Three-dimensional simulations of ion thruster beam neutralization

Motivated by the recently revived interest in electric propulsion, the neut ralization regime of an ion thruster is investigated by means of a three-di mensional particle-in-cell simulation. Electrons enter the simulation box w ith a half-Maxwellian velocity distribution, while the ions are injected wi th 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 injecti on velocity ratio eta:=nu (th)(e0)/nu (i0) between electron thermal velocit y and ion bulk velocity turned out to be a crucial parameter for the electr on dynamics within the plasma beam: For eta <1.7 a moving electrostatic sho ck forms with a potential jump of a few kT(e0)/e. Downstream of the shock f ront, the electron plasma becomes fully Maxwellian and drifts with the ion bulk velocity. For <eta>>1.7, the electrons still obtain a drift velocity r oughly equal to v(i0) within a few electron inertia lengths behind the emit ting 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 electr on 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 e xplained. (C) 2000 American Institute of Physics. [S1070-664X(00)04112-4].