Nonexistence of Quiescent Plasma States in Ion Propulsion

The problem of neutralizing an ion beam is analyzed within the framework of a one-dimensional collision-free theory. The character of steady-state solutions obtained depends upon the ratio between the mean thermal speed of the electrons and the ion velocity. When the ions are faster, excess electrons are returned to their source from a finite distance behind the electron emitter and the flow of charges is unidirectional (beam-like) further downstream. When the electrons are faster, a ``floating target'' must be postulated downstream to intercept electrons and ions at equal rates and to return excess electrons to the emitter. In this case the flow pattern of the electrons is ``plasma-like.'' Both beam-like and plasma-like states are found to be unstable with respect to small perturbations. Thus the plasma driving a space vehicle cannot be in a quiescent state.