Effect of ion-neutral collisions on plasma flow in the edge region of stellarators

The paper presents a study of the effect of charge exchange and elastic scattering collisions between ions and neutrals on the plasma flow along the divergent magnetic field lines in the divertor region of a stellarator. In the analysis, which is kinetic, an indirect solution to the kinetic equation for the ions is obtained using a Monte Carlo description for the ion dynamics together with the assumption of Boltzmann electrons and quasi-neutrality. Ion-neutral collisions are studied in a three-dimensional velocity space using a Monte Carlo algorithm. The results show that charge exchange and elastic scattering collisions play a significant role in reducing the drop of the electrostatic potential in the divertor chamber below that obtained from collisionless calculations. This, in turn, reduces the electrostatic barrier, which may be useful in retarding the flow of high Z impurity ions from the target plate towards the confined plasma. The calculations for the ion energy distribution suggest that, owing to ion-neutral collisions, most of the impact energy of ions on the divertor plate is gained while they cross the sheath region of the plate. (author). 14 refs, 16 figs