Session MW4: Electric Propulsion II

Live

A magnetic nozzle rf plasma thruster for space propulsion includes many aspects of physics [1]. The concept is very simple; the high density plasma produced in the source is transported along the magnetic field lines toward the source exit, is expanded along the magnetic nozzle, and has to be detached from the magnetic nozzle. This talk will review our laboratory experiments on the thruster physics. In the source, a portion of the charged particles are lost to the radial wall, where the ions accelerated by the sheath transfer their radial momentum. Measurement of the axial force to the radial wall shows that non-negligible axial momentum is simultaneously transferred to the wall, where the ions are axially accelerated in the core plasma and are lost to the wall [2]. The plasma entering the magnetic nozzle are spontaneously accelerated during the expansion process. The direct force measurement to the magnetic field demonstrates that a Lorentz force arising from the azimuthal internal plasma current and the radial magnetic field can increase the thrust [3]. More downstream, magnetic field lines are observed to be stretched [4] when the plasma flow velocity exceeds 0.2VA, where VA is the Alfven velocity. In the series of the plasma expansion physics, the electron internal energy would be the energy and momentum sources. The thermodynamic behavior of the electrons [5] is also discussed via a measurement of electron energy probability functions. [1] K. Takahashi \textit{et al.}, Rev. Mod. Plasma Phys., \textbf{3}, 3 (2019). [2] K. Takahashi \textit{et al.}, Phys. Rev. Lett., \textbf{114}, 195001 (2015). [3] K. Takahashi \textit{et al.}, Phys. Rev. Lett., \textbf{110}, 195003 (2013). [4] K. Takahashi \textit{et al.}, Phys. Rev. Lett., \textbf{118}, 225002 (2017). [5] K. Takahashi \textit{et al.}, Phys. Rev. Lett., \textbf{120}, 045001 (2018).   

Live

Small satellite missions often require on-board propulsion for orbital insertion, station keeping, and deorbit. While many technologies are under development to address this need, none has emerged as a definitive solution. Magnetic nozzle thrusters are an ideal architecture for small satellite propulsion, as they offer operational simplicity, durability, and flexibility in terms of propellant, input power, and thrust level. These thrusters operate by using RF or microwave power to ionize, heat, and expel propellant through an expanding magnetic field. Despite their promise, testing has shown very low efficiencies at powers under 100 W. The most promising of these devices have used ECR to reach a thrust efficiencies over 10{\%}. Drawing from this work, our experiment seeks to improve performance by using custom input waveforms to heat the plasma. Specifically, we mix and pulse two microwave signals, with frequencies from 800 to 2500 MHz and combined power under 40W, and measure thrust using a hanging pendulum thrust stand. The thrust values for each test point are fed into a global optimization algorithm, which automatically selects new test points. The results of this experiment will be presented and discussed in detail in this work.   

Live

The design and optimization of an EHD thruster needs a good knowledge of the flow pattern imposed by the electrode (and its nature) geometry and the morphology of the electric potential. The aim of this work is to optimize a previously developed self-consistent model of single-stage electrohydrodynamic (EHD) thrusters for space applications [1]. The EHD thruster structural components are a needle-type anode and cylindrical cathode. The propellant gas is argon, at a pressure of 10Torr and a temperature of 300K. It was investigated the sliding effect of the electric field in a dielectric surface by introducing a chimney shape dielectric beneath our cathode. With the increase of the aperture angle, the morphology of the electric potential and field lines inside the cathode are changed and the species distributions vary as well. In these circumstances, there is an increase of the net thrust. We also study some aspects of the cylindrical intrinsic geometry considered for the cathode under the action of a voltage range 3-20 kV: inner radius and cylindrical height. Finally, the influence of the secondary electron emission coefficient was also analyzed. We found that in the maximized cases, Ar develops a thrust of 2.75 $\mu $N and a thrust-to-power of 295 mN/kW.   

Live

An equivalent circuit model is presented for the formation and propagation of a plasmoid in a rotating magnetic field field-reversed configuration thruster. The system consists of two rotating magnetic field (RMF) antennae that drive the azimuthal plasma current, an external coil that produces the necessary radial magnetic field, and the plasma itself. The plasma is treated as a conducting slug that translates downstream of the thruster due to an axial Lorentz force caused by the azimuthal plasma current and the external magnetic field. The mutual inductance terms vary with time as the slug changes position and are calculated by modeling the plasmoid in COMSOL. The plasmoid is moved to different axial positions along the centerline of the thruster to calculate and derive empirical expressions for the mutual inductances. The system of equations is solved numerically to calculate the impulse and efficiency of the thruster. The numerical model is compared to experimental performance data measured using the University of Michigan RMF thruster. The validity of the model and the observed trends of the results are discussed in the context of thruster performance.   

On Demand

We have conducted two-dimensional particle-in-cell simulations with Monte Carlo collisions (PIC-MCC) for a magnetic nozzle plasma thruster, where the simulation area consists of both the source tube and the downstream region with a convergent-divergent magnetic nozzle. The simulation results clearly show the axial Lorentz force exerted to the plasma on the basis of the distributions of the internal plasma current due to a diamagnetic effect in the downstream region, which is qualitatively consistent with previous experiments. In addition, distributions of the momentum gain due to the electrostatic and electromagnetic forces have been obtained by counting the momentum increment of each particle. As a result, it is shown that both the ions and electrons obtain their axial momentum. The relationship between the momentum gain of plasmas and electrostatic and electromagnetic forces is discussed   

Experimental Results for Rotating Magnetic Field -- Driven Thruster with Constant Amplitude Field

The experimental setup and results are presented for the testing of a rotating magnetic field (RMF) -- driven thruster. Two antennae are pulsed out of phase at RF frequencies using a power processing unit developed in conjunction with Eagle Harbor Technologies. This supply maintains constant current amplitude throughout the pulse length by driving the circuit at resonance. This pulsing generates an RMF which entrains electrons in the plasma, inducing an azimuthal current. This current interacts via the Lorentz force with a steady radial magnetic field to produce thrust in the axial direction. Probe data is used to measure plasma temperature and ion energies to determine thermal losses in the device to inform a phenomenological efficiency model. These efficiencies, along with direct thrust data and current waveforms in the circuit, are analyzed and compared to previous testing which used a ringdown to pulse the RMF. From comparison, the minimum current amplitude for the RMF to fully penetrate the plasma can be determined. These results are discussed in the context of future design changes which could improve this thruster's performance.