Alternative Neutralization Techniques for In-Space Charged Particle Thrusters and Electrodynamic Tethers.

Abstract

Over the last few years, there has been growing interest in a new class of space propulsion for smaller satellites in low Earth orbit that utilize highly charged and accelerated nanoparticles (Nanoparticle Field Extraction Thruster or NanoFET) or liquid droplets (colloidal thrusters) to provide thrust. As with any electric propulsion system that emits charged particles, an issue to be examined is how to ensure charge neutral operation. Specifically, this work investigates potential methods to self-neutralize or use the surrounding ambient ionospheric plasma to avoid the complexity of an additional neutralization sub-system.

The NanoFET system operates by charging and expelling either positively or negatively charged nanoparticles drawn from a dry powder through a MEMS-based sieve and grid structure. Similarly, colloidal thrusters charge liquid droplet particles to either polarity. This ability to charge particles to either polarity offers the possibility to use the particles themselves to self-neutralize the system. Common in-space electric propulsion systems only emit positive ion propellant, which requires that electrons be separately emitted, using hollow cathodes or other electron emitters. If NanoFET and colloidal thrusters did not need a separate sub-system for neutralization, it would improve efficiency and reduce complexity.

Plausible options to neutralize NanoFET and colloidal thrusters with as little increase in complexity as possible are to emit equal amounts of oppositely charged particles by either (i) simultaneous emission from separate locations (ii) emission from a single location with periodic polarity change, or (iii) emission of a negatively charged nanoparticle beam into a background plasma that can provide an electron return current.

Even smaller than the satellites that NanoFET and colloidal thrusters are envisioned for providing propulsion to are a class of satellites called femtosatellites. Femtosatellites require unconventional means of propulsion due to their low mass, power, and size budget, with one possible propulsion technique being electrodynamic tethers. An area of study for the electrodynamically tethered femtosatellite system is the electron emission scheme either through a field emitter array cathode or a thermionic cathode/hot filament.