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.     

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.     

A 200-W permanent magnet Hall thruster discharge with graphite channel wall

A 200-W Hall thruster was designed using permanent magnets placing the axial maximum magnetic field outside of the channel, and the anode/gas distributor as an integrated U-shaped structure. A split structure is adopted for the discharge channel to conveniently change the wall material. With the initial conditions that the anode mass flow is maintained at 0.8 mg/s, 1.0 mg/s and 1.2 mg/s over a voltage range of 150–400 V (at 50 V intervals), the discharge properties were determined for the two thruster models with the channel walls composed of graphite and boron nitride (BN). The results demonstrate that under identical operating parameters, the properties of the thruster with the graphite channel walls are similar to the properties of the thruster with the BN channel walls. The maximum difference of the discharge current between the two wall materials is 6.2%; the maximum difference of the thrust and the specific impulse is 3.3%, and that of the anode efficiency is 1.7% (absolute value). These differences are smaller than the corresponding parameter differences observed from changing wall materials in other common Hall thrusters.     

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%。     

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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。     

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。     

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)     

外磁场作用下的磁等离子体动力学过程仿真

磁等离子体动力学推力器是空间高功率电推进装置的典型代表,磁等离子体动力学过程是其核心工作机制.为深入理解外磁场对其工作特性的影响,本文采用粒子云(particle in cell,PIC)方法结合基于自相似准则的缩比模型,进行外加磁场作用下磁等离子体动力学推力器工作过程的建模仿真,通过与实验结果对比验证模型和方法的可靠性,并重点分析推力器点火启动过程的等离子特性参数分布,以及外磁场和阴极电流对推力器工作性能的影响.研究结果表明:阴阳极放电电弧构建是推力器启动和高效工作的关键步骤;外磁场强度较低工况不利于构建稳定放电电弧,等离子体束流集中于轴线附近,推力主要产生机制是自身场加速;外磁场强度较高时,阴阳极放电电弧稳定,推力产生主要机制是涡旋加速,推力、比冲随外磁场强度线性增大;推力器效率随阴极电流和外磁场强度增大而增大;放电电压随阴极电流增大而增大,但随外磁场强度的增大表现出先减小后增大的趋势.     

外磁场作用下的磁等离子体动力学过程仿真

磁等离子体动力学推力器是空间高功率电推进装置的典型代表,磁等离子体动力学过程是其核心工作机制.为深入理解外磁场对其工作特性的影响,本文采用粒子云(particle in cell,PIC)方法结合基于自相似准则的缩比模型,进行外加磁场作用下磁等离子体动力学推力器工作过程的建模仿真,通过与实验结果对比验证模型和方法的可靠性,并重点分析推力器点火启动过程的等离子特性参数分布,以及外磁场和阴极电流对推力器工作性能的影响.研究结果表明:阴阳极放电电弧构建是推力器启动和高效工作的关键步骤;外磁场强度较低工况不利于构建稳定放电电弧,等离子体束流集中于轴线附近,推力主要产生机制是自身场加速;外磁场强度较高时,阴阳极放电电弧稳定,推力产生主要机制是涡旋加速,推力、比冲随外磁场强度线性增大;推力器效率随阴极电流和外磁场强度增大而增大;放电电压随阴极电流增大而增大,但随外磁场强度的增大表现出先减小后增大的趋势.     

Particle-in-cell simulation for different magnetic mirror effects on the plasma distribution in a cusped field thruster

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.     

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.     

霍尔推进器中磁化二次电子对鞘层特性的影响

为进一步研究霍尔推进器壁面二次电子发射对推进器性能的影响,采用流体模型数值模拟了二次电子磁化效应的等离子体鞘层特性.得到二次电子磁化鞘层的玻姆判据.讨论了不同的磁场强度和方向、二次电子发射系数以及不同种类等离子体推进器的鞘层结构.结果表明:随器壁二次电子发射系数的增大,鞘层中粒子密度增加,器壁电势升高,鞘层厚度减小;鞘层电势及粒子密度随着磁场强度和方位角的增加而增加;而对于不同种类的等离子体,壁面电势和鞘层厚度也不同.这为霍尔推进器的磁安特性实验提供了理论解释. 关键词： 霍尔推进器 磁鞘 二次电子     

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.     

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)     

微型电子回旋共振离子推力器离子源结构优化实验研究

微型电子回旋共振(ECR)离子推力器可满足微小航天器空间探测的推进需求. 为此, 本文开展直径20 mm的微型ECR离子源结构优化实验研究. 根据放电室内静磁场和ECR谐振区的分布特点, 研究不同微波耦合输入位置对离子源性能的影响, 结果表明环形天线处在高于ECR谐振强度的强磁场区域时, 微波与等离子体实现无损耦合, 电子共振加热效果显著, 引出离子束流较大. 根据放电室电磁截止特性, 结合微波电场计算, 研究放电容积对离子源性能的影响, 实验表明过长或过短的腔体长度会导致引出离子束流下降甚至等离子体熄灭. 经优化后离子源性能测试表明, 在入射微波功率2.1 W、氩气流量14.9 μg/s下, 可引出离子束流5.4 mA, 气体放电损耗和利用率分别为389 W/A和15%.     

Measurements of the Drift Current Oscillations in Thrusters with Closed Electron Drift

An induction method used to measure the drift current in thrusters with anode layer (TAL) has been applied for measuring the integral distortion of the magnetic field in the channel of a low-power stationary plasma thruster (SPT). Experiments are described in which the integral level of magnetic field oscillations in the narrow channel of a thruster with closed electron drift was determined. Experimental data show that when a discharge is ignited in an SPT, the drift current completely demagnetizes the discharge gap (the ratio between the self-magnetic and external fields reaches 90%). The same effect has been observed in TALs. In thrusters of both types, the total discharge current is one and a half order of magnitude higher than that typical of the stationary discharge and the plasma glow is observed not only inside but also outside the discharge channel. Drift current oscillations have been measured under steady-state conditions. It has been shown, in particular, that when the oscillations are intense (in weak magnetic fields on the order of 100 G), the drift-current-induced distortion of the magnetic field may reach 33%. Under rated operating conditions (200 G), the distortion does not exceed 8%. Beyond optimal operating conditions, the type of oscillation in SPTs and TALs and their performances differ. For example, in SPTs, a critical magnetic field at which the “turbulent” regime sets in is absent. In general, the discharge current is weakly dependent on the magnetic field.     

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     

霍尔效应推力器放电双稳态机理研究

实验发现霍尔效应推力器在自励磁模式下具有两个稳定的放电工作点,且运 行过程中在这两个工作点上往复跳变,很大程度上影响了推力器通道内等离子体的放 电物理过程及其综合性能.本文结合推力器放电磁安特性曲线与励磁电流曲线的相互关 系,给出了推力器放电双稳态特性形成的物理机理．在此基础上提出了通过改变励磁电 流曲线斜率,使推力器稳定工作于单放电工作点的方法,结合一维动态流体模型给予了物 理解释,并通过实验加以验证.     

高频感应等离子体流动模型与微型电推进器流场特性分析

微型射频离子推力器具有结构简单、 工作寿命长、 推力动态范围大、 性能调节响应灵敏等特点,是国际微电推进领域的研究热点之一.射频离子推力器电离室内的感性耦合放电等离子体特性和推力器的性能密切相关.为此,文章建立了低气压、小尺寸微型射频离子推力器电离室内感性耦合等离子体流体模型,开展了电磁场、流场、化学反应浓度场的多物理...     

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)