Influence of channel length on discharge performance of anode layer Hall thruster studied by particle-in-cell simulation

Hall thruster has the advantages of simple structure, high specific impulse, high efficiency, and long service life, and so on. It is suitable for spacecraft attitude control, North and South position keeping, and other track tasks. The anode layer Hall thruster is a kind of Hall thruster. The thruster has a longer anode area and a relatively short discharge channel.In this paper, the effect of the channel length on the performance of the anode layer Hall thruster is simulated by the PIC simulation method. The simulation results show that the change of the channel length has significant effect on the plasma parameters, such as potential and plasma density and so on. The ionization region mainly concentrates at the hollow anode outlet position, and can gradually move toward the channel outlet as the channel length decreases. The collision between the ions and the wall increases with the channel length increasing. So the proper shortening of the channel length can increase the life of the thruster. Besides, the results show that there is a best choice of the channel length for obtaining the best performance. In this paper, thruster has the best performance under a channel length of 5 mm.     

Particle-in-cell simulation for the effect of magnetic cusp on discharge characteristics in a cylindrical Hall thruster

The cylindrical Hall thruster has the good prospect of serving as a miniaturized electric propulsion device.A 2 D-3 V particle-in-cell plus Monte Carlo(PIC-MCC) method is used to study the effect of the magnetic cusp on discharge characteristics of a cylindrical Hall thruster.The simulation results show that the main ionization region and the main potential drop of the thruster are located at the upstream of the discharge channel.When the magnetic cusp moves toward the anode side,the main ionization region is compressed and weakened,moving upstream correspondingly.The ionization near the cusp is enhanced,and the interaction between the plasma and the wall increases.The simulation results suggest that the magnetic cusp should be located near the channel exit.     

霍尔推进器壁面材料二次电子发射及鞘层特性

霍尔推进器放电通道等离子体与壁面相互作用形成鞘层,不同壁面材料的二次电子发射对推进器鞘层特性具有重要影响,本文针对推进器壁面鞘层区域建立二维物理模型,研究了氮化硼(BN)、碳化硅(SiC)和三氧化二铝(Al_2O_3)三种不同壁面材料的二次电子发射特性,在改进SiC材料二次电子发射模型的基础上,采用粒子模拟方法,讨论了壁面二次电子发射系数与电子温度和磁场强度的关系,研究了三种材料(BN,SiC和Al_2O_3)的鞘层特性,结果表明:修正的二次电子发射模型拟合曲线与实验曲线几乎一致;在相同电子温度下,三种材料(BN,SiC和Al_2O_3)的二次电子发射系数和壁面电子数密度依次增大,而鞘层电场和鞘层电势降依次减小,BN材料具有合适的二次电子发生射系数,使得霍尔推进器能在低电流下稳态工作。     

霍尔推力器热模型研究

为了对霍尔推力器的热分析研究提供准确的能耗加载条件,开展了霍尔推力器稳态工况下的热模型研究。基于等离子体理论,分析放电室内各项能量损耗机理,并建立各能量损耗与推力器工作参数、性能参数和结构参数的相关函数,系统地得到了霍尔推力器的完整热模型。以LHT100推力器为研究对象,热模型计算结果显示:额定工况下束流能量损耗约889 W,壁面能量损耗约300 W,阳极能量损耗约44 W,电离能量损耗约43 W,辐射能量损耗约34 W等。以此能量损耗作为热边界条件进行有限元分析,并开展热平衡试验进行验证,计算结果与试验结果吻合较好,最大误差小于5%。     

Laser-induced fluorescence measurements of velocity within a Hall discharge

The results of a study of laser-induced fluorescence velocimetry of neutral and singly ionized xenon in the plume and interior portions of the acceleration channel of a Hall thruster plasma discharge operating at powers ranging from 250 to 725 W are described. Axial ion and neutral velocity profiles for four discharge voltage conditions (100 V, 160 V, 200 V, 250 V) are measured as are radial ion velocity profiles in the near-field plume. Ion velocity measurements of axial velocity both inside and outside the thruster as well as radial velocity measurements outside the thruster are performed using laser-induced fluorescence with nonresonant signal detection on the xenon ion 5d[4]_7/2–6p[3]_5/2 excitation transition while monitoring the signal from the 6s[2]_3/2–6p[3]_5/2transition. Neutral axial velocity measurements are similarly performed in the interior of the Hall thruster using the 6s[3/2]⁰ ₂–6p[3/2]₂transition with resonance fluorescence collection. Optical access to the interior of the Hall thruster is provided by a 1-mm-wide axial slot in the insulator outer wall. While the majority of the ion velocity measurements used partially saturated fluorescence to improve the signal-to-noise ratio, one radial trace of the ion transition was taken in the linear fluorescence region and yields a xenon ion translational temperature between 400 and 800 K at a location 13 mm into the plume. Received: 27 September 2000 / Revised version: 2 March 2001 / Published online: 9 May 2001     

On the Effect of the Surface State and Geometry of the Discharge- Chamber Edges and Sizes of Morozov’s Stationary Plasma Thruster upon its Long-Term Operation

The results of long-term tests of Morozov’s stationary plasma thrusters are presented. It is revealed how the surface state and geometry of the discharge chamber’s edges influence the thruster’s parameters. It is shown that, during the ground tests of thrusters with cylindrical geometry of the acceleration channel under initial stage of operation, material sputtered from the discharge chambers’ walls is deposited onto the nearanode segment of the walls. Films of deposited material fail during thruster operation causing fragment formation, which jut out towards the discharge volume and disturb the motion of drifting electrons in the area of their acceleration. As a result the thruster reactive force and specific impulse decrease. The way in which the forming fragments influence thruster performance and operation is examined. It is shown that it decreases under long-term operation and significant channel widening since the ion flux to the wall and the quantity of the sputtered material decrease, and since the profile of the walls changes due to their wear and cleaning effect of the discharge. As a result the thruster’s parameters are restored to a level close to the initial one. It is shown that the dynamics of thruster parameters variation in space and during ground tests is different. This means that it is necessary to simulate more properly the conditions of thruster operation in space when conducting ground development tests. Thrusters with a long lifetime should be designed with widening of the acceleration channel beyond the loop which surrounds the magnetic system so that areas of acceleration and the erosion of walls are located in the widened part of the channel.     

Surface‐Driven Asymmetry and Instability in the Acceleration Region of Hall Thruster

It has been shown experimentally that the channel wall material has a substantial effect on the behaviour of Hall discharges. For this reason, the radial profile inside the Hall thruster SPT‐100 is investigated in detail. This is done by a one‐dimensional fully kinetic self‐consistent Particle‐in‐Cell model between the two walls in the acceleration region of the channel. A detailed Monte Carlo probabilistic model for secondary electron emission is implemented as boundary module. Using the local field approximation, two different operative conditions (axial electric field E_z =100 V/cm and 300 V/cm) have been simulated. For high discharge voltage case, a strong radial asymmetry and a stream instability propagating all along the radial domain are detected, while in the low voltage case a stable classical situation is recovered. The critical parameters for triggering this unstable regime are the electron azimuthal drift energy and the induced secondary electron emission, while the saturation mechanism is the increasing of the temperature of the initially cold secondary‐electrons. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

离子推力器放电室能量平衡研究

为探索放电室能量损耗机制,开展了离子推力器放电室能量平衡研究。基于放电室零维模型,得到放电室电流平衡关系,结合放电室电势分布,分析放电室能量损耗并建立了能量平衡模型。应用模型计算LIPS200离子推力器放电室各项能量损耗,并进一步得到各能量损耗所占比例,所得结果与国外离子推力器NEXT具有较好的一致性;采用多工况试验参数(阳极电流4.0~4.4A,阳极电压34~38V)对放电室总能量损耗进行动态验证,结果表明:计算结果与试验结果误差小于3%。     

Particle‐in‐cell simulation of the effect of curved magnetic field on wall bombardment and erosion in a hall thruster

A curved, convex towards the channel bottom magnetic field is an important feature of an advanced Hall thruster that allows confining the plasma flow in the channel center, reducing the divergence angle of the ejected ion beam, and improving the discharge performance. In this article, the discharge behaviour of a Hall thruster in magnetic fields with different degrees of curvature is simulated with a particle‐in‐cell numerical method, and the effect of curved magnetic field on the ion bombardment and wall erosion and the associated mechanisms are studied and analysed. The results show that, as the curvature of the magnetic field increases, the propellant ionization becomes more confined at the channel center, the potential drop inside the channel decreases, and the acceleration region shifts outside the channel, which lead to the attenuation of the ion energy bombarding the wall and the deviation of the bombardment angle from the optimal sputtering angle. Conversely, the ion flux bombarding the wall near the channel exit increases. Nevertheless, the bombardment energy and angle are the dominant factors for the wall erosion, and the wall erosion rate clearly decreases with the increasing curvature of the magnetic field. These findings are closely related to the behaviour of electron conduction under a curved magnetic field; the relevant mechanisms are clarified in this article.     

Particle‐in‐Cell Simulations for Ion Thrusters

The Particle‐in‐Cell (PIC) method was used to study two different ion thruster concepts: Hall Effect Thrusters (HETs) and High Efficiency Multistage Plasma Thrusters (HEMPs), in particular the plasma properties in the discharge chamber due to the different magnetic field configurations. Special attention was paid to the simulation of plasma particles fluxes on the thrusters inner surfaces. In both cases PIC proved itself as a powerful tool, delivering important insight into the basic physics of the different thruster concepts.The simulations demonstrated that the new HEMP thruster concept allows for a high thermal efficiency due to both minimal energy dissipation and high acceleration efficiency. In the HEMP thruster the plasma contact to the wall is limited only to very small areas of the magnetic field cusps, which results in much smaller ion flux to the thruster channel surface as compared to HET. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental and numerical investigation of a Hall thruster with a chamfered channel wall

A discharge channel with a chamfered wall not only has application in the design of modern Hall thrusters, but also exists where the channel wall is eroded, and so is a common status for these units. In this paper, the laws and mechanisms that govern the effect of the chamfered wall on the performance of a Hall thruster are investigated. By applying both experimental measurement and particle-in-cell simulation, it is determined that there is a moderate chamfer angle that can further improve the optimal performance obtained with a straight channel. This is because the chamfering of the wall near the channel exit can enhance ion acceleration and effectively reduce ion recombination on the wall, which is favorable to the promotion of the thrust and efficiency. However, the chamfer angle should not be too large; otherwise, both the density of the propellant gas and the distribution of the plasma potential in the channel are influenced, which is undesirable for efficient propellant utilization and beam concentration. Therefore, it is suggested that the chamfer shape of the channel wall is an important factor that must be carefully considered in the design of Hall thrusters.     

Numerical study on the electron-wall interaction in a Hall thruster with segmented electrodes placed at the channel exit

Electron-wall interaction is always recognized as an important physical problem because of its remarkable influences on thruster discharge and performance. Based on existing theories, an electrode is predicted to weaken electron-wall interaction due to its low secondary electron emission characteristic. In this paper, the electron-wall interaction in an Aton-type Hall thruster with low-emissive electrodes placed near the exit of discharge channel is studied by a fully kinetic particle-in cell method. The results show that the electron-wall interaction in the region of segmented electrode is indeed weakened, but it is significantly enhanced in the remaining region of discharge channel. It is mainly caused by electrode conductive property which makes equipotential lines convex toward channel exit and even parallel to wall surface in near-wall re- gion; this convex equipotential configuration results in significant physical effects such as repelling electrons, which causes the electrons to move toward the channel center, and the electrons emitted from electrodes to be remarkably accelerated, thereby increasing electron temperature in the discharge channel, etc. Furthermore, the results also indicate that the discharge current in the segmented electrode case is larger than in the non-segmented electrode case, which is qualitatively in accordance with previous experimental results.     

Effect of permanent magnet configuration on discharge characteristics in cylindrical Hall thrusters

Magnetic field design is important in cylindrical Hall thrusters and using permanent magnets to generate magnetic field is very promising in the future. In two typical permanent magnet configurations (i.e., ring and cylindrical configurations) of cylindrical Hall thrusters, discharge characteristics are compared in this paper through the experiments and simulations. The study shows that the cylindrical configuration can bring about higher thruster performance in the same working condition. The reason is that the potential drop of the cylindrical configuration is mainly concentrated in the channel, which is beneficial for the electrons to obtain energy to promote the ionization of the propellant. However, the voltage regulation range of the cylindrical configuration is lower because the anode is more easily overheated.     

电子温度对霍尔推进器等离子体鞘层特性的影响

采用二维粒子模拟方法研究了霍尔推进器通道中电子温度对等离子体鞘层特性的影响, 讨论了不同电子温度下电子数密度、鞘层电势、电场及二次电子发射系数的变化规律. 结果表明: 当电子温度较低时, 鞘层中电子数密度沿径向方向呈指数下降, 在近壁处达到最小值, 鞘层电势降和电场径向分量变化均较大, 壁面电势维持一稳定值不变, 鞘层稳定性好; 当电子温度较高时, 鞘层区内与鞘层边界处电子数密度基本相等, 而在近壁面窄区域内迅速增加, 壁面处达到最大值, 鞘层电势变化缓慢, 电势降和电场径向分量变化均较小, 壁面电势近似维持等幅振荡, 鞘层稳定性降低; 电子温度对电场轴向分量影响较小; 随电子温度的增大, 壁面二次电子发射系数先增大后减少. 关键词： 霍尔推进器 等离子体鞘层 电子温度 粒子模拟     

Study on the Relation Between Discharge Voltage and Magnetic Field Topography in a Hall Thruster Discharge Channel

The relation between magnetic field topography and operating voltage is investigated in a 1kW Hall thruster discharge channel in order to focus the ion beam effectively and optimize the performance. The curvature of magnetic field line (α) is introduced to characterize the differences of topologies. The optimized magnetic field distribution under each operating voltage is obtained by experiment. Through the curvature transformation, we find that the area of (α > 1) in the channel gradually decreases with the increase of the operating voltage. In response to the results above, two dimensional plasma flows are simulated employing Particle‐in‐Cell method. The distributions of the electric potential, ion density and ion radial velocity are calculated to understand the important influence of the relation above on ion beam focusing. The numerical results indicate that magnetic field curvature and thermal electric field control the ion beam in the ionization and acceleration zone, respectively. The magnetic field topography and discharge voltage interact with each other and together form the focusing electric field. The ion radial mobility is suppressed effectively and the ion beam is focused to the channel centerline. In addition, for a given voltages, when the area of (α > 1) is larger than the optimal scope, the electric potential lines excessively bend to the anode causing ion over focus; contrarily, the electric potential lines will bend to the exit and defocus ions. All these results suggest the relation between magnetic field topography and discharge voltage is important to the ion radial flow control and performance optimization of the Hall thruster (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

圆柱形霍尔推进器的三维刻蚀模拟与实验研究

通过使用数值模拟和实验相结合的方法研究圆柱形霍尔等离子体推进器。应用蒙特卡洛方法和Particle In Cell (PIC)方法对放电通道内等离子体碰撞和行为进行模拟。建立圆柱形霍尔推进器的物理和数值模型;通过对放电和加速区等离子体的产生和输运进行模拟,掌握了等离子体放电和加速机理以及内磁极的刻蚀
情况。结果表明：随着电压的升高,内磁极刻蚀较为严重;推进器内部离子能量值约为放电电压值的50%左右。同时通过实验方法测定不同放电电压情况下推进器的放电性能。     

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通过使用数值模拟和实验相结合的方法研究圆柱形霍尔等离子体推进器。应用蒙特卡洛方法和Particle In Cell(PIC)方法对放电通道内等离子体碰撞和行为进行模拟。建立圆柱形霍尔推进器的物理和数值模型;通过对放电和加速区等离子体的产生和输运进行模拟,掌握了等离子体放电和加速机理以及内磁极的刻蚀情况。结果表明:随着电压的升高,内磁极刻蚀较为严重;推进器内部离子能量值约为放电电压值的50%左右。同时通过实验方法测定不同放电电压情况下推进器的放电性能。     

电子温度各向异性对霍尔推力器BN绝缘壁面鞘层特性的影响

建立多价态多组分等离子体一维流体鞘层模型,引入电子温度各向异性系数并考虑出射电子速度分布,研究了电子温度各向异性对霍尔推力器中的BN绝缘壁面鞘层特性和近壁电子流的影响.分析结果表明,相比于纯一价氙等离子体鞘层参数,推力器中的多价态氙等离子体鞘层电势降略有降低,电子壁面损失增加,临界二次电子发射系数减小.推力器中的电子温度各向异性现象可以显著地加大出射电子能量系数,进而降低鞘层电势降,增强电子壁面相互作用.数值结果表明,空间电荷饱和机制下电子温度各向异性对鞘层空间电势分布影响显著. 关键词： 霍尔推力器 电子温度各向异性 空间电荷饱和鞘层     

Experimental observations of steady anodic vacuum arcs withthermionic cathodes

Stationary plasma discharges have been investigated in a high vacuum ambient (background gas pressure <10^-2 Pa), with an externally heated cathode and a consumable hot evaporating anode. With various anode materials like chromium or copper, and electrode separations between 0.5 and 3 mm, the nonself-sustained discharge operates with DC arc currents in the range of 220 A. The waveform of the arc voltage is strongly influenced by the magnetic field of the cathode heating current, and arc voltages between a minimum of 3 V and a maximum exceeding 100 V have been observed. The voltage-current characteristics (V_CC) and the influence of the electrode separation have been measured separately for the minimum and the maximum of the arc voltages and show a different behavior. The metal plasma expands into the ambient vacuum toward the walls of the vacuum vessel and offers a macroparticle free deposition source of thin films. The arc voltage can be varied by external manipulations of the arc discharge, and the mean ion energy of the expanding metal plasma shows a linear dependence of the mean arc voltage     

Particle‐in‐Cell Simulation for the Effect of Segmented Electrodes Near the Exit of an Aton‐Type Hall Thruster on Ion Focusing Acceleration

The effect of floating conductive electrodes near the channel exit of an Aton‐type Hall thruster on ion focusing acceleration is studied by simulating the two‐dimensional plasma flow with a fully kinetic Particle‐in‐Cell method for the gas flow rate j_a ranged in 1～3 mg/s. Numerical results show that low‐emissive electrodes can reduce plume divergence if the electrode length is less than 2 mm due to the low secondary electron emissive characteristic, but widen plume in all the gas flow rate range if the electrode length is greater than 2mm since the conductive property of segmented electrodes trends to make equipotential lines convex toward channel exit and is even parallel to the wall surface in the near‐wall region. Further investigation predicts that the combination of high emissive dielectric wall and segmented low‐emissive dielectric wall is a promising way to reduce plume divergence (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)