Inductive Probe Measurements in an RMF FRC Thruster*

The Rotating Magnetic Field (RMF) thruster is an example of an inductive pulsed plasma thruster which employs a rotating magnetic field to generate an azimuthal current in a plasma. This current then interracts with magnetic fields, including an applied bias field and self-induced fields, via the Lorentz force to produce plasmoid compression and thrust. In an ideal setting, a Field-Reversed Configuration (FRC) plasmoid is formed before the plasma slug is accelerated, which has the potential to reduce wall interractions through its confining magnetic field structure. The RMF thruster is thought to share many common characteristics of other inductive pulsed propulsion devices, including high power density, throttleability, and compatibility with exotic propellants. However, recent performance measurements show poor total efficiency (<1%), prompting investigation into the acceleration mechanism. In this work, we experimentally investigate the induced magnetic field structure to quantify the magnitude of the induced azimuthal current throughout the pulse. We find evidence of high (>2500 A) driven current and plasmoid formation. We also investigate the impact of flux conserving surfaces on the magnetic field structure and quantify their effect on the Lorentz force on the plasma slug.