A ‘particle’ model of plasma behaviour, suitable for application to the study of plasma-optical Systems, has been developed. A plasma-optical System which removes macroscopic droplets from a neutralized beam of ions and electrons, produced by an arc, has been modelled and its performance has been analysed. The particle model employs an extremely efficient treatment of the electron motion through a background gas, which in this case consists of neutral particles. The distribution of distances travelled by electrons in the course of many collisions with the neutral background is generated using a Monte Carlo method and samples from the distribution are used to determine the electron motion at each integration step. In sheath regions, for instance, direct integration is still necessary, but for most electrons the simulation is very fast, largely removing the numerical ‘stiffness’ of the particle method. This method will be equally effective for time-dependent fields such as occur in RF discharges used in ‘plasma processing’. The variation in ion current to a substrate, where the ions deposit as a thin film, has been studied. The objective is to maximize the deposition rate whilst preventing macroscopic particles which arise at the are from striking the film. An alternative System whose calculated transmittal rate from source to target is higher, for long mean-free-paths, is also examined.