Ion current on a small spherical attractive probe in a weakly ionized plasma with ion-neutral collisions (kinetic approach)

The ion flux on a small attractive sphere (the sphere radius is much less than the Debye length) and the electric potential distribution around the sphere charged in a stationary weakly ionized plasma are calculated by the direct solution of the kinetic equation with the Bhatnagar-Gross-Krook collision term. Only a weak dependence of the ion flux on the electron temperature is found. An analytical approximation of the calculated ion current-voltage characteristics is proposed for intermediate collisionalities, where neither a collisionless theory nor a continuum fluid approach is valid. This approximation was used for calculation of the floating sphere potential. A reasonable agreement of the calculated floating sphere potential with prior numerical calculations is found. (C) 2008 American Institute of Physics.