Two-dimensional computer experiments on ion- beam neutralization

Abstract

The two-dimensional electron-ion mixing mechanism for ion propulsion has been programed into a computer. Electrons and ions are simulated by several thousand rods of negative and positive charge. They are accelerated step-by-step, the space-charge fields being evaluated at each step by a new superfast technique of integrating Poisson's equation. The technique employs Fourier analysis, a marching method, and a capacitance matrix characterizing the electrode system. Integration takes approximately one second for 2% linear resolution. A stack of strip ion beams is injected through an accel-grid. Thermal electrons are released from both sides into each beam. The electron emitters define the decel-potential but are not placed directly within the beams; free space conditions are imposed at the heads of the advancing beams. Trajectory and equipotential plots show that the electron supply and demand of the beams is regulated by fluctuating fields. A near-neutral plasma is formed at a potential within a few kT of the electron emitters. The thrust is thus maintained near a value corresponding in ion acceleration to this potential, beam spread remaining negligible. Changing parameters, such as masses, currents, and velocities produce expected results, such as ion turn around when electron emission is reduced to zero value.