Magnetic Thrust Research Articles

Experimental investigation of a water electrolysis Hall effect thruster

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 mgs−1 to 1.85 mgs−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 thrust efficiency increases linearly with power for all configurations with no obvious plateau. Greatest specific impulse and efficiency was found when mass flow was lowest. Although discharge voltage increases linearly with magnetic field strength, thrust, specific impulse and efficiency peaks at 480 Gauss. The greatest thrust measured was 38.63 ± 0.25 mN, with a maximum specific impulse of 4112 ± 36 s and a maximum anode thrust efficiency of 15.50 ± 0.27%.

Fluid-kinetic model of a propulsive magnetic nozzle

A kinetic-electron, fluid-ion model is used to study the 2D plasma expansion in an axisymmetric magnetic nozzle in the fully-magnetized, cold-ion, collisionless limit. Electrons are found to be subdivided into free, reflected, and doubly-trapped sub-populations. The net charge current and the electrostatic potential fall on each magnetic line are related by the kinetic electron response, and together with the initial profiles of electrostatic potential and electron temperature, determine the electron thermodynamics in the expansion. Results include the evolution of the density, temperature, and anisotropy ratio of each electron sub-population. The different contributions of ions and electrons to the generation of magnetic thrust are analyzed for upstream conditions representative of different thruster types. Equivalent polytropic models with the same total potential fall are seen to result in a slower expansion rate, and therefore to underpredict thrust generated up to a fixed section of the magnetic nozzle.

Analytical Modeling of Air Gap Magnetic Fields and Bearing Force of a Novel Hybrid Magnetic Thrust Bearing

In this article, an analytical method is presented for modeling the complex air gap field distribution and the magnetic force of a novel permanent magnet (PM) bias hybrid magnetic thrust bearing (HMTB). This bearing has a very compact structure and is proposed to replace the traditional magnetic thrust bearings (MTBs) for high-speed rotor systems. The proposed method introduces the notion of the complicated air gap field, calculated from the Schwarz–Christoffel transformation of the air gap geometry, to take into account the effect of slot fringing and leakage. As a result, accurate solutions of the flux distribution and the bearing force can be obtained, which shows good agreement with the results of the finite-element method (FEM). The prototype of the HMTB is designed, analyzed, and tested. The results of the analytical model are compared with experimental data.

Optimized design of a novel yokeless mover permanent magnet linear generator for free‐piston stirling engines applied to solar cogeneration systems

Considering the growing environmental issues, the use of renewable energy converters such as solar cogeneration systems has received more attention during recent years. Solar cogeneration systems convert the thermal energy of solar radiation into mechanical energy (by means of a free-piston Stirling engine) and then into electrical energy (by means of a linear generator). This paper introduces the novel yokeless mover permanent magnet linear generator (YMPMLG) to reduce mover mass, increase power density, increase thrust and decrease the normal force on the mover, which can make this generator a more suitable structure for operating in solar cogeneration systems. This paper is initiated with a comparative study of a YMPMLG with two reported prototypes in this field. Three generators with similar characteristics are simulated by the finite element method (FEM). The magnetic field flux density, power density, thrust coefficient, normal force, and efficiency parameters are analysed and compared in three generators. Following that, some dimensional parameters of the YMPMLG, including generator diameter and length and permanent magnet dimensions are optimized to achieve the highest possible power density and thrust coefficient using the ‘hybrid genetic and pattern search’ algorithm. The highest power density and efficiency are 0.58 kW/kg and 90.3%, respectively. In the end, the YMPMLG prototype has been tested to validate the results of the FEM method.

Three-dimensional neutralizer effects on a Hall-effect thruster near plume

The near plume of an SPT100-like Hall thruster is simulated with a hybrid particle-in-cell/fluid code to assess the three-dimensional effects of the neutralizer for two different asymmetric positions. The electron fluid model accounts for the effects of both magnetization and collisions, and closes the momentum equation with a simple polytropic closure for the electron temperature. The observed asymmetries are small in the majority of the plasma plume properties and in the achieved magnetic thrust and torque, which also show consistent trends with the available experimental characterizations. The only exception is represented by the 3D electric and electron current densities, which show persistent asymmetries that tend to diminish as the neutralizer is placed both farther downstream and radially outward, outside of the magnetic separatrix. These findings permit to justify the use of axisymmetric codes for Hall-effect thruster performance estimation. The results on the neutralizer asymmetry effects are found robust to different polytropic coefficients (i.e. electron temperature gradients). Finally, the comparison of the adopted polytropic closure model against an advanced one including the energy balance equation shows relevant differences in the electron temperature but still a good agreement in the electron current density map.

Kinetic insights into thrust generation and electron transport in a magnetic nozzle

Axisymmetric fully kinetic particle-in-cell simulations are carried out to study the expansion of plasma in a propulsive magnetic nozzle (MN). The simulation results reveal that the MN can boost the total thrust while the on-axis ion acceleration may be suppressed, demonstrating the necessity of including two-dimensional effects if the propulsive performance is the primary concern. The magnetic thrust resulting from the interaction of azimuthal electric current with the applied magnetic field is identified as the only source of enhancing the plasma plume thrust. It is found that the role of electron transport, in connection with the stress tensor, cannot be neglected in fluid modeling. The electron transport effects will induce a paramagnetic drift current and thus undermine the propulsive performance of MN.

A Three-Coil Setup for Controlled Divergence in Magnetic Nozzle

Magnetic nozzles with their contactless feature have opened new grounds for long-distance space travels. For quicker mission completions along with improving system reliability, it becomes necessary to design robust systems that exhibit the ability to confine thrust. A semianalytical numerical method is proposed for rapid and assumption-less calculations of the magnetic field because of a thick coil with rectangular cross section. The method is extended for the proposal of a three-coil setup, where divergence angle of the magnetic field lines is readily controlled in-flight to provide the flexibility to modify the output magnetic thrust and the nozzle's efficiency.

Design and Modeling of a Novel Permanent Magnet Width Modulation Secondary for Permanent Magnet Linear Synchronous Motor

High-speed linear motor with low thrust ripple is one of the research focuses of high-performance systems. This article proposes a novel secondary topology for permanent magnet (PM) linear synchronous motor (PMLSM) with high speed, to suppress the thrust ripple. PM width modulation secondary adopts several PMs to excite the main magnetic field per pole. The issues from a big pole pitch, which is generally used by high-speed PMLSM, are solved. These PMs have various longitudinal lengths based on the pulsewidth modulation principle, to achieve a magnetic field with excellent sinusoidal features. First, the topology structure of PM width modulation secondary is designed. The analytical model of the main magnetic field excited by PMs in this novel secondary is established, and the electromagnetic thrust is calculated. Furthermore, the influences of PM width modulation secondary on the main magnetic field and thrust properties are analyzed. The comparisons between two PMLSMs employing PM width modulation secondary and Halbach secondary, respectively, are provided. Finally, prototypes are manufactured and tested, to verify the effectiveness of the topology proposed in this article.

Analytical Airgap Field Model and Experimental Validation of Double Sided Hybrid Excited Linear Flux Switching Machine

Linear Flux Switching Machines (LFSMs) are suitable candidates for long stroke applications as they confines all excitation sources to primary thus leaving completely passive, robust, and low cost secondary. Permanent Magnet LFSMs (PMLFSMs) enables high thrust force density and efficiency. However, deficiency of controllable air-gap flux, risk of PM demagnetization, and increasing cost of rare earth PM materials diverted researchers towards Field Excited LFSMs (FELFSMs). FELFSMs wiped out aforementioned PMLFSM’s shortcomings at the cost of low thrust force density. In this paper, merits of PMLFSM and FELFSM are combined by proposing a novel Hybrid Excited LFSM (HELFSM). Proposed machine is excited by PMs, Field Excitation Coils (FECs), and Armature Windings (AWs). However, complex magnetic circuit of poly-excited HELFSM compels designers to adopt FE Analysis (FEA) for design, analysis, and optimization. To decrease dependency on computationally complex and time consuming FEA, an analytical model combining lumped parameter magnetic equivalent circuit, Fourier analysis, Laplace equation, and Maxwell Stress Tensor method is proposed to predict open-circuit flux linkage, B-EMF, normal and tangential components of no-load and on-load magnetic flux density, detent, and thrust force performance. Finally, predictions of the developed analytical model are validated with corresponding FEA and experimental results.

Analysis of Standby Losses and Charging Cycles in Flywheel Energy Storage Systems

Aerodynamic drag and bearing friction are the main sources of standby losses in the flywheel rotor part of a flywheel energy storage system (FESS). Although these losses are typically small in a well-designed system, the energy losses can become significant due to the continuous operation of the flywheel over time. For aerodynamic drag, commonly known as windage, there is scarcity of information available for loss estimation since most of the publications do not cover the partial vacuum conditions as required in the design of low loss energy storage flywheels. These conditions cause the flow regime to fall between continuum and molecular flow. Bearings may be of mechanical or magnetic type and in this paper the former is considered, typically hybridized with a passive magnetic thrust bearing. Mechanical bearing loss calculations have been extensively addressed in the open literature, including technical information from manufacturers but this has not previously been presented clearly and simply with reference to this application. The purpose of this paper is therefore to provide a loss assessment methodology for flywheel windage losses and bearing friction losses using the latest available information. An assessment of windage losses based on various flow regimes is presented with two different methods for calculation of windage losses in FESS under rarefied vacuum conditions discussed and compared. The findings of the research show that both methods closely correlate with each other for vacuum conditions typically required for flywheels. The effect of the air gap between the flywheel rotor and containment is also considered and justified for both calculation methods. Estimation of the bearing losses and considerations for selection of a low maintenance, soft mounted, bearing system is also discussed and analysed for a flywheel of realistic dimensions. The effect of the number of charging cycles on the relative importance of flywheel standby losses has also been investigated and the system total losses and efficiency have been calculated accordingly.

Edge effect of multi-degree-of-freedom oscillatory actuator driven by vector control

AbstractThis article proposes a thrust equation model of two-degree-of-freedom oscillatory actuator considering its edge effect. The proposed thrust equation model clarifies that asymmetric permanent magnet flux linkage and inductance characteristics cause undesirable magnetic thrust and reluctance thrust, respectively.

Design and optimization of permanent magnetic thrust bearing used in vessels

Design and optimization of permanent magnetic thrust bearing used in vessels

Analysis of plasma detachment in the magnetic thrust chamber using full particle-in-cell simulation

A magnetic nozzle, which is a convergent-divergent magnetic field to control a plasma flow, has been investigated for application to plasma propulsion systems in spaceships. In the magnetic nozzle, plasma thermal energy is converted to one-directional kinetic energy by Lorentz force to generate thrust. Although magnetic field structure and strength are optimized for improvement of the thrust performance, it is essential to understand physical processes of plasma ejection from the nozzle, because the plasma may flow back along lines of magnetic field if the directed plasma continues being strongly magnetized. It is assumed, for one of the scenarios to explain a plasma detachment, that a plasma detaches from magnetic field when a cyclotron radius exceeds a scale length of magnetic field in size. Hence, we investigate a plasma detachment condition by analyzing parameters of the plasma for unmagnetization. We assumed individual particle motion of a fully ionized plasma in the magnetic nozzle and conducted a full particle-in-cell simulation in a two-dimensional coordinate system. We calculated a ratio of cyclotron radius to a scale length of the magnetic field. The ratio increased in a downstream due to variation of magnetic field induced from a diamagnetic cavity, suggesting unmagnetization.

Optimization and dynamic characteristics of a new type of permanent magnet bias bearing

Unlike traditional magnetic thrust bearings, a new type of permanent magnet biased bearing structure made of soft magnetic composites in the bevel gear coupled rotor system was proposed that requires both lower eddy currents and greater thrust force. This paper presents the optimization of structure parameters, analysis of magnetic field and dynamic stiffness for two kinds of permanent magnet biased bearing. Firstly, the structural parameters for the bearing were optimized based on the adaptive particle swarm optimization. Secondly, the dynamic magnetic flux distribution for the permanent magnet biased bearing made of carbon steel at 1000 Hz was obtained by finite element method. Then, the equivalent reluctance models for the new type of permanent magnet biased bearing considering the effect of eddy currents are derived. Finally, based on the equivalent reluctance models, the dynamic force–current factor and force–displacement factor are given. The results show that the permanent magnet biased bearing made of soft magnetic composites can provide more stable electromagnetic force and larger bandwidth in the frequency range of 1000 Hz, and have greater electromagnetic force at the same frequency comparing with that made of carbon steel.

Mechanism Analysis of Eddy-Current Effects on Switching Power Amplifier of a Nonlaminated Magnetic Bearing

For thrust magnetic bearings (TMB), the stators and thrust disks are commonly made of nonlaminated material. Therefore, eddy currents are inevitable in solid ferromagnetic cores. However, the mechanisms of eddy-current effects on switching power amplifiers have never been investigated. Firstly, the fractional-order model of effective reluctance of a nonlaminated TMB is built. And the time domain solution of current ripples of switching power amplifiers is calculated. Then we define the effective inductance and present the equivalent circuit of current drive mode. Through model reduction, the main impact factor of eddy currents is found, and the simplified model as well as simplified equivalent circuit is obtained. Furthermore, the effects of eddy currents on two current control modes and control parameter design are analyzed. Through finite element method (FEM) and experiments, the established models are verified, and the current tracking performance is compared between laminated and nonlaminated TMB with different control parameters, which demonstrates the theoretical results.

Multi-Objective Optimization of Double Primary Tubular Permanent Magnet Synchronous Linear Motor in Wide Temperature Range Environment Based on Pareto Front Method

In order to improve the thrust density of the electric actuator in aircraft, a double primary tubular permanent magnet synchronous linear motor is designed. By establishing the magnetic circuit model, the air gap magnetic density and thrust characteristics of outer and inner primary motors are analyzed. Considering that it works in the wide temperature range environment, a multi-objective optimization of motor is carried out. The surrogate model established by neural network is used to simplify the mathematical model of the motor. In order to improve the average thrust, restrain the thrust fluctuation and reduce the difference of output thrust under different environment temperatures, the Pareto front method is used to determine the target range of the optimal parameters, and gradually strengthen conditions, so as to obtain the optimal scheme of motor design.

Analysis of eddy current field for new type of thrust bearings based on single-objective genetic algorithm

Based on the working condition of the magnetic thrust bearing (MTB), the single-objective optimization model (SOOM) is built. The optimum design of structure parameters for two types of magnetic bearings including traditional carbon bearing and soft magnetic composite (SMCs) bearing has been carried out using genetic algorithm. The analysis of eddy current field for two types of bearings are obtained by FEM. It is found that the maximum eddy current field of SMCs bearing is less than that of carbon steel one, and the air gap magnetic density is larger than that of carbon steel bearing at the same frequency. SMCs obtained by the compression of insulated soft magnetic powders have low eddy current. Thus, under the same magnetic force conditions, the input current of SMCs bearings is lower than that of carbon steel ones. The analysis shows that SMCs can replace carbon steel and has superiority in bearing application.

Rational Approximation of the Analytical Model of Nonlaminated Cylindrical Magnetic Actuators for Flux Estimation and Control

Eddy currents can significantly hamper the electromagnetic dynamics, especially those of nonlaminated actuators, and have thus been of great interest in actuator control for many years. Although there are high-fidelity models describing phenomena inside the solid core, such as the magnetic skin effect, these models have never been applied directly within the control circuit. The main reason for this is the lack of an appropriate approximation suitable for time-discrete digital implementation on a microcontroller. This article aims to provide a complete review about possible approaches to obtain a rational approximation of the frequency-dependent effective reluctance and inductance of an electromagnetic actuator. The rational approximation is then used to design a common infinite impulse response filter (IIR-filter). Using the example of a flux-controlled magnetic thrust bearing, we conclude with two novel approximations: a mathematically complex Pade approximation of a sum of continued fractions and a simple Matsuda approximation of an implicit equivalent fractional-order system. On the other hand, the former is the most accurate, and the latter allows wider approximation bandwidths. Depending on possible sampling and switching frequencies as well as the bandwidth of the actuator and computing capacity of the control unit, the most appropriate approximation for almost any use case can be selected.

Magnetic sail-based displaced non-Keplerian orbits

This paper deals with the problem of determining the requirements for the maintenance of circular, displaced, non-Keplerian orbits around the Sun by means of a magnetic sail-based spacecraft. The magnetic sail is an exotic propellantless propulsion system that gains thrust from the magnetostatic interaction between the solar wind and an artificial magnetic field, which is generated on board by an electrical current flowing through a loop of conducting material. The propulsive requirements are given in terms of characteristic acceleration and thrust (cone) angle. The analysis is performed using a recent mathematical model in which the magnetic sail thrust vector is expressed as a function of the Sun-spacecraft distance and the sail attitude. Moreover, a linear stability analysis is carried out to identify the (marginally) stable displaced orbits when an error in the orbital insertion is assumed.

Permanent magnet thrust bearings for flywheel energy storage systems: Analytical, numerical, and experimental comparisons

A new type of flywheel energy storage system uses a magnetic suspension where the axial load is provided solely by permanent magnets, whereas active magnetic bearings are only used for radial stabilization. This means that the permanent magnet bearing must provide all the axial damping. Furthermore, it must have as low a negative radial stiffness as possible to reduce the workload on the radial active magnetic bearings. Many different mathematical models for determining force, stiffness, and damping of permanent magnet bearings are available in the literature. This work will further develop the most applicable analytical and numerical methods in order to make them directly implementable for designing permanent magnet thrust bearings for flywheel energy storage systems. The outcome is a fast and efficient method for determining force, stiffness, and damping when the bearing setup contains magnetic materials with relative permeability higher than one as well as when it does not. The developed method is validated against numerical and experimental results with good agreement.