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Conference paperFil P, Rosati Azevedo E, Knoll A, 2024,
Investigation of Transient Behavior in Water Propelled Hall-effect Thruster
, The 38th International Electric Propulsion Conference -
Conference paperRosati Azevedo E, Berhe M, Jones-Tett K, et al., 2024,
Joint Development of a Water Electrolysis Propelled Hall Effect Thruster and LaB6 Hollow Cathode
, The 38th International Electric Propulsion Conference -
Conference paperRosati Azevedo E, Fil P, Knoll A, et al., 2024,
Early Performance Characterization of a 2 kW Water Propelled Hall Effect Thruster
, The 38th International Electric Propulsion Conference -
Conference paperMunoz Tejeda JM, Schwertheim A, Moloney R, et al., 2024,
First End-to-End Technological Demonstration of a Water Electrolysis Hall Effect Thruster Operating with a Water Electrolyser
, The 38th International Electric Propulsion Conference -
Journal articleRose 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: 0022-3727Unsteady 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.
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Journal articleFaraji 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: 0021-8979The 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.
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Journal articleReza 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: 0963-0252High-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
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Journal articleReza 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: 0021-8979Partially 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.
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Journal articleReza 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: 0022-3727Reduced-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
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Journal articleTejeda 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: 2150-766XWater 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.
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Journal articleFaraji 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: 2158-3226In 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.
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Journal articleTejeda 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: 0094-5765An 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
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Journal articleReza 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: 2731-4596Plasma 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.
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Conference paperAbbi 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 -
Conference paperRosati 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 -
Conference paperMunoz Tejeda JM, Knoll A, 2022,
A Water Electrolysis Hall Effect Thruster Computational Model with Radiofrequency Excitation
, International Electric Propulsion Conference, no 313A 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.
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Journal articleFaraji 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: 0021-8979The 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.
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Conference paperMunoz Tejeda JM, Knoll A, 2022,
Water as an Environmentally Friendly Propellant for a Multi-functional Spacecraft Architecture
, Space Propulsion Conference, no 00272We 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.
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Journal articleMunoz Tejeda JM, Reza M, Faraji F, et al., 2022,
Performance enhancement of Hall Effect Thrusters using radiofrequency excitation
, Acta Astronautica, Vol: 194, ISSN: 0094-5765Radiofrequency 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
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Journal articleSchwertheim A, Knoll A, 2022,
Experimental investigation of a water electrolysis Hall effect thruster
, Acta Astronautica, Vol: 193, Pages: 607-618, ISSN: 0094-5765We 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
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