents caused by ion-neutral collisions.
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