Small-scale Instability Driven Electron Transport in Hall Thrusters

Abstract

There is an increasing demand for efficient electric propulsion technologies for orbital station keeping and deep-space missions. Hall effect thrusters, as a leading form of electric propulsion, exhibit superior propellant efficiency through high specific impulse values, resulting in significant reductions in mission costs and propellant mass. This has led to their extensive utilization for spacecraft applications, including orbit-raising maneuvers, attitude control, and interplanetary propulsion. However, despite their widespread use the underlying physics governing the operation of Hall thrusters are not fully understood. Due to the lack of understanding of key physical processes, currently Hall thrusters cannot be readily simulated, and the development of new systems heavily relies on costly and time-consuming experimental testing.

Our research aims to delve into the first principles of Hall thruster physics and address several deficiencies in our understanding that impede predictive simulations. Notably, the transport of electrons across the magnetic field lines of Hall thrusters is orders of magnitude greater than predicted by simple fluid models. The Hall thruster modeling community has recently reached a consensus that plasma turbulence is the most likely cause of this anomalous cross-field transport. In this work, we experimentally validate the role of electron drift instability (EDI) in electron transport within Hall thrusters. Despite widespread consensus on its significance, this topic has predominantly relied on numerical simulations with limited experimental validation. These simulations exhibit notable disparities concerning the formation of the EDI, relevant oscillation frequencies and wavelengths, and the extent of the resulting electron transport. Such uncertainties impede the development of precise and universally applicable low-fidelity models that accurately represent electron transport. To address these ambiguities, we employ experimental methodologies, including the direct measurement of EDI using electrostatic probes inserted into a Hall thruster. These probes measure high-speed plasma density oscillations, and subsequent spectral analyses of these measurements offer insights into the dispersion relation of the EDI, its growth and saturation patterns, and the level of induced electron transport. Our measurements identified the presence of plasma waves characteristic of the electron drift instability. Furthermore, through bispectral analysis, an inverse energy cascade was identified whereby the EDI initially grows following its linear dispersion relation at discrete resonance frequencies. Subsequently, the resonances couple together, transferring energy from high frequency and small wavelength to low frequency and long wavelength. This energy cascade occurs as the waves propagate downstream of the Hall thruster, where eventually most of the wave energy belongs to the long-wavelength component. These experimental findings serve as validation for several simulation and modelling effort the first proposed these mechanisms. Moreover, we utilized these measurements of plasma wave properties to calculate the wave-driven anomalous cross-field transport and validated these calculations through laser-based measurement of the true cross-field transport levels. This provides the first experimental proof that the EDI is the mechanism controlling electron transport in Hall thruster plasma plumes. Overall, this investigation enhances the understanding of the EDI's characteristics, advances electron transport models, and brings the field one step closer to predictive Hall thruster modeling.