Characterization of simulated ion beam neutralization stability in two and three dimensions

Summary form only given. The ready neutralization of ion beams given a source of electrons is well known, but the physics of that process are not clearly understood. Ion and electron currents must be matched to prevent spacecraft charging while space charge neutrality is required in the beam to prevent reflection. These are often thought of as two separate processes, yet the reality that the system is quite robust indicates that there is a physical mechanism to couple them. With electric propulsion moving into both the micro and macro ranges with colloids, FEEPs and thruster arrays, predicting the behavior of neutralizing electrons and plume ions is of increasing importance. Simulations using 2D and 3D Particle-in-Cell (PIC) codes are presented, detailing starting and steady state conditions of an ion beam. It is shown that at steady state, simulations are ultimately nearly space charge neutral regardless of ion or electron currents, while simulations using starting conditions require detailed current balance to propagate without space charge barriers. Further investigation of the steady state reveals that standard PIC provides no mechanism for imparting bulk ion velocity to electrons to provide both space charge and current coupling without a careful balance of the source conditions, such as the electron temperature, contrary to observations.