High current beam emissions from spacecraft

During high current charged particle beam emissions from a spacecraft, the spacecraft potential floats relative to the ambient plasma. There are important questions about the upper limits of beam current emitted into the ambient plasma and of the spacecraft potential. The usually accepted view is that the beam current emitted reaches its upper limit when the spacecraft potential reaches the beam energy. In this view, any attempt to emit a current beyond the critical value would result in partial beam return while both the effective beam current emitted and the spacecraft potential would remain unaffected. Surprisingly, results from three recent beam experiments in space show that the spacecraft potential can exceed the beam energy when very high beam currents are emitted. These results show that the usual limits on the beam current emitted and the spacecraft potential achieved are incorrect. Study of space charge beam divergence shows that the usual limits can be exceeded.