On supercharging: Electrostatic aspects

Abstract

Charged‐beam emissions control the spacecraft potential if the beam current is higher than the ambient plasma currents. There are important questions as to the upper limit of the beam current that can be emitted into the ambient plasma and the upper limit of the spacecraft potential that can be induced. The usually accepted view is that the beam current emitted reaches its upper limit when the spacecraft potential energy 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 energy can exceed the beam energy when very high beam currents are emitted. This phenomenon is called supercharging. The experimental results show that the expected limits on the beam current emitted and the spacecraft potential achieved appear to be incorrect. We offer two physical explanations. In the first one the beam energy is higher than expected. For example, the extra energy can be due to the beam space charge or the nonmonoenergetic beam energy distribution. For an unneutralized beam the beam space charge near the beam exit point can be significant. The potential energy of the beam space charge is converted into kinetic energy as the beam diverges. The total beam particle kinetic energy becomes more than its initial value. Scattering of beam particles with each other can spread the beam energy to lower and higher values. The beam particles with higher energies than the spacecraft potential escape. The second explanation concerns a common measurement technique using long booms for measuring spacecraft potential. The measurement represents the potential difference between the tip of a boom and that of a spacecraft. The tip of a long boom is commonly assumed to be at the ambient plasma potential. We stress that the boom can be charged, however, especially during high beam current emissions. As a result of boom charging, the measurement exceeds the true potential difference between the spacecraft and the ambient plasma. Both explanations concern possible misinterpretations of the measurements.