Performance Characterization of a Low Power Magnetic Nozzle

Abstract

The thrust and efficiency performance of a low-power magnetic nozzle test article is analytically and experimentally investigated. In the last two decades the demand for new forms of in-space propulsion for small spacecraft has increased interest in low-power (< 200 W) magnetic nozzle thrusters. The inherent advantages of these devices, including the electrodeless design and the potential to be propellant-agnostic, coupled with the potential to efficiently accelerate the propellant makes low-power magnetic nozzles attractive propulsion options for small satellites. However, the measured performance of both the low and moderate power versions of these thrusters has compared unfavorably to existing state-of-the-art propulsion technologies. A theoretical model was developed to predict low-power magnetic nozzle performance and identify fundamental differences in operation between these devices and their higher power counterparts.

An experiment was designed to inform the theoretical model and to provide insight into the fundamental dynamics of plasma flowing through a low-power magnetic nozzle. This test article consisted of a reconfigurable inductively-coupled plasma source and an electromagnet. A suite of electrostatic probes and laser induced fluorescence is used to measure the plasma properties throughout the nozzle and map the plasma structures present in the plume. Using the experimental measurements, it is found that the plasma expansion follows a polytropic law, as predicted in the literature. The observed increase in the ion velocity confirms that the test article accelerates the propellant. By coupling the experimental results with the theoretical framework, two novel effects that reduce device performance are identified: 1) a low ion fraction, and corresponding neutral-collisional effects, impedes ion acceleration and shifts the nozzle throat downstream, and 2) non-uniform power deposition enhances the plasma density adjacent to the liner wall, resulting in degraded source (the ratio of power flowing into the diverging nozzle section to the total power deposited in the plasma) and divergence efficiency (the fraction of the kinetic energy in the thrust direction). These effects arise from the low input power and the thruster design parameters. Experimental characterization of a reconfigured test article demonstrates that performance can be recovered by accounting for these two effects when designing the thruster and selecting the operating parameters.