Publications
88 results found
Munoz Tejeda JM, Potrivitu G, Rosati Azevedo E, et al., 2024, Experimental demonstration of a water electrolysis Hall Effect Thruster (WET-HET) operating with a hydrogen cathode, Acta Astronautica, ISSN: 0094-5765
Faraji F, Reza M, Knoll A, et al., 2024, Dynamic mode decomposition for data-driven analysis and reduced-order modeling of E × B plasmas: II. Dynamics forecasting, Journal of Physics D: Applied Physics, Vol: 57, ISSN: 0022-3727
In part I of the article, we demonstrated that a variant of the dynamic mode decomposition (DMD) algorithm based on variable projection optimization, called optimized DMD (OPT-DMD), enables a robust identification of the dominant spatiotemporally coherent modes underlying the data across various test cases representing different physical parameters in an E × B simulation configuration. We emphasized that the OPT-DMD significantly improves the analysis of complex plasma processes, revealing information that cannot be derived using conventionally employed analyses such as the fast Fourier transform. As the OPT-DMD can be constrained to produce stable reduced-order models (ROMs) by construction, in this paper, we extend the application of the OPT-DMD and investigate the capabilities of the linear ROM from this algorithm toward forecasting in time of the plasma dynamics in configurations representative of the radial-azimuthal and axial-azimuthal cross-sections of a Hall thruster and over a range of simulation parameters in each test case. The predictive capacity of the OPT-DMD ROM is assessed primarily in terms of short-term dynamics forecast or, in other words, for large ratios of training-to-test data. However, the utility of the ROM for long-term dynamics forecasting is also presented for an example case in the radial-azimuthal configuration. The model's predictive performance is heterogeneous across various test cases. Nonetheless, a remarkable predictiveness is observed in the test cases that do not exhibit highly transient behaviors. Moreover, in all investigated cases, the error between the ground-truth and the reconstructed data from the OPT-DMD ROM remains bounded over time within both the training and the test window. As a result, despite its limitation in terms of generalized applicability to all plasma conditions, the OPT-DMD is proven as a reliable method to develop low computational cost and highly predictive data-driven ROMs in systems with a quasi-
Faraji F, Reza M, Knoll A, et al., 2024, Dynamic mode decomposition for data-driven analysis and reduced-order modeling of E × B plasmas: I. Extraction of spatiotemporally coherent patterns, Journal of Physics D: Applied Physics, Vol: 57, ISSN: 0022-3727
The advent of data-driven/machine-learning based methods and the increase in data available from high-fidelity simulations and experiments has opened new pathways toward realizing reduced-order models for plasma systems that can aid in explaining the complex, multi-dimensional phenomena and enable forecasting and prediction of the systems' behavior. In this two-part article, we evaluate the utility and the generalizability of the dynamic mode decomposition (DMD) algorithm for data-driven analysis and reduced-order modeling of plasma dynamics in cross-field E × B configurations. The DMD algorithm is an interpretable data-driven method that finds a best-fit linear model describing the time evolution of spatiotemporally coherent structures (patterns) in data. We have applied the DMD to extensive high-fidelity datasets generated using a particle-in-cell (PIC) code based on the cost-efficient reduced-order PIC scheme. In this part, we first provide an overview of the concept of DMD and its underpinning proper orthogonal and singular value decomposition methods. Two of the main DMD variants are next introduced. We then present and discuss the results of the DMD application in terms of the identification and extraction of the dominant spatiotemporal modes from high-fidelity data over a range of simulation conditions. We demonstrate that the DMD variant based on variable projection optimization (OPT-DMD) outperforms the basic DMD method in identification of the modes underlying the data, leading to notably more reliable reconstruction of the ground-truth. Furthermore, we show in multiple test cases that the discrete frequency spectrum of OPT-DMD-extracted modes is consistent with the temporal spectrum from the fast Fourier transform of the data. This observation implies that the OPT-DMD augments the conventional spectral analyses by being able to uniquely reveal the spatial structure of the dominant modes in the frequency spectra, thus, yielding more accessible, compr
Reza M, Faraji F, Knoll A, 2024, Reduced-order particle-in-cell scheme: redefining kinetic modelling for electric plasma propulsion, AIAA SCITECH 2024 Forum, Publisher: AIAA
Particle-in-cell (PIC) simulations are powerful tools for detailed, high-fidelity study of the plasma behavior in electric propulsion technologies such as Hall thrusters, in which the multiscale, nonlinear, and kinetic processes exist that not only locally affect the plasma, but can also lead to the formation of large-scale, coherent, global structures and oscillations. However, due to the significant computational cost of traditional PIC codes without any speed-up techniques and/or physics-altering parameter scaling, they are most often applied over a limited set of conditions and/or simulation parameters. Indeed, traditional PIC codes cannot be deemed as tools suitable for extensive parametric studies within practical timeframes. The reduced-order PIC scheme, the cost-efficiency and accuracy of which have been verified in prior publications, changes the current notion regarding the applicability and the use cases of a PIC code toward investigating the science behind plasma propulsion. Through this article, we demonstrate the capability of the reduced-order PIC to enable investigating the underlying plasma phenomena over a broad parameter space that is of significance from both a fundamental and an applied perspective. The parametric simulations assess the influence that the magnitude of the applied electromagnetic field and the curvature of the magnetic field can have on the plasma behavior in a radial-azimuthal configuration representative of a Hall thruster when using different propellants - xenon, krypton, and argon. The results show that the studied factors induce notable variations in the spectra of the azimuthal instabilities. These variations consequently affect the macroscopic distribution of the plasma properties, the electrons’ mobility, and the divergence of the ion beam. The derived unique insights majorly advance our understanding of the physics of Hall-thruster-like cross-field plasma discharges.
Reza M, Faraji F, Knoll A, 2024, Latest verifications of the reduced-order particle-in-cell scheme: Penning discharge and axial-radial Hall thruster case, AIAA SCITECH 2024 Forum, Publisher: AIAA
Reduced-order particle-in-cell (PIC) scheme is a novel kinetic modeling method for plasma systems that has been developed at Imperial Plasma Propulsion Laboratory. Since its conception, the scheme has become progressively more mature and has undergone a rigorous step-by-step verification to ensure its generalizability, accuracy, and reliability. In this article, we expand our verification efforts to further ascertain the applicability and versatility of the reduced-order PIC and its underpinning dimensionality-reduction approach in new simulation configurations and test cases. We first present the verification results against a recent Penning discharge benchmark case. This test case assesses the ability of the reduced-order PIC to resolve the underlying physics in a setup where the principal direction of the involved phenomena is constantly changing with respect to the coordinate system. Second, we present results from reduced-order quasi-2D (Q2D) axial-radial Hall thruster simulations with various approximation orders of the 2D problem. These results complement the verifications carried out so far in the axial-azimuthal and radial-azimuthal Hall thruster coordinates. As a precursor to the Q2D code verification in the axial-radial coordinates, we demonstrate that the methodology underlying the reduced-order PIC can also enable reduced-order Direct Simulation Monte Carlo studies for the neutral gas dynamics. This is crucial because: (1) a kinetic-DSMC neutral dynamics module is an essential component of Hall thruster PIC simulations in axial-radial geometries and, (2) this module needs to be compatible with the reduced-order definition of the PIC simulation itself. The latest verifications and tests reported in this work demonstrated that the reduced-order PIC and its enabling underlying dimensionality-reduction technique are fully versatile and generalizable. This paves the way toward reliably extending the method to 3D to achieve a quasi-3D PIC code that realizes c
Faraji F, Reza M, Knoll A, 2024, Machine-learning-enabled plasma modeling and prediction, AIAA SCITECH 2024 Forum, Publisher: American Institute of Aeronautics and Astronautics
In line with significant rise over the last decade in the body of data available from high-fidelity simulations and experiments, there has been an increasing interest across fields of science and engineering to employ data-driven methods and machine learning to develop interpretable and generalizable reduced-order models (ROMs) that can aid in explaining the observed complex, multidimensional phenomena and/or to enable prediction and forecasting of the systems’ behavior. Despite major advances in data-driven (DD)/machine-learning (ML) algorithms for physics modeling and dynamics discovery in the past years and the promising applications demonstrated across multiple scientific domains, particularly in fluid dynamics, the data-driven methods have not yet found a rigorous, widespread application for plasma physics, especially within the low-temperature technological plasmas communities. In this article, we aim to demonstrate the potential of two highly promising data-driven algorithms for plasma state forecasting and to report on the other research directions being pursued at Imperial Plasma Propulsion Laboratory (IPPL) on the broader subject of machine-learning-enabled/enhanced plasma modeling. First, we present results from linear-time-dynamics ROMs obtained from the Optimized Dynamic Mode Decomposition (OPT-DMD) method toward predicting plasma properties’ evolution in two test cases, one representing a radial-azimuthal cross-field discharge similar to that of a Hall thruster, and the other one corresponding to a Penning discharge configuration. Second, we will introduce a novel in-house developed approach, named the “Phi Method”, and demonstrate its capability for simultaneous forecasting of a coupled system of plasma state variables in a 1D azimuthal problem. Third and last, we provide an overview of the ongoing efforts on the “Sparse Identification of Nonlinear Dynamics” (SINDy) algorithm to develop reduced-order PDE/ODEs for pl
Reza M, Faraji F, Knoll A, 2023, Influence of the magnetic field curvature on the radial–azimuthal dynamics of a Hall thruster plasma discharge with different propellants, Journal of Applied Physics, Vol: 134, ISSN: 0021-8979
The topology of the applied magnetic field is an important design aspect of Hall thrusters. For modern Hall thrusters, the magnetic field topology most often features curved lines with a concave (negative) curvature upstream of the field’s peak and a convex (positive) curvature downstream. Additionally, the advent of the magnetic shielding technique has resulted in Hall thruster designs with non-conventional field topologies that exhibit high degrees of concavity upstream of the field’s peak. In this article, a detailed study is carried out on the effects that the magnetic field curvature has on the plasma discharge in a 2D configuration representative of a Hall thruster’s radial-azimuthal cross-section. The analyses are performed for discharges of three propellants of high applied interest: xenon, krypton, and argon. For each propellant, high-fidelity electrostatic reduced-order particle-in-cell (PIC) simulations are performed with various degrees of positive and negative curvatures of the magnetic field. Corresponding 1D radial PIC simulations are also performed for xenon to compare the observations against the 2D results. Most notably, it is observed that the instability spectra in the positive-curvature cases are mostly dominated by the Electron Cyclotron Drift Instability, whereas the Modified Two Stream Instability is dominant in the negative-curvature cases. The distributions of the electron and ion temperatures, in particular, as well as the contribution of various mechanisms to electrons’ cross-field transport shows notable variations between the positive and negative curvature values. Finally, the field curvature is observed to majorly influence the ion beam divergence along the radial and azimuthal coordinates.
Munoz Tejeda JM, Knoll A, 2023, A water vapour fuelled Hall Effect Thruster: characterization and comparison with oxygen, Acta Astronautica, Vol: 211, Pages: 702-715, ISSN: 0094-5765
A Hall Effect Thruster propelled by water vapour is investigated at the Imperial Plasma Propulsion Laboratory. For that purpose,a water vapour feed system is designed, optimised and tested, with the major objective of keeping water in vapour state at all times.This system primarily consists of a mass flow controller, a flow restrictor, and a heating and pressure monitor system capable ofidentifying under which conditions water condensation occurs. A hanging pendulum thrust balance is used to measure the thruston the power range of Pd = 600 − 1600 W. Different magnetic field strengths and mass flows are investigated to determine theconditions in which the highest efficiency can be achieved. Then, a comparison between water vapour and oxygen (intended tobe the propellant of a water electrolysis Hall Effect Thruster) is included. The results show that oxygen is approximately 20 %more efficient than water vapour under the same operating conditions. Overall, the highest thrust measurement recorded with watervapour was 20.0 ± 0.2 mN; with a specific impulse of 2039 ± 20 s and an anode efficiency of 12.5 ± 0.3 % at the largest dischargepower of investigation (Pd = 1600 ± 1 W).
Rose B, Knoll A, 2023, Manipulating plasma turbulence in cross-field plasma sources using unsteady electrostatic forcing, Journal of Physics D: Applied Physics, Vol: 56, Pages: 1-14, ISSN: 0022-3727
Unsteady electrostatic forcing is investigated as a method for manipulating turbulent plasma behaviour within Hall effect thrusters and similar cross-field plasma devices using a simplified 1D-3V azimuthal electrostatic particle-in-cell simulation. A wide range of axial electric field forcing frequencies from 1 MHz up to 10 GHz at amplitudes of 10 V/cm, 50 V/cm and 100 V/cm are applied to the plasma and the response is evaluated against a baseline case defined by community benchmark LANDMARK Test Case 1. 'Tailoring' of plasma parameters such as the electron cross-field mobility is demonstrated via manipulation of the electron drift instability using unsteady forcing. Excitation of the unstable electron cyclotron modes of the electron drift instability is shown to be able to produce a reduction of the resultant electron cross-field mobility of the plasma by up to 50% compared to the baseline value. Additionally, forcing at the electron cyclotron frequency appears to be capable of increasing cross-field mobility by up to 2000%. Implications of the results for direct drive electric propulsion systems and improved current utilisation efficiencies for Hall effect thrusters are discussed.
Faraji F, Reza M, Knoll A, 2023, Effects of the neutral dynamics model on the particle-in-cell simulations of a Hall thruster plasma discharge, Journal of Applied Physics, Vol: 133, Pages: 1-26, ISSN: 0021-8979
The dynamics of the neutral atoms in Hall thrusters affects several plasma processes, from ionization to electrons' mobility. In the context of Hall thruster's particle-in-cell (PIC) modeling, the neutrals are often treated kinetically, similar to the plasma species, and their interactions with themselves and the ions are resolved using the direct-simulation Monte–Carlo (DSMC) algorithm. However, the DSMC approach is computationally resource demanding. Therefore, modeling the neutrals as a 1D fluid has been also pursued in simulations that do not involve the radial coordinate and, hence, do not resolve the neutrals' radial expansion. In this article, we present an extensive study on the sensitivity of the PIC simulations of Hall thruster discharge to the model used for the neutral dynamics. We carried out 1D axial PIC simulations with various fluid and kinetic models of the neutrals as well as self-consistent quasi-2D axial-azimuthal simulations with different neutrals’ fluid descriptions. Our results show that the predictions of the simulations in either 1D or 2D configurations are highly sensitive to the neutrals' model, and that different treatments of the neutrals change the spatiotemporal evolution of the discharge. Moreover, we observed that considering the ion-neutral collisions causes a significant variation in the neutral temperature, thus requiring that the neutrals' energy equation to be included as well in their fluid system of equations. Finally, the self-consistent axial-azimuthal simulations highlighted that a neutrals’ model based on the continuity conservation equation only is not an appropriate choice and leads to physically unexpected high-frequency global discharge oscillations.
Reza M, Faraji F, Knoll A, et al., 2023, Reduced-order particle-in-cell simulations of a high-power magnetically shielded Hall thruster, Plasma Sources Science and Technology, Vol: 32, Pages: 1-24, ISSN: 0963-0252
High-power magnetically shielded Hall thrusters have emerged in recent years to meet the needs of the next-generation on-orbit servicing and exploration missions. Even though a few such thrusters are currently undergoing their late-stage development and qualification campaigns, many unanswered questions yet exist concerning the behavior and evolution of the plasma in these large-size thrusters that feature an unconventional magnetic field topology. Noting the complex, multi-dimensional nature of plasma processes in Hall thrusters, high-fidelity particle-in-cell (PIC) simulations are optimal tools to study the intricate plasma behavior. Nonetheless, the significant computational cost of traditional multi-dimensional PIC schemes renders simulating the high-power thrusters without any physics-altering speed-up factors unfeasible. The novel reduced-order “quasi-2D” PIC scheme enables a significant reduction in the computational cost requirement of the PIC simulations. Thus, in this article, we demonstrate the applicability of the reduced-order PIC for a cost-efficient, self-consistent study of the physics in high-power Hall thrusters by performing simulations of a 20 kW-class magnetically shielded Hall thruster along the axial-azimuthal and radial-azimuthal coordinates. The axial-azimuthal quasi-2D simulations are performed for three operating conditions in a rather simplified representation of the thruster’s inherently 3D configuration. Nevertheless, we have resolved self-consistently an unprecedented 650 µs of the discharge evolution without any ad-hoc electron mobility model, capturing several breathing cycles and approximating the experimental performance parameters with an accuracy of 70 to 80 % across the operating conditions. The radial-azimuthal simulations, carried out at three cross-sections corresponding to different axial locations within the discharge channel, have casted further light on the evolution of the azimuthal instabilities
Reza M, Faraji F, Knoll A, 2023, Parametric investigation of azimuthal instabilities and electron transport in a radial-azimuthal E×B plasma configuration, Journal of Applied Physics, Vol: 133, Pages: 1-25, ISSN: 0021-8979
Partially magnetized low-temperature plasmas (LTP) in an E×B configuration, where the applied magnetic field is perpendicular to the self-consistent electric field, have become increasingly relevant in industrial applications. Hall thrusters, a type of electrostatic plasma propulsion, are one of the main LTP technologies whose advancement is hindered by the not-fully-understood underlying physics of operation, particularly, with respect to the plasma instabilities and the associated electron cross-field transport. The development of Hall thrusters with unconventional magnetic field topologies has imposed further questions regarding the instabilities’ characteristics and the electrons’ dynamics in these modern cross-field configurations. Accordingly, we present in this effort a detailed parametric study of the influence of three factors on the plasma processes in the radial-azimuthal coordinates of a Hall thruster, namely, the magnetic field gradient, Secondary Electron Emission, and plasma number density. The studies are carried out using the reduced-order particle-in-cell (PIC) code developed by the authors. The setup of the radial-azimuthal simulations largely follows a well-defined benchmark case from the literature in which the magnetic field is oriented along the radius and a constant axial electric field is applied perpendicular to the simulation plane. The salient finding from our investigations is that, in the studied cases corresponding to elevated plasma densities, a long-wavelength azimuthal mode with the frequency of about 1 MHz is developed. Moreover, in the presence of strong magnetic field gradients, this mode results from an inverse energy cascade and induces a significant electron cross-field transport as well as a notable heating of the ions.
Reza M, Faraji F, Knoll A, 2023, Concept of the generalized reduced-order particle-in-cell scheme and verification in an axial-azimuthal Hall thruster configuration, Journal of Physics D: Applied Physics, Vol: 56, Pages: 1-18, ISSN: 0022-3727
Reduced-order particle-in-cell (PIC) scheme is a novel modeling approach that enables computationally efficient electrostatic kinetic simulations of plasma. In our previous publications, we demonstrated that a proof-of-concept implementation of this novel PIC scheme resolves the multi-dimensional plasma processes and their interactions in a Hall thruster in a manner close to traditional electrostatic PIC codes. In this work, we extend our efforts on this topic and present a mathematically mature formulation for the dimensionality reduction of Poisson's equation in the Vlasov-Poisson system, which enables the generalized reduced-order "quasi-multi-dimensional" PIC scheme. The applicability of the dimensionality-reduction approach to solve general 2D Poisson problems is numerically verified. Next, we present several reduced-order quasi-2D simulations of a well-defined axial-azimuthal simulation case from the literature using approximation orders of the 2D problem whose computational costs are 2-15 % of a full-2D simulation. It is shown that these reduced-order simulations allow us to recover the same characteristics, behaviors and effects reported in the literature regarding the azimuthal instabilities in Hall thrusters. Moreover, in terms of the time-averaged plasma properties, it was found that, when increasing the approximation order, the error associated with the quasi-2D simulations' predictions decreases from 15 to 4 % for the electric field and from 20 to 2 % for the ion number density. We have additionally discussed a series of sensitivity analysis results, including the influence of the initial number of macroparticles per cell on the predictions of the quasi-2D simulations. According to the detailed results and analyses presented, we conclude that the generalized reduced-order PIC scheme serves as a rigorous foundation for eventual cost-effective and comprehensive three-dimensional kinetic studies of the physics in Hall thrusters and similar electr
Tejeda J, Schwertheim A, Knoll A, 2023, Water as an environmentally friendly propellant for a multi-functional spacecraft architecture, International Journal of Energetic Materials and Chemical Propulsion, Vol: 22, Pages: 21-33, ISSN: 2150-766X
Water can be utilized as spacecraft propellant to dramatically reduce the environmental impact of constructing and operating a satellite. In this work, a multi-mode chemical-electrical propulsion system, in which water was used as the propellant in both high thrust chemical and high specific impulse electrical maneuvres, was studied. This type of system allows the spacecraft architecture community to divest from traditional propellants such as hydrazine and xenon, thus reducing the production of highly toxic chemicals and dramatically reducing the carbon footprint of propulsion systems. Water has the lowest toxicity, carbon footprint, and price of any current or proposed propellant, and has been shown in laboratory testing to be a feasible alternative compared to traditionally used propellants. The unique role it can play across multiple spacecraft subsystems suggests that the commercial adoption of water as a propellant will reduce cost and mass while also reducing the environmental impact of the satellites of tomorrow. This technology has the ability to enable the development of modular, multifunctional, competitive, and environmentally friendly spacecraft architectures.
Faraji F, Reza M, Knoll A, 2023, Verification of the generalized reduced-order particle-in-cell scheme in a radial-azimuthal E×B plasma configuration, AIP Advances, Vol: 13, ISSN: 2158-3226
In this article, we present an in-depth verification of the generalized electrostatic reduced-order particle-in-cell (PIC) scheme in a cross electric and magnetic field configuration representative of a radial-azimuthal section of a Hall thruster. The setup of the simulations follows a well-established benchmark case. The main purpose of this effort is to demonstrate that our novel PIC scheme can reliably resolve the complex two-dimensional dynamics and interactions of the plasma instabilities in the radial-azimuthal coordinates of a Hall thruster at a fraction of the computational cost compared to full-2D PIC codes. To this end, we first present the benchmarking of our newly developed full-2D PIC code. Next, we provide an overview of the reduced-order PIC scheme and the resulting “quasi-2D” code, specifying that the degree of order reduction in the quasi-2D PIC is defined in terms of the number of “regions” along the simulation’s directions used to divide the computational domain. We compare the predictions of the quasi-2D simulation in various approximation degrees of the 2D problem against our full-2D simulation results. We show that, by increasing the number of regions in the Q2D simulations, the quasi-2D results converge to the 2D ones. Nonetheless, we also highlight that a quasi-2D simulation that provides a factor of 5 reduction in the computational cost resolves the underlying physical processes in an almost indistinguishable manner with respect to the full-2D simulation and incurs a L2-norm error of only about 2 % in the ion number density and below 1 % in the electron temperature.
Tejeda JM, Knoll A, 2023, An oxygen-fuelled Hall Effect Thruster: Channel length, ceramic walls and anode material experimental analyses, Acta Astronautica, Vol: 203, Pages: 268-279, ISSN: 0094-5765
An oxygen-fuelled Hall Effect Thruster is investigated at the Imperial Plasma Propulsion Laboratory vacuum chamber facilities over a different range of discharge channel axial lengths, ceramic walls and anode materials. The purpose of using oxygen as a propellant is to better understand the principles of water electrolysis Hall Effect Thrusters, which are envisaged to use oxygen to propel the thruster. These studies aimed to answer whether if for molecular plasmas, a larger channel length would benefit the overall performance of the thruster by increasing the length of the ionization region, or if a shorter channel would be more beneficial due to a reduction in the energy losses associated with the plasma-wall interactions. Experimentally, it is found that channel lengths of 13.1 mm performed the best amongst the lengths tested in terms of thrust, specific impulse and thrust efficiency. Larger channels (59.8 mm, 44.8 mm and 34.8 mm) showed a reduction in thruster performance with increasing channel length. A very short channel length of 5.5 mm is found to be less efficient than the best performing case (13.1 mm), possibly indicating that the ions are being formed within or downstream of the peak acceleration region due to the constrained length of the channel. These behaviours appear to be more evident the higher the discharge power. The impact of the walls material is also investigated. In the past, changing the thruster walls from Alumina (Al2O3) to Boron Nitride (BN) made a significant improvement on the performance, generally because of the lower Secondary Electron Emission of the BN walls. In this study, two different grades of BN walls are used: 99% purity BN (grade AX05) and a BNSiO2 compound (grade M26). Although BNSiO2 walls are said to have slightly lower Secondary Electron Emission than BN, the thrust measurements obtained using these walls are very similar. Finally, anodes made out of different materials are also tested. The main goal is to identify a su
Reza M, Faraji F, Knoll A, 2022, Resolving multi-dimensional plasma phenomena in Hall thrusters using the reduced-order particle-in-cell scheme, Journal of Electric Propulsion, Vol: 1, ISSN: 2731-4596
Plasma in Hall thrusters exhibits a complex behavior, characterized by the interplay between various dominant processes in each of the thruster’s coordinates. The emergence of high-power Hall thrusters in the recent years and the design modifications intended to extend the lifetime of these devices have further amplified the three-dimensional nature of the plasma behavior. In this regard, the novel reduced-order particle-in-cell (PIC) scheme provides the possibility to resolve the multi-dimensional interactions in a Hall thruster at a computational cost up to two orders of magnitude lower than current multi-dimensional PIC simulations. To demonstrate this point, we present in this article the results from a series of pseudo-two-dimensional simulations we performed in three configurations: axial-azimuthal, azimuthal-radial, and axial-radial. We show that, in each configuration, the pseudo-2D PIC scheme provides a significantly improved picture of the involved physics compared to a one-dimensional PIC simulation and captures self-consistently the coupling between the plasma processes in different directions, notably similar to the observations from full-2D kinetic simulations.
Rosati Azevedo E, Jones-Tett K, Larsen H, et al., 2022, Sizing and Preliminary Design of a 2-kW Water Propelled Hall Effect Thruster, The 37th International Electric Propulsion Conference
Munoz Tejeda JM, Knoll A, 2022, A Water Electrolysis Hall Effect Thruster Computational Model with Radiofrequency Excitation, International Electric Propulsion Conference, no 313
A Hall Effect Thruster operating with the products of water electrolysis (oxygen for the anode and hydrogen for the cathode) is modelled using a pseudo 2-dimensional full Particle-In-Cell code capable of tracking five different species (diatomic neutrals, monoatomic neutrals, diaotomic ions, ions and electrons). The diatomic model developed for that purpose is verified against an analytical solution from the fluid governing equations of the system. Then, the complete code is validated against experimental data collected at the Imperial Plasma Propulsion Laboratory from a non-excited Hall Effect Thruster operating on oxygen. Once the code is verified and validated, electrostatic excitation is studied as a possible mechanism to enhance the performance of this technology, and its influence on the reactive model is analyzed. The proposed excitation mechanism is based on high frequency oscillations of the ground reference potential of the neutralizing hollow cathode. This radiofrequency excitation induces electromagnetic waves into the Hall Effect Thruster channel, whose electrostatic solution is known as the ’Bernstein Modes’. The resonance frequencies of these waves are chiefly found at the Electron Cyclotron Resonance and upper harmonics, which can be excited by setting the right power and oscillation frequency coming from that hollow cathode. Several spectral analyses confirm the presence of these waves at Electron Cyclotron Resonance within the channel. For an excited simulation, it is found that the ionizing and dissociating rates increase, together with the electron temperature and overall potential. In turn, this can potentially boost the thruster performance compared to a non-excited thruster, which can unlock an innovative satellite architecture where the microwave generator hardware is shared between the communications-payload and the propulsion subsystems.
Abbi M, Munoz Tejeda JM, Reza M, et al., 2022, Investigation into the Wall Interactions of a Hall Effect Thruster Using Water Vapor as a Propellant, The 37th International Electric Propulsion Conference
Faraji F, Reza M, Knoll A, 2022, Enhancing one-dimensional Particle-in-Cell simulations to self-consistently resolve instability-induced electron transport in Hall thrusters, Journal of Applied Physics, Vol: 131, Pages: 1-20, ISSN: 0021-8979
The advent of high-power Hall thrusters and the increasing interest toward their use as a primary propulsion system for various missions have given a new boost to the efforts aiming at self-consistent predictive modeling of this thruster technology. In this article, we present a novel approach, which allows enhancing the predictive capability of one-dimensional particle-in-cell (PIC) simulations to self-consistently capture the wave-induced electron transport due to the azimuthal instabilities in Hall thrusters. The so-called “pseudo-2D” PIC scheme resulting from this approach is extensively tested in several operating conditions. The results are compared against a well-established 2D3V axial–azimuthal reference case in terms of the axial profiles of the time-averaged plasma properties, the azimuthal electric field fluctuations and their dispersion features, and the contributions of the force terms in the electron azimuthal momentum equation to the cross-field mobility. We have demonstrated that the pseudo-2D PIC provides a prediction of the above aspects that compares very closely in almost all conditions with those from the full-2D simulation. In addition, the sensitivity of the pseudo-2D simulation results to the numerical parameters associated with our approach is assessed in detail. The outcomes of these analyses have casted light on the next steps to further improve the approach.
Munoz Tejeda JM, Knoll A, 2022, Water as an Environmentally Friendly Propellant for a Multi-functional Spacecraft Architecture, Space Propulsion Conference, no 00272
We propose water can be utilized as spacecraft propellant to dramatically reduce the environmental impact of constructing and operating a satellite. We present a multi-mode chemical-electrical propulsion system where water acts as the propellant for both high thrust chemical manoeuvres, and high specific impulse electrical manoeuvres. Such a system would allow the community to divest from traditional propellants such as hydrazine and xenon, reducing the production of highly toxic chemicals and dramatically reducing the carbon footprint of the propulsion system. Water has the lowest toxicity, carbon footprint and price of any current or proposed propellant and has demonstrated both feasibility and competitiveness in laboratory testing. The unique role it can play across multiple spacecraft subsystems suggests that the commercial adoption of water as a propellant will reduce cost and mass while also reducing the environmental impact of the satellites of tomorrow. This technology has the ability to enable the development of a modular, multi-functional, competitive and environmentally friendly spacecraft architecture.
Munoz Tejeda JM, Reza M, Faraji F, et al., 2022, Performance enhancement of Hall Effect Thrusters using radiofrequency excitation, Acta Astronautica, Vol: 194, ISSN: 0094-5765
Radiofrequency excitation in single-stage Hall Effect Thrusters is proposed as a method to increase the performance of these devices. The topology of the magnetic and electric field within Hall Effect Thrusters makes it possible to excite quasistatic waves which travel longitudinally through the channel across a magnetostatic field, whose resonances are found at the electron cyclotron gyrofrequency and its upper harmonics. An in-house pseudo 2-dimensional axial–radial Particle-In-Cell software developed at the Imperial Plasma Propulsion Laboratory called PlasmaSim is verified and validated against the Russian thruster SPT-100 and adapted to be used as a computational tool to analyze plasma-wave interactions. A benchmark case study for 2-dimensional axial–radial plasma simulation codes is proposed, and PlasmaSim plasma in-channel properties are evaluated in this analysis. In terms of performance, comparison between simulated and experimental measurements shows average values in agreement with thrust, specific impulse and anode efficiency, over the full range of discharge power conditions of the SPT-100. The proposed method of radiofrequency excitation is to vary the ground reference potential of the neutralizing hollow cathode at high frequency. A range of potential excitation frequencies is established on the basis of hot plasmas’ theory, with candidate frequencies varying between 0.1 GHz to 2 GHz for the plasma conditions within a SPT-100 device. Simulation’s results give a deeper insight into the nature of these waves and their propagation in the plasma. Quantitative analyses as a function of power and excitation frequency are reported, showing the impact on thruster performance and in-channel plasma properties. The thruster’s total power is taken as the sum of the DC discharge power and AC radiofrequency power, which is calculated numerically from the simulation results based on the time varying discharge current and voltage. Taking
Schwertheim A, Knoll A, 2022, Experimental investigation of a water electrolysis Hall effect thruster, Acta Astronautica, Vol: 193, Pages: 607-618, ISSN: 0094-5765
We conceptualise an electric propulsion system in which water is utilised as a propellant for a Hall effect thruster using in situ electrolysis. By supplying the generated oxygen to the thruster anode and the hydrogen to the neutralising cathode, poisoning of the cathode emitters is mitigated. Not only does such a system benefit from the low cost, high storability and in situ resource utilisation potential of water, but synergies with water electrolysis chemical propulsion systems allow for multi-mode chemical-electrical propulsion architectures. The water electrolysis Hall effect thruster (WET-HET) has been optimised to operate on oxygen as a proof of this concept. We perform direct thrust measurements on the WET-HET using a hanging pendulum thrust balance. The thruster was operated using oxygen mass flow rates ranging from 0.96 mg s−1 to 1.85 mg s−1, and discharge powers ranging from 490 W to 2880 W. The cathode used in this test was supplied with krypton rather than hydrogen, due to laboratory restrictions preventing compressed hydrogen and oxygen cylinders being used in close proximity. Two channel wall materials were investigated — alumina and boron nitride. It was found that the wall material had a significant impact on the thrust, with an increase of approximately 40% for boron nitride. Reconfiguration of the magnetic components of the WET-HET allows us to alter the thickness of the magnetised region within the thruster channel. We test the device in three different magnetic configurations, ranging from a traditionally thin magnetic region to complete magnetisation of the discharge channel. We find that increasing the thickness of the magnetic region reduces thrust, specific impulse, and thrust efficiency of the device. We assess the change in performance as we change the discharge channel depth of the thruster. The best performance was achieved with the shallowest channel of depth 35 mm. We find the that thrust, specific impulse and anode t
Fabris AL, Young C, Knoll A, et al., 2022, Evidence of a free-space ion acceleration layer in the plume of a quad confinement plasma source, Journal of Applied Physics, Vol: 131, Pages: 1-11, ISSN: 0021-8979
The Quad Confinement Plasma Source is a novel plasma device developed for space propulsion applications, whose core is an E×Bdischarge with open electron drift. The magnetic field is produced by independently powered electromagnets able to generate different magnetic field topologies with the ultimate aim of manipulating the ion flow field for achieving thrust vectoring. In this work, we map the ion velocity in the plasmaejected from the Quad Confinement Thruster with different magnetic configurations using non-intrusive Laser-Induced Fluorescence diagnostics. Measurements show asteep ion acceleration layer located 8 cm downstream the exit plane of the discharge channel, detached from any physical boundary of the plasma source. In this location, the ion velocity increases from 3 km/s to 10 km/s within a 1 cm axial region. The ion acceleration profile has been haracterised under multiple testing conditions in order to identify the influence of the magnetic field intensity and topology on this peculiar ion acceleration layer.
Schwertheim A, Knoll A, 2022, Low power thrust measurements of the water electrolysis Hall effect thruster, CEAS Space Journal, Vol: 14, Pages: 3-17, ISSN: 1868-2502
We propose that a Hall effect thruster could be modified to operate on the products of water electrolysis. Such a thruster would exploit the low cost and high storability of water while producing gaseous hydrogen and oxygen in-situ as they are required. By supplying the anode with oxygen and the cathode with hydrogen, the poisoning of the cathode is mitigated. The water electrolysis Hall effect thruster (WET-HET) has been designed to demonstrate this concept. The dimensions of the WET-HET have been optimized for oxygen operation using PlasmaSim, a zero-dimensional particle in cell code. We present the first direct thrust measurements of the WET-HET. A hanging pendulum style thrust balance is used to measure the thrust of the WET-HET while operating in the Boltzmann vacuum facility within the Imperial Plasma Propulsion Laboratory. For this test the beam was neutralized using a filament plasma bridge neutralizer operating on krypton. We find thrust, specific impulse, and thrust efficiency all increase linearly with power for values between 400 and 1050 W. Increasing the mass flow rate from 0.96 to 1.85 mg/s increases thrust at the expense of specific impulse. Changing mass flow rate was found to have little impact on the thrust efficiency over this range. An optimal radial magnetic flux density of 403 G at the exit plane is found. Further increases to the magnetic field beyond this point were found to decrease the thrust, specific impulse and thrust efficiency, whereas the discharge voltage increased monotonically with increasing magnetic field for a given input power. It was found that the experimental thruster performance was lower than the simulation results from PlasmaSim. However, the general trends in performance as a function of power and propellant mass flow rate were preserved. We attribute a portion of this discrepancy to the inability of the simulation to model the energy absorbed by the covalent bond of the oxygen molecule. For the powers and mass flow rat
Staab D, Longhi E, Garbayo A, et al., 2021, ICE: A MODULAR WATER ELECTROLYSIS PROPULSION SYSTEM, SPACE PROPULSION CONFERENCE 2020+1
Masillo S, Lucca Fabris A, Karadag B, et al., 2021, EXPERIMENTAL CHARACTERISATION OF THE NOVEL HALO PLASMA THRUSTER FOR SMALL SATELLITE APPLICATIONS, SPACE PROPULSION 2020+1
Schwertheim A, Knoll A, 2021, PERFORMANCE CHARACTERISATION OF THE WATER ELECTROLYSIS HALL EFFECT THRUSTER (WET-HET) USING DIRECT THRUST MEASUREMENTS, SPACE PROPULSION 2020+1
Muir C, Ma C, Knoll A, 2021, The design, fabrication and test progress summary of the iridium catalysed electrolysis thruster, Space Propulsion 2020+1
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