Effect of neutral atoms on tokamak edge plasmas

Neutral atoms can significantly influence the physics of tokamak edge plasm as, e.g., by affecting the radial electric field and plasma flow there, whi ch may, in turn, be important for plasma confinement. Earlier work [Fulop , Phys. Plasmas 5, 3969 (1998)], assuming short mean-free path neutrals and Pfirsch-Schluter ions, has shown that the ion-neutral coupling through char ge-exchange affects the neoclassical flow velocity significantly. However, the mean-free path of the neutrals is not always small in comparison with t he radial scale length of densities and temperatures in the edge pedestal. It is therefore desirable to determine what happens in the limit when the n eutral mean-free path is comparable with the scale length. In the present w ork a self-similar solution for the neutral distribution function allowing for strong temperature and density variation is used, following the analysi s of Helander and Krasheninnikov [Phys. Plasmas 3, 226 (1995)]. The self-si milar solution is possible if the ratio of the mean-free path to the temper ature and density scale length is constant throughout the edge plasma. The resulting neutral distribution function is used to investigate the neutral effects on the ion flow and electrostatic potential as this ratio varies fr om much less than one to order unity. (C) 2001 American Institute of Physic s.