Dr Aaron Knoll

Lecturer in Plasma Propulsion

Phone: Work: 01483 68 6444
Room no: 01 BA 00

Office hours

9:00 - 17:30, Monday - Thursday

9:00 - 17:00, Friday

Further information


Aaron Knoll is a lecturer in the field of plasma propulsion for spacecraft within the Surrey Space Centre. Aaron received his Bachelors of Aerospace Engineering (2003) and a Masters of Applied Science in Aerospace (2005) from Carleton University in Ottawa, Canada. During his undergraduate education he performed an internship with the Canadian Space Agency as part of the Software and Ground Segment from 2001 – 2002. Aaron received his Ph.D. (2010) from Stanford University, where he was involved with the research of instability driven electron transport within Hall Effect Thrusters. The focus of Aaron’s research at Surrey is toward the development of novel low power plasma propulsion technologies for small spacecraft.

Research Interests

Aaron's research interests involve experimental and computational plasma physics related to electric propulsion for spacecraft applications.  His Ph.D. research focused on Hall Thrusters, and was aimed at understanding the high frequency plasma oscillations that occur in these devices and their relevance to the efficiency and performance. He also has personal research interests in Fluid/Aerodynamic simulations and computational techniques for solution of non-linear partial differential equations.

Research Collaborations

EADS, Astrium, Surrey Satellite Technology Ltd


Journal articles

  • Knoll AK, Harle T, Lappas VJ, Pollard M. (2014) 'Experimental Performance Characterization of a Two-Hundred-Watt Quad Confinement Thruster'. Journal of Propulsion and Power, 30 (6), pp. 1445-1449.
  • Pottinger SJ, Lamprou D, Knoll AK, Lappas VJ. (2012) 'Impact of plasma noise on a direct thrust measurement system.'. Rev Sci Instrum, United States: 83 (3)


    In order to evaluate the accuracy and sensitivity of a pendulum-type thrust measurement system, a linear variable differential transformer (LVDT) and a laser optical displacement sensor have been used simultaneously to determine the displacement resulting from an applied thrust. The LVDT sensor uses an analog interface, whereas the laser sensor uses a digital interface to communicate the displacement readings to the data acquisition equipment. The data collected by both sensors show good agreement for static mass calibrations and validation with a cold gas thruster. However, the data obtained using the LVDT deviate significantly from that of the laser sensor when operating two varieties of plasma thrusters: a radio frequency (RF) driven plasma thruster, and a DC powered plasma thruster. Results establish that even with appropriate shielding and signal filtering the LVDT sensor is subject to plasma noise and radio frequency interactions which result in anomalous thrust readings. Experimental data show that the thrust determined using the LVDT system in a direct current plasma environment and a RF discharge is approximately a factor of three higher than the thrust values obtained using a laser sensor system for the operating conditions investigated. These findings are of significance to the electric propulsion community as LVDT sensors are often utilized in thrust measurement systems and accurate thrust measurement and the reproducibility of thrust data is key to analyzing thruster performance. Methods are proposed to evaluate system susceptibility to plasma noise and an effective filtering scheme presented for DC discharges.

  • Knoll AK, Cappelli M. (2008) 'A simple isentropic model of electron transport in Hall thrusters'. Journal of Physics D: Applied Physics, 41 (16)


    A simple model is presented for the time-averaged electron mobility within a Hall thruster. The model is predicated on the experimental evidence for isentropic electron flow and, when used in a one-dimensional simulation, captures plasma properties that are in reasonable agreement with experiment.

Conference papers

  • Ahmed OD, Knoll AK, Lappas V. (2014) 'Hybrid Propulsion System for CubeSat Mission Applications'. Cologne, Germany: Space Propulsion Conference 2014
  • Knoll AK, Harle T, Lappas V, Pollard M, Bianco P. (2014) 'Influence of Cathode Position on the Performance of the Quad Confinement Thruster'. Cologne, Germany: Space Propulsion Conference 2014
  • Harle T, Knoll AK, Lappas V. (2014) 'Thrust Balance Characterization of the Halo Thruster using a Radio-Frequency Cathode Neutralizer'. Cologne, Germany: Space Propulsion Conference 2014
  • Harle T, Knoll AK, Lappas V, Bianco P. (2013) 'Performance Measurements of a High Powered Quad Confinement Thruster'. Washington DC: 33rd International Electric Propulsion Conference


    The Quad Confinement Thruster employs a convex magnetic field bounded by four cusps to weakly con fine electrons and thus create a high density plasma. An electric field sustained between a rear anode and an external hollow cathode provides ion acceleration. In this study the first performance measurements of a permanent magnet high powered QCT (QCT1500) are reported. Direct thrust measurements were made, using a pendulum type thrust balance, as a function of the anode power up to maximum power of 800 W. A symmetric quadrupole field strength of 950 G was used throughout and the krypton propellant flow was varied from 10-30 sccm. Thrust levels between 3-10 mN at specific impulses of 200-1600s were recorded.

  • Knoll AK, Lamprou D, Lappas V, Pollard M, Bianco P. (2013) 'Thrust Balance Characterization of a 200W Quad Confinement Thruster for High Thrust Regimes'. Washington DC: 33rd International Electric Propulsion Conference


    A thrust balance characterization of a low powered Quad Confinement Thruster is presented for high levels of propellant flow. The nominal flow rate for this device is between 1sccm and 2sccm of Xenon propellant. This study extends the operating range, and investigates the performance at two high flow conditions of 10sccm and 20sccm. Power is varied incrementally between 20W and 200W in order to characterize the performance versus power trends of the device. It was found that for these high flow regimes the propellant is underutilized, and a proportion of the increased thrust can likely be attributed to a hot gas expansion of the neutral Xenon rather than the generation of additional accelerated ions. The thrust was increased from 1mN (nominal) to 3.3mN at 200W of input power for the 20sccm condition. However, the performance penalty in terms of the specific impulse was considerable. The specific impulse under these conditions dropped below 200s, where the nominal condition is 1000s. A compromise between increased thrust and decreased performance was found at 10sccm of flow: 3mN of thrust at 300s specific impulse.

  • Knoll AK, Shafiq U, Lappas V, Perren M. (2012) '3-Dimensional Mapping of Plasma Properties in the Plume Region of the Quad Confinement Thruster'. Bordeaux, France: Space Propulsion 2012


    A 3-axis translating Langmuir probe system has been developed in order to characterize the plasma properties in the plume region of the Quad Confinement Thruster. These experiments have provided a volumetric map of the electron density, plasma potential, and electron temperature for three operating conditions of the device. The operating points investigated in this study were for a symmetric magnetic field and two magnetic steering scenarios where alternate sets of the thruster’s four electromagnets were powered. The most significant outcome of these experiments was a quantitative analysis of the thrust vectoring capabilities of the device showing an 11 degree angular offset of the plasma beam from the thruster axis for each of the two steering scenarios investigated. It was also found that there was a 90 degree change in orientation between the two steering conditions. This study has also contributed insight into the 3-dimensional structure of the plasma plume. The results indicate that the electron density and plasma potential peak near the centre axis whereas the electron temperature is at a minimum.

  • Knoll AK, Melly B, Lappas V. (2011) 'The Quad Confinement Thruster - Preliminary Performance Characterization and Thrust Vector Control'. Wiesbaden, Germany: 32nd International Electric Propulsion Conference


    A new variety of electric thruster, the Quad Confinement Thruster, has been designed and tested during a preliminary one year development effort. This thruster utilizes an innovative magnetic cusp topology. Eight electromagnets are used to create a convex magnetic field structure with a center cusp, and four outlying cusps along the periphery. The thrust produced by the device is derived from the momentum of ions accelerated through a Hall effect static electric field. Direct thrust measurements show a specific impulse up to 700s at powers of less than 100W. This specific impulse was achieved using a propellant flow rate of 5sccm of Krypton, and corresponded to a thrust of approximately 2.1mN. One of the key motivations for this research is the ability of this thruster to actively control the direction of thrust through manipulation of the magnetic field by regulating power to the individual solenoid magnets. Preliminary experiments, which measure the profile of the plasma density at two axial stations downstream of the channel exit, appear to show a 14 degree thrust vectoring capability of the device.

  • Knoll AK, Cappelli M. (2009) 'A multi-fluid 2-D simulation of a co-axial Hall plasma discharge'. Saratoga Springs, New York, USA: 62nd Annual Gaseous Electronics Conference


    A multi-fluid 2-D simulation of a co-axial E x B plasma discharge is presented, resolving the azimuthal dynamics leading to the growth and saturation of high-frequency (0.5 -- 10 MHz) azimuthally-propagating fluctuations. The simulation accounts for finite-rate ionization kinetics, with associated losses of particles and energy to the bounding ceramic walls. These discharges are typical of Hall thruster plasma accelerators, which are increasingly being used in space propulsion applications. The simulations presented are for full scale thrusters that operate in the 1 kW power levels, capturing the entire azimuthal domain. The simulations focus on the role played by these fluctuations in establishing the cross-field electron current in regions of relatively strong magnetic fields (50-200 Gauss). The time-average predictions for plasma properties are in qualitative and quantitative agreement with experiments, and the findings seem to be supportive of the experimental results that indicate that high frequency fluctuations may be more important at defining electron current at lower discharge voltages, where the azimuthal electron shear is small.

  • Lam C, Knoll AK, Cappelli M. (2009) 'Two-Dimensional (z-theta) Simulations of Hall Thruster Anomalous Transport'. Ann Arbor, Michigan: 31st International Electric Propulsion Conference


    This paper presents results on the development of both hybrid and multifluid simulations of Hall thrusters that resolve azimuthal electron flow dynamics. Simulations are carried out for a laboratory, nominally 90 mm channel diameter discharge with an extended acceleration region for which a modest collection of experimental data exists. The simulations are intended as a tool to better understand the mechanism behind azimuthal wave-driven electron transport. Both the hybrid and fluid simulations capture azimuthal fluctuations which appear to be consistent with quasi-neutral disturbances predicted by linear analysis. The impact of such disturbances on the cross-field transport is discussed.

  • Knoll AK, Cappelli M. (2009) 'Experimental Characterization of High Frequency Instabilities within the Discharge Channel of a Hall Thruster'. Ann Arbor, Michigan: 31st International Electric Propulsion Conference


    This study examines the detailed dispersion properties of high frequency (1- 15 MHz) plasma fluctuations within a Hall thruster. The results reveal the existence of two strong modes in this frequency range, which are predominantly axially propagating, with weaker azimuthal waves near the exit and near field region. Both modes show generally similar dispersion characteristics. Each propagates predominantly towards the anode within the discharge, and towards the cathode in the near field. Each is characterized by low wavenumber cut-offs (near 4.5 MHz and 10.5 MHz respectively). In all cases, within the channel, the results seem to be only weakly dependent on axial position, indicating that these disturbances are highly non-local. The wavenumber lies within a range of 100-700 rad/m, and the phase velocities fall in the range of 105 5x105 m/s. The azimuthal components near the exit of the discharge have phase velocities approaching the azimuthal electron drift velocity, but their dispersive characteristics fall short of what is expected for a typical beam plasma mode. We suspect that the because the frequency of these waves lie close to the lower-hybrid resonance, that these waves are the result of an interaction between the lower hybrid waves and the electron stream mode, driven and coupled by the relatively strong transverse electron shear flow, the scale length of which corresponds closely to the measured wavelengths.

  • Knoll AK, Gascon N, Cappelli M. (2007) 'Numerical Simulation of High Frequency Wave Coupling within a Hall Thruster'. Cincinnati, OH, USA: 43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit


    A 2-dimensional Hall thruster simulation has been developed in the axial-azimuthal coordinate plane. The goal of this simulation is to numerically model high frequency plasma waves within the discharge channel of the Hall thruster, and study the contribution of these waves to the time-averaged axial electron drift. This model uses a continuum (fluid) representation for both the electrons and ions. In order to simulate oscillations in the electron field it was necessary to model the electrons dynamically, as opposed to assuming a steady state solution at each time step. The electron momentum equations also include electron inertia terms that are normally neglected in typical Hall thruster models. These inertia terms provide a wave coupling mechanism between axially and azimuthally propagating waves. This numerical model was able to reproduce two dominant high frequency plasma oscillations in the Hall thruster: a 74MHz Kelvin-Helmholtz type shearing instability, and a 7MHz oscillation in the plasma density that has also been observed experimentally. The simulation was successful at predicting the axial electron drift in good agreement with experiment. The results of this study suggest that the plasma oscillations play a dominant role in the electron transport process. In particular, contributions to the electron transport resulting from perturbations in the azimuthal electron velocity were found to be greater than 300% of classical collisional transport.

  • Fernandez E, Knoll AK, Cappelli M. (2006) 'An Axial-Azimuthal Hybrid Simulation of Coaxial Hall Thrusters'. AIAA Sacramento, California, USA: 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit


    We report on progress towards the development of a Hall thruster simulation in the axial-azimuthal (z - θ) computational space. Unlike most computational studies of closed-drift Hall accelerators which have been in one dimension (1D) along the axial direction or in two dimensions (2D) in the axial and radial dimensions, and which require some specification of the axial transport mechanism, this z - θ numerical simulation developed here self-consistently evolves the azimuthal electron drift velocity. The simulation is, in principal, capable of capturing correlated azimuthal disturbances in plasma properties which may give rise to cross-field transport, and makes no use of ad-hoc transport models. Preliminary analysis of the results indicates that azimuthal plasma instabilities may contribute to the axial electron transport process.

  • Knoll AK, Thomas N, Gascon N, Cappelli M. (2006) 'Experimental Investigation of High Frequency Plasma Oscillations Within Hall Thrusters'. Sacramento, California, USA: 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit


    An experimental setup has been developed to measure high frequency plasma oscillations within the acceleration channel of a laboratory Hall thruster. The plasma oscillations are measured with three Langmuir probes separated by small axial and azimuthal offsets. This configuration permits the oscillations to be correlated with direction and wave number. This work is motivated by the anomalous electron transport phenomena, as plasma instabilities may play a crucial role in this transport process. Preliminary data has been gathered downstream of the exit plane of the thruster and suggests high frequency oscillations in the 1 to 10MHz range predominately in the axial direction. Work is currently underway to measure the high frequency oscillations within the acceleration channel at various axial locations.

  • Fernandez E, Knoll AK, Tarik K, Cappelli M. (2005) 'A Two-Dimensional Hybrid (z-theta) Simulation of Coaxial Hall Thrusters'. Princeton University, USA: The 29th International Electric Propulsion Conference


    This paper describes a 2-dimensional simulation of a coaxial Hall thruster that was developed in the axial-azimuthal (z - θ) computational space. Most computational studies of closed-drift Hall accelerators have been in one dimension (1D) along the axial direction or in two dimensions (2D) in the axial and radial dimensions. These 1D and 2D models have had reasonable success in describing the overall behavior of the plasma discharge. However, in these descriptions, the axial transport of electrons is modeled in an ad hoc fashion, usually with a prescribed cross-field mobility. The cross-field electron mobility is likely to be influenced/established by the azimuthal dynamics. Azimuthal perturbations arise from the established equilibrium and, if properly correlated, result in a net axial transport of electrons. The numerical model developed in this study self-consistently evolves the azimuthal electron drifts, and makes no use of ad hoc transport models. Preliminary analysis of the results indicates that azimuthal plasma instabilities do contribute to the axial electron transport process. However, both numerical and theoretical challenges still need to be addressed as there were notable discrepancies in terms of the time averaged ion velocity and electron density characteristics as compared with experimental findings. These differences are partly attributed to spurious spikes in the plasma potential, the origins of which are yet to be identified.

  • Saraf S, Knoll AK, Pelletier F, Tafazoli M. (2002) 'Investigating Formation Flying and COTS in an Integrated Simulation Environment'. Houston, Texas: SpaceOps 2002


    The Canadian Space Agency’s (CSA) Software and Ground Segment (SGS) section has the mandate to develop innovative software and ground segment technologies. The implementation of formation flying concepts for Canadian missions is also currently under investigation at CSA's Spacecraft Engineering section. To that end, there is an ongoing development of a simulation environment to test COTS (Commercial-Off-The-Shelf) and formation flying technologies. Some of today's spacecraft are laboriously custom designed for a specific mission and a limited set of tasks. Development time can be lengthy (several years), which means that designs do not take advantage of the most recent technology. Designs also tend to be extremely inflexible, creating a spacecraft that cannot be easily adapted to future missions. A design ethic that promotes reusability is much more cost-effective and increases the time available to advance new technologies. COTS offer advantages such as a reduced development time, an increased product selection, faster and cheaper parts replacement, and extensively tested advanced designs. The main drawbacks to COTS use in space are susceptibility to radiation and in some cases decreased reliability. Since one of the main advantages of formation flying is the reduced mission sensitivity to a spacecraft failure, the risk associated with COTS, which has hindered its use in conventional space missions, is less concerning in the context of a multiple spacecraft mission. Achieving some level of standardization is a problem currently confronting the space industry, which must be addressed to realize the cost savings that can come from mass production and spacecraft interoperability. The use of standard components with standard interfaces also reduces development time. As well, since part of the goal is to have spacecraft already in orbit regroup and possibly join with new spacecraft to accomplish other missions, some forwards and backwards compatibility between generations of spacecraft will be necessary. This paper describes an integrated simulation environment that uses COTS spacecraft and simulation components to investigate formation flying scenarios and their benefits and challenges. A few of the well-known industry software and hardware tools incorporated into the environment include Analytical Graphic's STK, Mathwork's Matlab/Simulink, CAE Electronic's Real-time Object-Oriented Simulation Environment (ROSETM), Intel's StrongARM processor, and the PC-104

  • Saraf S, Knoll AK, Melanson P, Tafazoli M. (2002) 'Development of a simulation environment to test space missions COTS technologies'. Proceedings of DASIA 2002, Dublin, Ireland: DASIA 2002


    The Canadian Space Agency's (CSA) Software and Ground Segment Section (SGS) has the mandate to develop innovative emerging software and on-board satellite and ground segment computer technologies. To that end, there is an ongoing development of a simulation environment to test COTS (Commercial-Of-The-Shelf) technologies. There are severe cost constraints in all aspects of many space missions due to the limited return on investment and scarce commercialization opportunities that come with many science missions. There is an opportunity to explore the innovative implementation of COTS technologies to reduce the mission cost and maximize performance available from COTS components. However, using COTS technologies in the space environment has ist constraints and therefore designing a spacecraft mission has to involve some new techniques that allow implementation of these components and minimize the risk of failure. The goal of our project is to develop a simulation environment, itself using COTS components, and then to allow the seamless integration of various components to test spacecraft mission concepts. For example, one of the aspects of using COTS processors in space is to protect them from the radiation environment. The current state of the simulation tests an innovative software EDAC (Error Detection and Correction) package and a redundant processor configuration to investigate protection against the effects of radiation and other failures on a generic mission. It also includes the capability to test formation-flying concepts that have the potential to revolutionize cost reduction efforts for space missions and to enable new space applications. This paper describes the simulation environment in detail and illustrates some of the technologies being tested for possible future space missions. The paper concludes with a look at the future development of the simulation environment and possible benefits of its use as well as the lessons learned to date.

  • Saraf S, Knoll AK, Tafazoli M. (2001) 'A Selection Methodology for Radiation-Hardened Processors'. Bristol, UK: Avionics 2001
  • Saraf S, Knoll AK, Tafazoli M. (2001) 'Radiation Hardened Processors: A Comparative Study'. Nice, France: Commercialization of Military and Space Electronics


  • Knoll AK. (2011) PLASMA THRUSTERS. Article number PCT/GB2011/051016


    A plasma thruster comprises a plasma chamber having first and second axial ends, the first of which is open, an anode located at the second axial end, and a cathode. The cathode and anode are arranged to produce an electric field having at least a component in the axial direction of the thruster. A magnet system comprising a plurality of magnets is spaced around the thruster axis, each magnet having its north and south poles spaced around the axis.

  • Auerbach P, Carter J, Fuller L, Haylett D, Knoll AK, Reitenberg J, Smith A, Thorsell E. (2010) Avalanche rescue device. Article number US 12/660425


    A rescue device includes a control module to sense an avalanche, sense the direction of the surface and establish a target path to the surface. A nozzle is selected or oriented by the control module along the target path. A fluid reservoir is connected to the nozzle to force a fluid through the nozzle along the target path to the surface. This allows rescuers to identify the location of a victim and also provides an air path to the victim.

Theses and dissertations

  • Knoll AK. (2010) Plasma oscillations and associated electron transport within hall thrusters. Stanford University, California, USA


    The Hall thruster is a type of plasma propulsion system for space vehicle applications. The thrust produced by this device is derived from the momentum of ions, which are accelerated to high exit velocities by the action of an electric field sustained within the plasma. The advantage of the Hall thruster compared to conventional chemical rocket propulsion is a significantly higher exhaust velocity, which leads to better utilization of propellant mass. Since the early days of Hall thruster research, experiments have suggested that the mobility of electrons along the axis of the thruster, perpendicular to an imposed magnetic field, is higher than can be explained by classical collision transfer processes alone. A lack of understanding regarding the mechanism for this enhanced mobility has proved a significant challenge toward the development of reliable simulations capable of predicting the performance of these devices. This thesis examines the role of high frequency plasma oscillations on the electron mobility using a combination of experimental studies on a laboratory Hall thruster, and numerical simulations capable of capturing these oscillations and quantifying their impact on the electron mobility. Two high frequency oscillations were consistently observed in the experiments: a 10MHz mode which appeared strongest in the vicinity of the anode, and a 4.5MHz mode which was strongest in the mid-channel region of the thruster. These were relatively low wave number (long wavelength) oscillations: approximately 6cm for the 4.5MHz oscillation and 3cm for the 10MHz oscillation. The angle of these waves varied considerably depending on the operating conditions of the thruster. They were found to be closely aligned to the axis of the thruster for experiments conducted with Xenon propellant, and were aligned with the circumference of the thruster (in the direction of electron drift) for experiments conducted with Krypton. A Hall thruster simulation, formulated in the axial-azimuthal coordinates of the thruster, was able to capture high frequency oscillations in reasonable agreement with experimental findings: 13MHz near the anode and 5MHz in the mid-channel region of the thruster for 160V discharge conditions. The simulation results demonstrated the crucial role of these oscillations in regulating the electron transport. In the vicinity of these oscillations the electron mobility was increased by a factor of five or more. The central finding of this thesis is that

  • Knoll AK. (2005) Simulation of High Frequency Plasma Oscillations within Hall Thrusters. Carleton University, Ottawa, ON, Canada

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