Time evolution of ion swarm transport in a constant mean free time encounter region

Ion swarm transport is simulated by a Monte Carlo technique in the case of a constant mean free time collision model with isotropic scattering after the ions have been released into a gas with a delta -function distribution in space and a steady-state velocity distribution. The time behaviour of the transport coefficients is investigated for ion-molecule mass ratios m/M of 0.1, 1.0 and 10 under the condition that the field energy is one hundred times greater than the thermal energy. In particular, discussion is given for the skewness and excess of the density distribution. It is also shown that these higher-order coefficients become increasingly important with decreasing m/M.