Characterization of Performance and Current Drive Mechanism for the Rotating Magnetic Field Thruster

Abstract

The Rotating Magnetic Field (RMF) Thruster is a propulsion concept which, could enable a wide variety of mission architectures. The RMF thruster can be operated either in the pulsed-mode, ejecting dense field-reversed configuration (FRC) plasmoids, or in the continuous wave (CW)-mode, producing a steady jet of plasma. Operation in pulsed-mode causes the RMF thruster to resemble an inductive pulsed plasma thruster, though avoiding the very high voltage and current transients which those devices require and which make power supply design especially difficult. Operation in the CW-mode causes the RMF thruster to resemble an electrodeless applied field magnetoplasmadynamic thruster, with the lack of electrodes removing a key lifetime limitation. Critically, in either case the RMF thruster’s lack of plasma wetted electrodes make it ideal for in-situ resource utilization, air-breathing, or multimode propulsion architectures, all of which demand the capability to operate on unconventional, often oxidizing, propellants. Despite these potential advantages, however, performance characterization of this thruster design remains sparse, with relatively few groups having made thrust and efficiency estimates. Indeed, no direct thrust measurements to our knowledge have been published. Scaling laws for the RMF thruster, too, are unconvincing, with no realistic upper bound to performance being suggested from the theory employed to date. We seek to remedy this gap in knowledge by designing, building, and directly characterizing the RMF thruster with an eye towards performance trends. Observations of those trends can then help to inspire further insight into the physics behind RMF thruster behavior and eventually provide intuition to optimize this device. Two devices are designed and used in this work, with results from the first directly informing the design of the second. We first employ a first-principles approach based on existing theory for RMF current drive to design for the PEPL RMFv2 thruster. We then make the first ever direct performance measurements for this class of device. While efficiency is found to be low, the RMFv2 thruster’s verified performance allows us to employ probing techniques to understand loss mechanisms and whether the RMF current drive is indeed functioning as intended. Satisfied that current is being produced and having gained insight into how power is inefficiently converted into kinetic energy in the plume, we present the updated PEPL RMFv3 thruster. The enhanced flexibility of this improved test article—including capability to operate in the CW-mode—allows us to explore operating parameter space even further. Our results for this characterization campaign contradict existing RMF current drive theory and inspire a re-examination of current generation scaling laws. Our updated model is compared to experimental data and is found to match quantitatively. This new model presents a much different picture for the ideal operational regime for the RMF thruster. We present throughout this work the first direct performance measurement of an RMF thruster, direct evidence of FRC plasmoid formation, acceleration, and ejection in an RMF thruster, as well as a significant update to the analytic model used for this device which has gone largely unchallenged for over 60 years. Ultimately, we have greatly improved our understanding of the physics behind RMF current generation and plasma acceleration, and have grounded this intuition in experiment. The knowledge produced in this work will help guide future research into the RMF thruster toward realizing the advantages this device has to offer.