[en] A model was developed for the propagation of a focussed ion beam through the gas that may exist in the target chamber of a reactor. The model contains the effects of beam ion stripping, background plasma generation by the beam ions, and electron avalanching in the electric fields produced by the ion pulse. Charge and partial current neutralization, together with stability conditions and the effects of preionization have been investigated, with most recent emphasis being on the role of filamentation instabilities