Session DR4: Plasma Propulsion II

Data-Driven Estimation of Electrical Facility Effects on Anomalous Electron Transport in Hall Effect Thrusters

Past studies have demonstrated that a global ionization oscillation, referred to as the breathing mode, is one of the dominant oscillations in Hall effect thrusters over a wide range of operating conditions. It has been experimentally observed that plasma properties, including the anomalous electron mobility, and current collected at the conductive walls of the vacuum chamber test facility oscillate periodically in time. To study the coupling that occurs between the anomalous electron mobility and the test facility electrical configuration, a zero-dimensional (0D) global plasma ionization oscillation model of a Hall effect thruster is combined with a circuit model of a vacuum chamber test facility. The ion and neutral number densities, electron temperature, ion and electron velocities, and a minimal set of circuit parameters (i.e., current or voltage) are estimated. An extended Kalman filter (EKF) allows for the estimation of anomalous electron mobility using time-dependent experimental data. The effects of the electrical circuit on the plasma oscillations will be discussed.   

Implementation of a xenon collisional radiative model with neural network for non-invasive determination of plasma parameters in Hall effect thrusters

The increasing number of in-orbit deployment of satellites is driven by the lowered costs brought by the advances in miniaturizing satellites' sub-systems, in particular propulsion systems. Xenon based thrusters are today widely used for their high specific impulse and low-energy consumption. However, estimation of their performance and lifetime is still performed through time-consuming stress campaigns. This work attempts to predict the operating conditions of a mid-power Hall thruster from the plume's emission spectrum, using neural network (NN) techniques. To this end, a xenon collisional radiative (CR) model based on the 5s and the highly radiative 6p states of xenon was developed. Insights from a sensitivity analysis of the xenon CR, using Morris method, backed by experimental observations allowed to identify a list of electron temperature-sensitive neutral lines relevant for diagnostic and machine learning purposes. Using these lines, a neural network model was trained and tested based on the output of the CR. Then the NN was used to make predictions over a new experimental dataset. Preliminary results show the potential of the method for applications that require a fast estimation of the plasma parameters of the thrusters, however, improvements of the CR are still required.   

Radiofrequency plasma thrusters and related studies

The Space Plasma, Power and Propulsion (SP3) laboratory provides a creative and collaborative environment for industry and academe related projects spanning a wide range of topics such as space plasmas (solar corona and exoplanet research), space missions (QB50, mission to Mercury), space propulsion systems (for low earth orbit, geostationary and deep space) as well as focused ion beams (materials characterisation, forensic studies). Over the past decade the development of a range of electrodeless plasma thrusters based on geometric and magnetic plasma nozzles (i.e. Helicon thruster, Pocket Rocket thruster, Naphthalene thruster) has provided a wonderful platform for a better understanding of basic plasma physics with some prototypes near ready for space use. We use similar radiofrequency plasma technologies for our various thruster concepts and for our focused ion beam studies. Thousands of nano and micro-satellites are expected to be launched over the next decade, many in constellations, and rideshare opportunities are increasing. There is interest in developing miniaturised communications filters which may be achieved using the Coaxial Stepped Impedance Resonator topology. The destructive ‘multipactor’ effect of avalanche electrons in such filters can be further assessed using plasma technology. The development of the Pocket Rocket thruster into a laminar nozzle capable of producing high vibrational temperatures for molecular gases is carried out for implementation onto the SMAUG exoplanet research apparatus which produces non-LTE (Local Thermodynamic Equilibrium) spectra of various molecules characterised using cavity ringdown spectroscopy. Expanding nearly collisionless plasmas can be used to investigate out-of-equilibrium thermodynamics via polytropic index studies both in the laboratory and in space. 

    

Effect of electron-neutral collisions on plasma transport enhancement by kinetic instability in a Hall-effect thruster

Hall thrusters are space propulsion systems that have been used in many missions. Nevertheless, the physical phenomenon called the "anomalous electron transport" is not well understood and is still actively studied. In this study, a particle-in-cell method is used to reproduce the plasma in a discharge channel to investigate the plasma transport phenomena. In previous calculations, a wavy structure with the same scale as the Debye length appeared in the plasma. The analysis of the calculation results suggests that the non-thermal velocity distribution of electrons is important to elucidate the transport phenomena by these waves. In this presentation, we will discuss how electron-neutral collisions affect the electron velocity profile and anomalous transport phenomena.   

Investigation of cross-field electron transport in Hall Effect Thrusters using 1D axial PIC/MCC simulation

A one-dimensional (1D) particle-in cell Monte Carlo collision (PIC/MCC) model is developed to investigate kinetic effects of cross-field electron transport in Hall effect thrusters. The inertia terms such as the shear term are evaluated from the non-Maxwellian velocity distribution function (VDFs). While the discharge current obtained from the PIC/MCC model is in good agreement with the five-moment fluid model, the plasma properties in the cross-field discharge are compared. In particular, we discuss the anisotropic pressure tensor that arise from non-Maxwellian VDFs near the anode sheath and in the region where the gradient of  E×B drift exists.   

Facility Effects Associated with Ion Beam Neutralization

Gridded ion thrusters utilize a separate electron source to neutralize the thrust producing positive ion beam. The neutralizer coupling voltage drives electrons to the beam. The coupling voltage is dependent on the relative ease for electrons to reach the beam as well as facilitate the neutralization process therein. The neutralization process however is complex and remains poorly understood. Neutralization processes that take place in ground tests can differ appreciably from those that occur in space owing to the presence of a relatively denser charge exchange plasma and the proximity to ground potential surfaces. Variations in neutralizer-chamber wall voltage coupling, neutral background pressure, and external magnetic fields are important facility effect considerations that must be considered when attempting to replicate space conditions. In this particular study, we characterize electron transport from the neutralizer to three potential loss surfaces—the beam, the chamber walls, and the thruster plasma screen. Through such a study, we aim to better clarify the facility effect impacting neutralizer beam coupling and to pose potential solutions for better mimicking space conditions in ground test facilities.   

Investigation of ion back flow by Hybrid-PIC simulation considering experimental current density distribution at the conductive surface for microwave discharge ion thruster

10 cm class microwave discharge ion thruster has been installed in the asteroid probe Hayabusa2. From the operational history, it was found that the conductive surfaces were sputtered. It is expected that the main cause of sputtering is the ion backflow. As ground-experiment have shown that the ion sputtering is detrimental to cathode, it is crucial to investigate the physical process of the ion back flow. In this work, both experimental and numerical approaches are performed. In numerical simulation, electron is treated as fluid model with quasi-neutral approximated drift-diffusion, and ion is treated as particles, i.e., Hybrid-PIC. To consider the boundary condition with finite current density at the conductive surface, the distribution of current density at the conductive surface is experimentally measured. The combination of experimental and numerical simulation can reproduce the potential profile, and capture kinetic ion dynamics with low computational cost.