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  • Journal article
    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.

  • Journal article
    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.

  • Journal article
    Munoz Tejeda JM, Reza M, Faraji F, Knoll Aet 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

  • Journal article
    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

  • Journal article
    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

  • Conference paper
    Staab D, Longhi E, Garbayo A, Swar K, Segovia-Guerrero L, Larsen H, Muir C, Knoll A, Ma C, Wilson S, Sadler J, Sheppard-Alden J, Rodier Cet al., 2021,


  • Conference paper
    Rosati Azevedo E, Tirila V, Schwertheim A, Knoll Aet al., 2021,

    Magnetic Field Enhancement of the Quad Confinement Thruster (QCT): Design and Early Development of the QCT Phoenix

    , The 7th Space Propulsion Conference
  • Conference paper
    Masillo S, Lucca Fabris A, Karadag B, Potterton T, Knoll A, Bianco Pet al., 2021,


  • Conference paper
    Muir C, Ma C, Knoll A, 2021,

    The design, fabrication and test progress summary of the iridium catalysed electrolysis thruster

    , Space Propulsion 2020+1
  • Conference paper
    Schwertheim A, Knoll A, 2021,


  • Journal article
    Schwertheim A, Rosati Azevedo E, Liu G, Bosch BorrĂ s E, Bianchi L, Knoll Aet al., 2021,

    Interlaboratory validation of a hanging pendulum thrust balance for electric propulsion testing

    , Review of Scientific Instruments, Vol: 92, Pages: 1-11, ISSN: 0034-6748

    A hanging pendulum thrust balance has been developed by Imperial College London in collaboration with the European Space Agency (ESA) to characterize a wide range of static fire electric propulsion and chemical micro-propulsion devices with thrust in the range of 1 mN to 1 N. The thrusters under investigation are mounted on a pendulum platform, which is suspended from the support structure using stainless steel flexures. The displacement of the platform is measured using an optical laser triangulation sensor. Thermal stability is ensured by a closed loop self-compensating heating system. The traceability and stability of the calibration are ensured using two separate calibration subsystems: a voice coil actuator and a servomotor pulley system. Two nearly identical thrust balances have been constructed, with one being tested in the Imperial Plasma Propulsion Laboratory and the other in the ESA Propulsion Laboratory. Both balances show a high degree of linearity in the range of 0.5 mN–100 mN. Both instruments have demonstrated a stable calibration over several days, with an estimated standard deviation on thrust measurements better than 0.27 mN for low thrust measurements. The same electric propulsion test article was used during both tests: a Quad Confinement Thruster (QCT) variant called QCT Phoenix. This thruster differed from previous QCT designs by having a newly optimized magnetic topology. The device produced thrust up to 2.21 ± 0.22 mN with a maximum specific impulse of 274 ± 41 s for an anode power range of 50 W–115 W.

  • Journal article
    Williams RD, Fabris AL, Knoll A, 2020,

    Insight into the plasma structure of the Quad Confinement Thruster using electron kinetic modelling

    , Acta Astronautica, Vol: 173, Pages: 111-118, ISSN: 0094-5765

    The behaviour of plasma within the discharge channel of the Quad Confinement Thruster is studied on the basis of electron kinetics. Here we propose that E × B drift of electrons drives the formation of unusual quadrant dependent light emitting structures observed experimentally in the discharge channel of the Quad Confinement Thruster. This assertion is made on the basis of a theory-based analysis and a computational model of the Quad Confinement Thruster. A particle orbit model of electron motion under the influence of applied electric and magnetic fields was used to assess electron transport. Structures strongly resembling that of the observed visible emission regions were found in the electron density distribution within the channel. While the motion of electrons cannot be decoupled from the motion of ions, as in this simple electron kinetic approximation, the results of this analysis strongly indicate the physical mechanism governing the formation of the non-uniform density distributions within the Quad Confinement Thruster channel.

  • Patent
    Knoll A, Harle T, Peter S, Frame T, Wantock Tet al., 2020,

    Plasma generation

    , US10595391B2

    A plasma torch having an open end from which a plasma plume is emitted in use is disclosed. The plasma torch includes a central cathode rod, a grounded conductive tube having an open end and being arranged around the cathode and spaced therefrom to form a first cylindrical cavity open at one end; and a high voltage electrode having a dielectric barrier material at a radially inward-facing surface thereof and being arranged around the grounded conductive tube and spaced apart therefrom to form a second annular cylindrical cavity open at one end. A constant direct current (DC) electrical power plus a high voltage pulsed electrical power is provided to the cathode producing an arc discharge in the first cavity between the cathode and grounded tube to generate a central thermal plasma emitted at an open end of the first cylindrical cavity. A high voltage alternating current electrical power or pulsed electrical power is provided to the high voltage electrode producing a dielectric barrier discharge in the second annular cylindrical cavity to generate a non-thermal plasma emitted from an open end of the second cavity as a halo around the central thermal plasma.

  • Conference paper
    Schwertheim A, Knoll A, 2020,

    The Water Electrolysis Hall Effect Thruster (WET-HET): Paving the Way to Dual Mode Chemical-Electric Water Propulsion

    , 36th International Electric Propulsion Conference
  • Conference paper
    Karadag B, Masillo S, Moloney R, Lucca Fabris A, Potterton T, Knoll A, Bianco Pet al., 2020,

    Experimental Investigation and Performance Optimization of the Halo thruster

    , 36th International Electric Propulsion Conference
  • Conference paper
    Mörtl M, Knoll A, Williams V, Shaw P, Argyriou V, Zamattio J, Pugliese Let al., 2020,

    Enhancing Hall Effect Thruster Simulations with Deep Recurrent Networks

    , 36th International Electric Propulsion Conference
  • Conference paper
    Williams V, Argyriou V, Shaw P, Montag C, Herdrich G, Knoll A, Mortl Met al., 2019,

    Development of PPTNet a Neural Network for the Rapid Prototyping of Pulsed Plasma Thrusters

    , 36th International Electric Propulsion Conference
  • Journal article
    Muir C, Knoll A, 2019,

    Catalytic Combustion of Hydrogen and Oxygen for an Electrolysis Micro-Propulsion System

    , Journal of the British Interplanetary Society, ISSN: 0007-084X
  • Journal article
    Schwertheim A, Knoll A, 2019,

    In situ Utilization of Water as a Propellant for a Next Generation Plasma Propulsion System

    , Journal of the British Interplanetary Society, ISSN: 0007-084X

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