Study of RF Plasma Technology Applied to Air-Breathing Electric Propulsion.

Abstract

Satellites in low Earth orbit experience an aerodynamic drag force due to the finite density of gas in the thermosphere. Left unchecked, this drag force acts to reduce satellite altitude, eventually causing re-entry. To maintain a satellite in its intended orbit, an onboard propulsion system is typically implemented to counteract the drag. The initial satellite mass delivered to a particular orbit is fixed by the launch vehicle, and the propulsion system requires a significant portion of this mass to be allocated to propellant. These and other altitude-dependent factors strongly affect spacecraft design and cost of the mission. Electric propulsion systems use electric and magnetic fields rather than chemical energy to accelerate propellant to high exhaust velocities. The physical mechanisms that produce thrust are independent of propellant species, and so the ambient gas in the thermosphere may be used as propellant. However, the atmosphere consists of gases that are not typically used in electric propulsion systems, and previous studies have shown that operating with these gases may reduce thruster performance and lifetime. Radio frequency (RF) plasma systems are used by the semiconductor manufacturing industry to efficiently create a dense plasma source from a wide variety of gases, and therefore may be capable of increasing the performance of electric propulsion systems operating with atmospheric gases. This dissertation presents an experimental investigation into the use of RF plasma in an air-breathing electric propulsion system. Based on the requirements for such a system, two novel thrusters are tested in the laboratory. The first thruster uses only RF power and a magnetic field to create thrust. The second is a two-stage thruster that uses an RF ionization stage to increase the propellant utilization efficiency of a traditional Hall thruster. Thruster performance measurements are presented in the context of an air-breathing system, as well as plasma probe measurements of the exhaust to characterize the major loss mechanisms. The results suggest that an air-breathing satellite is feasible with currently available technology.