Three-dimensional simulations of ion thruster beam neutralization

Motivated by the recently revived interest in electric propulsion, the neutralization regime of an ion thruster is investigated by means of a three-dimensional particle-in-cell simulation. Electrons enter the simulation box with a half-Maxwellian velocity distribution, while the ions are injected with a uniform bulk velocity. Focus is put on the dynamics of the electrons, and the actual electron to ion mass ratio of 1:250 000 is used. The injection velocity ratio η≔ve0th/vi0 between electron thermal velocity and ion bulk velocity turned out to be a crucial parameter for the electron dynamics within the plasma beam: For η<1.7 a moving electrostatic shock forms with a potential jump of a few kTe0/e. Downstream of the shock front, the electron plasma becomes fully Maxwellian and drifts with the ion bulk velocity. For η>1.7, the electrons still obtain a drift velocity roughly equal to vi0 within a few electron inertia lengths behind the emitting plane. However, a shock front does not form, and the electron velocity distribution does not become Maxwellian. On the basis of a tentative model, in which the plasma beam is regarded as a self-similarly expanding electron diode, the shock front can be identified with a kind of virtual cathode and the dependence of the shock velocity on the beam cross section can be explained.