Session DT3: Electric Propulsion II

Review of SPT and TAL type Hall thrusters’ performance and operation through database analysis

Hall effect thrusters are widely adopted as advanced electric spacecraft propulsion systems and play a crucial role in satellite operation today. The analysis of the collated performance data along with thruster dimensions provides insight into the overall trends and characteristics of Hall thrusters over the past four decades.

 

In this study, we compare two prominent variants of Hall thrusters: the stationary plasma thrusters (SPT),  and the thruster with anode layer (TAL). We utilize a comprehensive database of >1800 data points for approximately 50 Hall thrusters from published data to discuss the operation and geometry of these two variants.

 

The analysis of the database allows for a detailed exploration of these design differences. Our goal is to facilitate discussion and research on the influence of thruster type and geometry on performance. In addition, TAL variants are also investigated in comparison to SPT to better understand how the underlying operation of these thrusters vary.

In conclusion, the database analysis of SPT and TAL variant Hall thrusters provides valuable insights into their operation and enhances our understanding of their performance and design characteristics. This analysis enables a deeper exploration into how thruster variants, geometry, propellant, channel dimensions and propellant flow rates effect performance which will have significant impact in future scaling considerations for Hall thrusters.

Analysis of the Breathing Mode dynamics in Hall thrusters using a hybrid simulation

The Breathing Mode (BM) is a macroscopic oscillation of the plasma in Hall Thrusters (HTs). In recent years, several studies have tried to explain theoretically and numerically the origin and development of this instability. Yet, some efforts are still required to fully characterize it. In the current work, we use a hybrid code, in which the neutral dynamics is treated with 1D Euler equations, while the dynamics of the charged species (electrons and single-charged positive ions) is reproduced by a 2.5D Particle-in-Cell (PIC) module. The ionization is self-consistently calculated within the Monte-Carlo collision (MCC) module. Thus, the consequent coupling of the neutral species dynamics with the charged species allows for replicating the BM mechanism. The data provided by the hybrid simulation allows us to analyze the microscopic processes that originate the BM and the relation between the variation of the various parameters. In particular, we analyze which is the relationship of the ionization with the charge and neutral densities and electron temperature variation at different stages of the BM cycle. From our results, it emerges that the temperature increment alone is not capable of sustaining the ionization. The particle creation appears to be dependent on the variation of both the gas and the plasma densities, along with the ionization rate coefficient. The hybrid code results allow us also to study the evolution of some quantities at different times of the BM cycle. For example, by analyzing the parameters fluctuations during a BM, we show that the diamagnetic drift is always lower than the ExB one and that the isothermal approximation is not valid in HT conditions. Moreover, we show that the effect of the anomalous transport is particularly strong during the BM growing phase, while it is much less marked when the current is low.   

Data-Driven Estimation of Background Pressure Effect on Thrust in Hall Effect Thrusters

Previous studies showed a correlation between the measurement (e.g., thrust and discharge current) of Hall Effect Thrusters (HET) and the background pressure in testing facilities. Generally, as the background pressure decreases, the thrust generated by the HET also decreases. However, since the experimental conditions within these testing facilities cannot perfectly replicate the conditions in space, we typically rely on extrapolating the facility data to a zero-background pressure to estimate the thruster performance in space. In contrast to current approaches that directly extrapolate experimental thrust values obtained from testing facilities, this research focuses on examining the time-dependent electron mobility. A zero-dimensional (0D) global plasma model of a HET discharge plasma is used to estimate the electron mobility over time at different background pressures. The average value, oscillation amplitude, and frequency of the electron mobility are then extrapolated to zero pressure. This extrapolation allows for the estimation of the thrust and discharge current oscillations under in-space vacuum conditions.   

A Quasi-Neutral Full PIC Model for the Study of Electron Thermodynamics in a Plasma Thruster Plume

Classical electron fluid formulations for the simulation of electric propulsion plumes employ the simplification of a diagonal and isotropic temperature tensor. However, experimental and kinetic numerical data point to the presence of relevant temperature anisotropies. To overcome this limitation, a 2D 3V full PIC model has been developed, in which ion and electron particles are moved in a potential map obtained by assuming quasi-neutrality and solving the inertialess electron momentum and continuity equations. To account for an anisotropic temperature tensor, a revisited definition of the thermalized potential is used, permitting a reduced noise in the solution for the potential. The quasi-neutrality assumption allows to overcome the constraint of the discretization of the Debye length and plasma frequency typical of classical full PIC codes, largely reducing the otherwise prohibitive computational costs of full-scale plume simulations. The model is applied to the case of a plasma thruster plume expanding into vacuum; and is used to evaluate the accuracy and limitations of an anisotropic polytropic closure. The simulation results show that an anisotropic formulation of the polytropic closure is fundamental to capture the evolution of the electron temperature tensor in a plume; and optimal values of the global polytropic coefficients are obtained through the comparison with the kinetic electron population.   

Development and Validation of a Collisional Radiative Model for Neutral Xenon in a Hall Effect Thruster

A neutral Xe collisional radiative model based on the 6s and 6p levels was developed and validated against other models available in the literature and Langmuir probe measurements.

The model solves the non-linear steady-state balance equations of the 6s and 6p levels of Xe considering electron impact excitation, spontaneous emission as well as absorption based on Mewe approximation. Synthetic spectra are generated from the calculated densities of the excited species at steady state, which are then compared with experimental spectra to estimate the plasma parameters, with an emphasis on the electron temperature and electron density.

Optical emission measurements were conducted on a mid-power Hall effect thruster for different voltages at the anode and different Xe injection flows, allowing to scan of the plume along three axes: axial, lateral, and radial. An optimization strategy based on 15 neutral lines was used to predict the electron temperature from the collected spectra.

The developed model successfully reproduced the expected trends in electron temperature and electron density. Comparison between the LP and optical emission spectroscopy measurements demonstrated a high level of agreement between the two methods. Additionally, a comparison between temperature profiles obtained from Particle-in-Cell simulations and the predicted results yielded a favorable agreement in terms of trends and order of magnitude.   

Validation of an adjustable sensitivity inverted pendulum thrust stand with a novel external plasma thruster

The design and testing of a variable sensitivity inverted pendulum thrust stand (VST) has been undertaken at the University of Southampton. The newly designed thrust balance, in a displacing configuration, has been validated via the measurement of thrust levels with a novel External Discharge Plasma Thruster (XPT).  Previously, the External Discharge Plasma Thruster (XPT) has been proposed to solve the lifetime and low efficiency problems of low power Hall thrusters. The operational power is below 500W with 25% maximum recorded anode efficiency and the thrust ranges from 2 to 17 mN. It has the potential to extend the lifetime of Hall thrusters and reduce failure rates, as well as decrease thruster head mass by one order of magnitude. The performance characterisations conducted using the VST were replicated against a torsional thrust balance developed by Added Value Solutions (AVS) UK Ltd and the Surrey Space Centre at the University of Surrey vacuum facilities to validate the measurements taken . The new thrust stand has shown to be calibratable to a thrust resolution of 11 µN with a calibration error of <3 % and has currently been tested with simulated thrusts up to 16 mN. However, testing with the XPT is expects to subject the stand to a thrust range of 0.2 – 20 mN. Calibration and test data will be presented and a discussion on the accuracy and validation of this high accuracy thrust stand will be given.