Cusped Field Thruster Research Articles

Simulation study of the influence of leak electrons on the discharge characteristics of a cusped field thruster

Previous studies have shown that leak electrons in cusped field thrusters can move along the channel axis to the anode after crossing the magnetic cusp on the exit. In this paper, a one-dimensional fluid model is built along two typical electron paths to study the influence of leak electrons on the discharge characteristics of a cusped field thruster, considering the electron temperature equation. It is found that the frequencies of low-frequency oscillations increase with a decrease in the proportion of leak electrons, which is related to an increase in the ion speed in the channel. Simulation results show that the position of the peak electron temperature is near the magnetic cusp on the exit and the position of the peak electron density is located downstream from the middle magnetic tip. With a decrease in the proportion of the leak electrons, the peak electron temperature and peak electron density decrease and the position of the peak electron density moves away from the exit, which is related to a decrease in the potential fall on the exit and an increase in confinement of electrons to the middle magnetic cusp.

Experimental study on the ionization regions in a multi-cusped field thruster

The multi-cusped field thruster is a novel electrostatic thruster with significant performance merits in spacecraft propulsion. In this paper, the ion energy distribution function (IEDF) in the plume region and its changing characteristics with anode voltage are tested to study the ionization regions in a two-stage cusped field thruster. Firstly, the IEDFs in two different operating modes are studied, the test result indicates that the plume region could be divided into three angle zones by the discontinuity of IEDFs. Secondly, the changing characteristics of IEDF in each angle zone are studied to analyze the location and the forming process of the corresponding ionization regions. The results show that the two-stage cusped thruster contains three ionization regions. The ions in zone II (from 15 to 40 degrees) are generated in columnar ionization region in discharge channel. The ions in zone III (above 40 degrees) are generated in the exit ionization region. The ions near axis in zone I (from 0 to 15 degrees) are generated in two significant different ionization regions. The high-energy ions are generated in the exit ionization region and the low-energy ions are generated in the plume ionization region. In this study, the number of the ionization regions is validated and the relationships between ion current in plume zones and ionization regions are specified, which is helpful to clarify the complicated ionization processes in the multi-cusped thruster.

The influence of anode position and structure on cusped field thruster

A cusped field thruster is a kind of electric propulsion device using multi-stage cusped fields to realize plasma discharges and produce thrust. A previous study showed that plasma discharges in this thruster are non-uniform. In this work, a multi-annulus anode is used to measure the radial distribution of anode current density at different anode positions. The experimental results reveal that some electrons may reach the anode along the axis after they accelerate from the final cusp regardless of the anode positions. To further validate this idea and find out the mechanism of this central path along the axis, the central part of the anode is replaced with ceramics. This results in an increase in the total current with larger contributions at larger radii. The current oscillations also get larger. This brief letter is helpful to further understand the movement of electrons in cusped field thrusters and provide guidance on reducing the non-uniform degree of current density.

Experimental investigation of the effects of variable expanding channel on the performance of a low-power cusped field thruster

Due to a special magnetic field structure, the multi-cusped field thruster shows advantages of low wall erosion, low noise and high thrust density over a wide range of thrust. In this paper, expanding discharge channels are employed to make up for deficiencies on the range of thrust and plume divergence, which often emerges in conventional straight cylindrical channels. Three thruster geometries are fabricated with different expanding-angle channels, and a group of experiments are carried out to find out their influence on the performance and discharge characteristics of the thruster. A retarding potential analyzer and a Faraday probe are employed to analyze the structures of the plume in these three models. The results show that when the thrusters operate at low mass flow rate, the gradually-expanding channels exhibit lower propellant utilization and lower overall performance by amounts not exceeding 44.8% in ionization rate and 19.5% in anode efficiency, respectively. But the weakening of magnetic field intensity near the exit of expanding channels leads to an extended thrust throttling ability, a smaller plume divergence angle, and a relatively larger stable operating space without mode converting and the consequent performance degradation.

Cusped field thruster using different propellants

Cusped field thruster is a new kind of thruster which confines plasma by magnetic mirror effect to produce thrust. It is characterized by long lifespan and adjustable thrust in a large range, which makes it have great potential applications in drag free satellites and commercial space satellites. It was put forward first by THALES Electron Devices in Germany and sponsored from European Space Agency. There are several institutions are engaged in the research of this thruster, including Massachusetts Institute of Technology, Stanford University and Technische Universiteit Delft. Now the test experiments on the cusped field thruster using Xe, Kr and Ar are being carried out in the laboratory of plasma propulsion of Harbin Institute of Technology to ascertain the ionization regulations of different propellants under the high voltage and strong magnetic field conditions. On this basis, it is significant to know the mechanism about how the performances change with propellant and provide the foundation for the cusped field thruster using different propellants. In this paper, the principle and design process of this thruster are presented. Then it can be found that the thruster can be ignited easily by using Xe compared with by using Kr and Ar under the same volume flux, which is caused by their differences in ionization energy and ionization section. Experiments show that the cusped field thruster can be ignited under 200 V while it cannot be ignited by using Kr and Ar even under 1000 V under the same volume flux. Then the performances of cusped field thruster using three propellants are tested. It can be found that there are obvious differences in anode current, thrust, efficiency and impulse using three propellants under the same conditions. The diagnosing of plume using Faraday probe shows that the propellant utilization causes the difference in performance which is related to ionization process. The experiments show that the utilization rate of Xe is over 90 percent, while the utilization rate of Kr is less than 60 percent and the utilization rate of Ar is less than 20 percent. The obvious difference in ionization voltage can reflect the difference in performance. The experimental results under the same flux show that the utilization rates of Kr and Ar can be improved by increasing flow density and reducing the collision free path between atoms. Experiments show that the peak utilization rate of Ar can be improved to 50 percent approximately. In the aspect of plume structure, the results of Faraday probe show that the hollow plume can be observed and the angle linked with peak ion current density decreases with atom mass decreasing.

Effects of cusped field thruster on the performance of drag-free control system

With increased measurement tasks of space science, more requirements for the spacecraft environment have been put forward. Those tasks (e.g. the measurement of Earth's steady state gravity field anomalies) lead to the desire for developing drag-free control. Higher requirements for the thruster performance are made due to the demand for the drag-free control system and real-time compensation for non-conservative forces. Those requirements for the propulsion system include wide continuous throttling ability, high resolution, rapid response, low noise and so on. As a promising candidate, the cusped field thruster has features such as the high working stability, the low erosion rate, a long lifetime and the simple structure, so that it is chosen as the thruster to be discussed in this paper. Firstly, the performance of a new cusped field thruster is tested and analyzed. Then a drag-free control scheme based on the cusped field thruster is designed to evaluate the performance of this thruster. Subsequently, the effects of the thrust resolution, transient response time and thrust uncertainty on the controller are calculated respectively. Finally, the performance of closed-loop system is analyzed, and the simulation results verify the feasibility of applying cusped field thruster to drag-free flight in the space science measurement tasks.

High Fidelity Multi-Objective Design Optimization of a Downscaled Cusped Field Thruster

The Cusped Field Thruster (CFT) concept has demonstrated significantly improved performance over the Hall Effect Thruster and the Gridded Ion Thruster; however, little is understood about the complexities of the interactions and interdependencies of the geometrical, magnetic and ion beam properties of the thruster. This study applies an advanced design methodology combining a modified power distribution calculation and evolutionary algorithms assisted by surrogate modeling to a multi-objective design optimization for the performance optimization and characterization of the CFT. Optimization is performed for maximization of performance defined by five design parameters (i.e., anode voltage, anode current, mass flow rate, and magnet radii), simultaneously aiming to maximize three objectives; that is, thrust, efficiency and specific impulse. Statistical methods based on global sensitivity analysis are employed to assess the optimization results in conjunction with surrogate models to identify key design factors with respect to the three design objectives and additional performance measures. The research indicates that the anode current and the Outer Magnet Radius have the greatest effect on the performance parameters. An optimal value for the anode current is determined, and a trend towards maximizing anode potential and mass flow rate is observed.

Performance Characterization of the Low-Power Halo Electric Propulsion System

Performance measurements have been obtained of a novel propulsion concept called the Halo thruster under development within the University of Surrey. The Halo thruster, a type of cusped-field thruster with close similarity to the cylindrical Hall thruster, is motivated by the need for low-power and low-cost electric propulsion for the small satellite sector. Two versions of the device are investigated in this study: a design using permanent magnets at high magnetic-field strength and a design using electromagnets with moderate field strength. While operating at 200 W discharge power, which is of particular interest to power-limited small satellite platforms, the permanent-magnet design achieved a maximum thrust efficiency of 8% at a specific impulse of approximately 900 s using a krypton propellant. By comparison, the electromagnet design achieved a maximum thrust efficiency of 28% at a specific impulse of approximately 1500 s at 200 W using a xenon propellant. For higher levels of power (tested up to 800 W), the performance of the electromagnetic design saturated at approximately 25% thrust efficiency using krypton and 30% using xenon. The thrust efficiency of the permanent-magnet design appeared to increase monotonically up to 600 W reaching a maximum value of 14%.

Design of a cusped field thruster for drag-free flight

Drag-free flight has played a more and more important role in many space missions. The thrust control system is the key unit to achieve drag-free flight by providing a precise compensation for the disturbing force except gravity. The cusped field thruster has shown a significant potential to be capable of the function due to its long life, high efficiency, and simplicity. This paper demonstrates a cusped field thruster's feasibility in drag-free flight based on its instinctive characteristics and describes a detailed design of a cusped field thruster made by Harbin Institute of Technology (HIT). Furthermore, the performance test is conducted, which shows that the cusped field thruster can achieve a continuously variable thrust from 1 to 20mN with a low noise and high resolution below 650W, and the specific impulse can achieve 1800s under a thrust of 18mN and discharge voltage of 1000V. The thruster's overall performance indicates that the cusped field thruster is quite capable of achieving drag-free flight. With the further optimization, the cusped field thruster will exhibit a more extensive application value.

Anode current density distribution in a cusped field thruster

The cusped field thruster is a new electric propulsion device that is expected to have a non-uniform radial current density at the anode. To further study the anode current density distribution, a multi-annulus anode is designed to directly measure the anode current density for the first time. The anode current density decreases sharply at larger radii; the magnitude of collected current density at the center is far higher compared with the outer annuli. The anode current density non-uniformity does not demonstrate a significant change with varying working conditions.

Effect of the variable cross-section channel on performance of a cusped field thruster at low power

The cusped field thruster has drawn much attention from many institutions due to its wide thrust range, high impulse, and long lifetime. However, lots of experimental results reveal that the cusped field thruster at low power has a poor performance. A cusped field thruster with a variable cross-section channel by putting a ceramic spacer in the channel is introduced in this paper, aimed at improving the thruster performance at low power. The DSMC results validate that the upstream atom density can be increased by a spacer, especially near the wall. Based on simulated results, spacers are put into different positions of the channel. The experimental results show that a suitable spacer can enhance thruster performance at low power, which can be confirmed by the results that the anode efficiency can achieve 40% at 400 V anode voltage and 20 sccm gas flow rate by contrast to 35% without a spacer under the same condition.

Particle-in-cell simulation for different magnetic mirror effects on the plasma distribution in a cusped field thruster**Project supported by the National Natural Science Foundation of China (Grant No. 51006028) and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51121004).

Magnetic mirror used as an efficient tool to confine plasma has been widely adopted in many different areas especially in recent cusped field thrusters. In order to check the influence of magnetic mirror effect on the plasma distribution in a cusped field thruster, three different radii of the discharge channel (6 mm, 4 mm, and 2 mm) in a cusped field thruster are investigated by using Particle-in-Cell Plus Monte Carlo (PIC-MCC) simulated method, under the condition of a fixed axial length of the discharge channel and the same operating parameters. It is found that magnetic cusps inside the small radius discharge channel cannot confine electrons very well. Thus, the electric field is hard to establish. With the reduction of the discharge channel’s diameter, more electrons will escape from cusps to the centerline area near the anode due to a lower magnetic mirror ratio. Meanwhile, the leak width of the cusped magnetic field will increase at the cusp. By increasing the magnetic field strength in a small radius model of a cusped field thruster, the negative effect caused by the weak magnetic mirror effect can be partially compensated. Therefore, according to engineering design, the increase of magnetic field strength can contribute to obtaining a good performance, when the radial distance between the magnets and the inner surface of the discharge channel is relatively big.

Simulations on the influence of the spatial distribution of source electrons on the plasma in a cusped-field thruster

Abstract: We present results from simulations on the influence of source electrons on the plasma properties in a magnetic cusps environment. Our simulations are based on the VSim/Vorpal particle-in-cell plasma simulation package. Magnetic cusps are a typical feature of High Efficiency Multistage Plasma Thrusters (HEMPTs). This research is part of an effort to downscale a HEMPT to thrust levels in the μN and sub-μN regime. The aim is to fulfill the requirements of upcoming formation flight satellites and probes. Those missions demand very precise attitude control. In order to get the necessary insight, the plasma of a section of the HEMPT discharge chamber is simulated with idealized boundary conditions. The results for such a section at two different distributions of source electrons are shown. A significant increase of the overall ion number is recognized for one of the distributions. Comparisons with published similar simulations are made. Factors that should be important for improvements of this thruster type are highlighted.

Fluid Simulation of a Cusped Field Thruster

AbstractThe cusped field thruster is a kind of newly developed electric propulsion device. The electric field at the channel exit and the low frequency oscillation were measured by former experiments. While the formation mechanism of them have not been fully interpreted yet. Through studying two distinguishing typical electron paths in the thrusters, a fluid model is built up along two electron paths, and then the model is completed by synthetically analyzing the effect of two electron paths. Time‐averaged electric potential distribution, anode current oscillation curve and time‐synchronized atom density and ion density distribution are obtained by simulation. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental study on a variable magnet length cusped field thruster

The cusped field thruster is an emerging kind of electric propulsion equipment employing a cusped magnetic field to achieve potentially long lifetime. As important factors contributing to the thruster performance, the optimal channel length and magnetic field design are still uncertain. In order to research how magnet length affects performance, a variable magnet length cusped field thruster (VML-CFT) was designed and tested in Harbin Institute of Technology (HIT). By using modular magnets, VML-CFT can vary the length of each cusp stage. The design of VML-CFT is demonstrated in this paper. Experimental results show that certain length increase of the middle stage can improve the thruster performance. By analyzing experimental results with different magnet lengths, the thruster internal physical process can be inferred.

Plume Control of a Cusped Field Thruster

The cusped field thruster uses several permanent ring magnets of alternating polarity to create a multistage cusp magnetic topology. It has demonstrated a long lifetime because of reduced plasma losses to the discharge channel surface. However, the high beam divergence problem has not been solved in this type of thruster. The purpose of this technical note is to describe a plume control method to reduce its plume angle. The results show that the magnetic field intensity in the plume region and the shape of the last separatrix have great influence on plume divergence.

Ion Velocimetry Measurements and Particle-In-Cell Simulation of a Cylindrical Cusped Plasma Accelerator

The Stanford Cylindrical Cusped Field Thruster (CCFT) has been experimentally and numerically investigated with particular focus on the exit plane acceleration region near the top magnetic cusp. Time-averaged xenon ion laser-induced fluorescence measurements using the $5d[{4}]_{7/2} - 6p[{3}]_{5/2}$ ( $\lambda = 834.72$ -nm air) Xe II transition have mapped the total ion velocity vectors in this region. The thruster is also simulated using the fully kinetic 3-D particle-in-cell code F3MPIC. The consistent experimental and numerical results give physical insight into the mechanisms of ion acceleration and the role of the magnetic field topology in determining ion trajectories and plume divergence. The electrons are strongly magnetized and follow the magnetic field structure, grouping near the cusps. A steep potential drop over a few millimeters near the exit plane follows the magnetic separatrix of the top cusp, and is consistent with measured ion velocity vectors. A characteristic conical region of high ion density, peak ion velocity, and visible emission is observed in the experimental and simulated plume, with an estimated divergence half-angle of 30°.

Time-synchronized continuous wave laser-induced fluorescence axial velocity measurements in a diverging cusped field thruster

Measurements are presented of time-synchronized axial ion velocities at three positions in the discharge channel and plume of a diverging cusped field thruster operating on xenon. Xenon axial ion velocities for the thruster are derived from laser-induced fluorescence measurements of the 5d[4]7/2–6p[3]5/2 xenon ion excited state transition centred at λ = 834.72 nm. The thruster is operated in a high-current mode, where the anode discharge current is shown to oscillate quasi-periodically. A sample-hold scheme is implemented to correlate ion velocities to phases along the current cycle. These time-synchronized measurements show that median axial ion velocities decrease as discharge current increases, and that the widths of ion velocity distributions increase with increases in discharge current for positions at the exit plane and outside the thruster channel.

Particle-in-cell simulations for the effect of magnetic field strength on a cusped field thruster

The particle-in-cell plus Monte Carlo method is used to simulate a cusped field thruster. Three different values of magnetic field strength are simulated and convergent results are obtained successfully. The simulation results indicate that with the increase of the magnetic field strength, electron density peak tends to move in the radial direction towards the axis of the discharge channel. The central gap area of electron density is decreased due to the stronger electron confinement, which correspondingly reduces the inevitable central electron leak in the cusped field thrusters. However, it is found that the stronger confinement of electrons reduces electron density near the wall, which increases the neutral gas leak there. It is also validated that the main potential drop in the discharge channel appears near the channel exit. With the enhancement of magnetic field strength, potential drops at separatrices increase correspondingly. At the same time, several potential wells and barriers can be found near the wall, which accords well with recent theoretical and experimental results.

Erosion Measurements in a Low-Power Cusped-Field Plasma Thruster

Results from a long duration test of a cusped-field plasma thruster operating at an anode power of 165 W are presented and discussed. Profile measurements of the boron nitride insulator were performed before and after the 204 h test, enabling the quantification of average erosion rates over a large portion of the interior. Unlike most Hall thrusters, the lifetime is not limited by erosion near the exit plane, due to the positioning of magnetic circuit elements. Additionally, the maximum erosion rate is found to be lower than rates measured in low-power Hall thrusters. The lifetime of this laboratory prototype is estimated at 920–1220 h based on the time needed to erode through the insulator in one of the ring cusps. However, as an indicator of the potential for cusped-field thruster designs to obtain longer lifetimes, the low erosion rate measurements are more significant. For example, fortifying cusps with additional or more durable material could increase lifetime. A summary of long duration tests and erosion measurements in Hall thrusters is also provided. During the tests discussed here, the thruster operated in the “high-current” mode, characterized by strongly oscillatory anode currents. Peak erosion rates are located upstream of the exit in this mode of operation. Reliable performance measurements were not acquired during this test, but anode current values suggest the thruster did not operate with the desired anode efficiency.