Effects of charge exchange in ion beams emitted from spacecraft

Summary form only given, as follows. Most ion beams devices to date generate high energy beam ions mixed with a low energy neutral gas. When such a positive ion beam is emitted from a spacecraft, the spacecraft potential often swings negative. The spacecraft potential is determined by the balance of the currents of the beam and the space plasma. Charge exchange between the ions and the neutrals generates low energy ions, which are attracted towards the spacecraft. This affects the current balance and decreases the magnitude of the spacecraft potential. When the ion beam current density is high, the newly generated low energy ions build up space charge which slows down the beam ions. The slow down, in turn, enhances the beam current density and the rate of charge exchange. Thus, the low energy ion return current is a non-monotonic function of the emitted beam current. This theory may explain the observed non-monotonic behavior of spacecraft potential during xenon ion beam emissions from the SCATHA satellite.