Collisional Scattering Into and Evaporative Cooling From a Potential Well

摘要

It can be shown that ion beams in electric propulsion devices create a potential well which acts to retain neutralizing electrons to a degree. To be trapped in the well, however, it is necessary for the energy and momentum that got the electrons to the well to 'bounce' off the other side of the well to bring their dwell time up to a point where they can be trapped. We demonstrate that conditions exist in normal electric propulsion plumes where a collisional scattering mechanism can be sufficient to scatter a neutralizing electron beam into the ion beam. Furthermore, once in the well for a sufficiently long period, collective instabilities such as the Buneman instability thermalize the electrons, dropping the bulk electron velocity to match that of the ions. While normally this would mean that electron temperature should be equal to the well depth, we show by means of a simple flux model that electrons thermalize only to a point where the flux of 'hot' electrons out of the well is matched by the ambient 'cold' electrons moving into the well.