3D simulations of shock-like ion thruster neutralization

Abstract

Ion thrusters are valued as a very efficient type of space propulsion. However, their plasma environment has raised some concerns, e. g. in terms of electromagnetic interference induced by plasma instabilities in the neutralization process. Recently, it was shown via numerical simulations that ion thrusters can undergo a shock-like neutralization process, where an emerging electrostatic shock generates a completely thermalized beam plasma with no free energy to drive unwanted instabilities. Based on this finding, we investigate the impact of an axial magnetic field on the shock-like neutralization scenario by means of a three-dimensional particle-in-cell simulation. In a simplified quasi-one-dimensional configuration, electrons and ions are injected through a common opening on one side of the simulation box. Similarly to previous results for the absence of a magnetic field, the injection velocity ratio 77 := Vgo/^o between electron thermal velocity and ion bulk velocity turned out to be the crucial parameter determining whether the plasma beam undergoes a shock-like or a shock-free neutralization. For the shock-like neutralization scenario, we further characterize the downstream plasma state as a function of applied magnetic field strength and beam diameter and thereby explore in how far the intentional use of strong axial magnetic fields can provide favourable conditions for the neutralization to be shock-like. Introduction charge fields at the electron plasma frequency provided the entropy increase needed for a proper mixIon engines derive their thrust from the electroing of electrons and ions, and thus lead to the crestatic acceleration of heavy ions, such as xenon ation of a stably streaming plasma, in the case of Deep Space 1 (DS1). Outside the Those investigations defined the state of the art thruster, the ejected ions form a dense beam with m understanding ion beam neutralization for about a characteristic kinetic energy of about 1 keV. In 35 years. In a recent paper, Othmer et a/. revisorder for the ion beam to propagate, it needs to ited this problem and started an in-depth analybe electrically neutralized. For this purpose, a hoisis of the physical processes that accompany ion low cathode that injects electrons into the beam is thruster beam neutralization. They applied a mounted on the thruster periphery. three-dimensional (3D) electromagnetic particleThe process of neutralization, i. e. the mixing of in-cell (PIC) code to a strongly simplified ion electrons and ions, presents a challenging problem thruster neutralization regime. They chose a quasiand the detailed physics that govern ion thruster one-dimensional geometry, where electrons and beam neutralization are only poorly understood. ions entered the simulation box through the same Early treatments of this problem go back to Bunerectangular opening. By using the actual electron man and Kooyers, Dunn and Ho, and Wadhwa to ion mass ratio of 1 : 250000, they focused on the et a/., who carried out oneand two-dimensional dynamics of the electrons. In their simulations, the computer simulations of the mixing between cold injection velocity ratio 77 := v^/v^ between the streaming ions and cold or hot electrons. These electron thermal velocity and the ion bulk velocity authors found that self-excited fluctuating spaceturned out to be a crucial parameter for the elec____________ tron dynamics within the plasma beam: for rj 1.7, * Institute of Geophysics and Meteorology the electrons still obtained a drift velocity roughly § Institute of Theoretical Physics equal to V& within a few electron inertia lengths ^ Computing Center