Virtual Anode in Ion Beam Emissions in Space: Numerical Simulations

Abstract

The plasma interactions associated with the emission of an unneutralized ion beam in space are discussed. Three-dimensional particle simulations, which follow the beam ions as well as the ambient electrons and ions in the self-consistent electric e eld, are performed for various emission conditions. Classical theories and laboratory experiments for one-dimensional beam e ow in diodes have shown that a potential hump will form when the beam current density exceeds a critical value. When the potential hump becomes so high that the beam kinetic energy is near zero there, it becomes a virtual electrode. We show that similar characteristics are also exhibited in high-density ion beams emitted from spacecraft to space plasma, although three-dimensional effects and the ambient plasma make the interaction more complex. Once a virtual anode has formed, it will partly block the beam transmission and discharge the spacecraft potential.