Simulation of self-neutralization techniques for charged particle thrusters

Summary form only given.There is an emerging class of nanospacecraft thrusters under development that use colloids or nanoparticles that can be charged either positively or negatively to provide thrust. An issue to be examined is how the ability to charge equal number of particles to both polarities is beneficial to the system in terms of being able to provide self-neutralization of the thruster. An important consideration here is that, unlike traditional ion thruster technology, which emit a single particle polarity and is separately neutralized by highly mobile electrons, this neutralization scheme will be predicated on equal charge, equal mass, massive, cold particles to provide neutralization. Thus, different issues need to be examined, such as strong electric fields near the spacecraft. We explore two approaches for charged particle thruster neutralization: spatially and temporally separated, oppositely charged populations of nanoparticles. Both approaches would result in equal amounts of oppositely charged particles being emitted, resulting in a net neutral spacecraft. Our investigation is accomplished through particle-in-cell simulation using XOOPIC™ and analytical modeling. Through simulations, we are able to observe the behavior of spatially separated, equal mass particles with opposite charge. We observe that even when equal numbers of oppositely charged particles are emitted simultaneously from spatially separated regions, there is still a local charge buildup on the spacecraft wall due to an image charge effect. This results in an electric field between the emitted beam and the nearby spacecraft surface, which can decrease particle beam velocity by a few percent. As expected, oppositely charged nanoparticle beams tend to converge. Thus collisions of equal mass particles where charge can be separated from the particle center-of-mass must be considered. For particle emissions that are non-neutral (temporally separated), analytical modeling enables us to estimate how quickly a 30 cm diameter nanospacecraft would charge up. For example, emitting 10 mA of current would result in the spacecraft potential reaching 5% of the equivalent beam energy in under 10 μs requiring oscillation of the charging and accelerating scheme at approximately 100 kHz.