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.     

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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。     

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。     

真空背压对霍尔推力器放电特性影响的实验研究

真空背压的变化会改变进入霍尔推力器放电通道内的背景气体量,对工质的电离、电子的传导等物理过程产生影响,从而进一步影响到推力器的宏观放电特性.为分析真空背压对推力器放电的影响规律,通过向真空罐输入流量可控的氪气改变真空背压,在不同真空背压下测量通道内原子、离子的发光特性以及出口处离子流的伏安特性.分析结果表明:背景气体返流对通道内工质放电过程具有全局性的影响,提高背压会使通道内的电子温度降低、电离效率降低,并会在通道内形成一个新的电离区,且背压越高,该电离区距推力器阳极越近.     

Characteristics of wall sheath and secondary electron emission under different electron temperatures in a Hall thruster

In this paper, a two-dimensional physical model is established in a Hall thruster sheath region to investigate the influences of the electron temperature and the propellant on the sheath potential drop and the secondary electron emission in the Hall thruster, by the particle-in-cell(PIC) method. The numerical results show that when the electron temperature is relatively low, the change of sheath potential drop is relatively large, the surface potential maintains a stable value and the stability of the sheath is good. When the electron temperature is relatively high, the surface potential maintains a persistent oscillation, and the stability of the sheath reduces. As the electron temperature increases, the secondary electron emission coefficient on the wall increases. For three kinds of propellants(Ar, Kr, and Xe), as the ion mass increases the sheath potentials and the secondary electron emission coefficients reduce in sequence.     

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.     

Laser-induced fluorescence study of a xenon Hall thruster

2 ⁰→6p[3/2]₂(³P₂-¹D₂) transition at 823.2 nm and the xenon-ion 5d[3]_7/2→6p[2]_5/2 ⁰(⁴D_7/2-⁴P_5/2) transition is used to measure plasma parameters in the plume of a laboratory-model xenon Hall thruster. The Hall discharge operates nominally at 62 V, 4.2 A, and 3.2 mg s^-1 xenon flow, with an overall thruster power of 320 W. A tunable semiconductor diode laser and an Ar⁺-pumped dye laser are used to probe the respective excited-state transitions. Axial velocity measurements are made at a number of axial and radial locations up to 4.5 cm downstream of the thruster-exit plane and under a variety of thruster operating conditions. Neutral velocities from 100 m s^-1 to 400 m s^-1 and ion velocities as high as 12 km s^-1 are calculated from measured Doppler shifts. The charge-exchange phenomenon evidently does not significantly affect the xenon neutrals. The spectral-line shapes of the ion indicate a spread in ion energies through a non-Maxwellian distribution of axial velocities. Neutral kinetic temperatures of 500 (±200) K are observed under standard operating conditions. Zeeman and Stark effects on the spectral-line shapes, from the thruster’s magnetic and electric fields, are not substantial. The measured line center of the ion transition is 16521.23 (±0.02) cm^-1. Received: 20 January 1997/Revised version: 12 May 1997     

激光推力器喷管热防护实验研究

 基于激光推力器喷管热烧蚀因素分析，初步提出了反射式和吸收式两种喷管热防护方法，并相应设计了两种喷管。以10 kW级脉冲式CO₂激光器为光源，三次反射激光推力器为对象，进行了两种方式下两种喷管的热防护初步实验研究。结果表明：两种喷管均发生不同程度烧蚀，与相对聚焦位置和激光作用时间有关；反射式热防护通过喷管光学表面对透射激光能量进行二次聚焦，以及良好的循环冷却，可能成为将来激光推力器喷管热防护的备选方案之一。     

壁面二次电子发射对霍尔推力器放电通道绝缘壁面双鞘特性的影响

为进一步揭示霍尔推力器放电通道绝缘壁面鞘层的特性, 利用考虑了壁面二次电子分布函数的一维稳态流体鞘层模型, 研究了壁面二次电子发射对近壁双鞘特性的影响. 分析结果表明, 由于壁面发射的二次电子对近壁鞘层中的电子密度有增加作用, 存在一个临界二次电子发射系数σ_dc使得: 当σ≤σ_dc时, 鞘层为单层的正离子鞘结构; 当σ>σ_dc时, 鞘层表现为双层的正离子鞘和电子鞘相连结构, 连接点对应于垂直于壁面方向上电势分布的拐点. 然而, 当σ进一步增大到0.999时, 鞘层转变为三层的正离子鞘-电子鞘-正离子鞘交替结构. 数值结果表明: 随着σ的增加, 电子鞘与离子鞘的连接点向远离壁面的方向移动, 电子鞘的厚度逐渐增加; 随着壁面出射电子能量系数a的增加, 近壁区鞘层的厚度也逐渐增加. 关键词： 霍尔推力器 双鞘 壁面二次电子发射     

Experimental and numerical investigation of a stagnation pulverised coal flame

A multi-stream Flamelet Progress Variable (FPV) model, specifically developed for coal combustion, is proposed. The model accounts for the different fuel streams associated with the volatile and char burnout products. The applicability of the new FPV model is investigated in a laminar stagnation pulverized coal flame. The flame considered is a premixed mixture of CH₄, O₂ and N₂, carrying pulverized coal particles, stabilized in an impinging wall. Spontaneous emissions of OH*, CH* and C${}_2^{*}$ are measured to identify the flame. The 1D numerical simulations of the experimental conditions are able to reproduce the main features of the flame. The applicability of the multi-stream FPV model to coal combustion is further evaluated with the a posteriori analysis of the FPV results, comparing the results with a reference model, where the species are fully transported and the chemistry directly evaluated. Then, with the budget analysis, the influence of the control variables used to build the look-up table is assessed by examining the conditional contributions to the overall transport terms of scalar quantities (e.g. species, temperature). The results of both analyses show that the proposed multi-stream FPV model can accurately predict the main features of coal combustion, with only minor issues related to the manifold used to build the look-up table.     

Experimental study of the effect of propellant asymmetrical distribution on anode current in a Hall effect thruster

This paper experimentally evaluated the effect of the disruption of the symmetrical distribution of the propellant on the characteristics of the anode current. The change in the asymmetry degree of the propellant distribution is achieved by supplying gas with a dual-cavity gas distributor. The results show that as the asymmetry degree increases, the magnitude of the anode current changes monotonically from a slow growth to a rapid growth, while the peak-to-peak value of the anode current exhibits a non-monotonic behavior. A preliminary analysis shows that the asymmetrical distribution of the propellant causes a nonuniform plasma generation along the azimuthal direction and consequently the appearance of an azimuthal electric field. The effect of the azimuthal electric field on the electron azimuthal drift as well as the induction of the electron axial drift are key factors that account for the change of the anode current and its oscillation.     

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.     

Influence of the erosion of the discharge channel wall on the efficiency of a stationary plasma thruster

The integral characteristics of stationary plasma thrusters are studied experimentally during their long-term operation. It is shown that the monotonic fall of the thrust efficiency and specific impulse within the initial 500?C1000 h of operation is explained, in particular, by a decrease in the propellant utilization efficiency. A physical mechanism underlying this phenomenon is suggested, and expressions predicting the variation of the output characteristics of the thrusters during the operating life are derived. Satisfactory agreement is observed between experimental and predicted results on the anode specific impulse variation in the course of testing several thrusters with different powers for endurance. An experimentally found correlation between the magnetic field configuration and the position of the erosion zone boundary on the discharge chamber wall is accounted for.     

Experimental and numerical investigation of friction-induced vibration of a beam-on-beam in contact with friction

The vibrations generated by friction are responsible for various noises such as squealing, squeaking and chatter. Although these phenomena have been studied for a long time, it is not well-understood. In this study, an experimental and numerical study of friction-induced vibrations of a system composed of two beams in contact is proposed. The experimental system exhibits periodic steady state vibrations of different types. To model and understand this experimental vibratory phenomenon, complex eigenvalue and dynamic transient analyses are performed. In the linear complex eigenvalue analysis, flutter instability occurs via the coalescence of two eigenmodes of the system. This linear study provides an accurate value of the experimental frequency of vibration. To understand what happens physically during friction-induced instability, a dynamic transient analysis that takes account of the non-linear aspect of a frictional contact is performed. In this analysis, friction-induced instability is characterized by self-sustained vibrations and by stick, slip and separation zones occurring at the surface of the contact. The results stemming from this analysis show that good correlation between numerical and experimental vibrations can be obtained (in time and frequency domains). Moreover, time domain simulations permit understanding the physical phenomena involved in two different vibratory behaviours observed experimentally.     

Experimental and numerical investigation of flow separation in a supersonic nozzle

The flow in a two-dimensional symmetric convergent-divergent supersonic nozzle with and without a deflector at the exit plane was tested experimentally and numerically. Combined optoelectronic devices were used to demonstrate the possibilities of the shadow and schlieren methods and holographic interferometry for the flow visualization especially in the separation region. A series of experiments was performed in a trisonic wind tunnel (VTI, Beograd). The experimental results obtained using optical methods are compared with the pressure measurements and numerical calculations. Numerical results were obtained using a code for solving the average Navier-Stokes equations. Five structured meshes with different resolutions and two turbulent models were employed. The coupled influence of the mesh resolution and the order of accuracy of the used numerical scheme is analyzed.     

Hall推力器内饱和鞘层下电子与壁面碰撞频率特性

利用二维粒子模拟方法研究Hall推力器内电子与壁面的碰撞频率.研究发现,饱和鞘层状态下的电子与壁面的碰撞频率较经典鞘层下大大增加,甚至高出经典鞘层状态下电子与壁面碰撞频率两个数量级,这样饱和鞘层状态下电子与壁面的碰撞频率对近壁电流的贡献将不容忽略.进一步分析造成饱和鞘层状态下电子与壁面碰撞频率增加的原因后认为,饱和鞘层状态下电子与壁面碰撞频率的增加是鞘层电势降过低和壁面发射的二次电子回流造成的. 关键词： 饱和 鞘层 碰撞 频率     

ATON型Hall推力器缓冲区预电离问题研究

ATON是20世纪90年代新提出的一种Hall推力器设计方案,其中的缓冲区是ATON新采用的一种结构.本文对缓冲区的作用进行分析,在此基础上提出了采用附加电源提高缓冲区预电离率的方法.并采用PIC(Particle-in-cell)粒子模拟方法对这种设计方式的等离子体分布进行数值模拟.结果表明,通过增加缓冲区内的附加电压,能够有效地提高缓冲区预电离率.     

Near-wall conductivity effect under a space-charge-saturated sheath in the Hall thruster

In this paper, we adopt the modified Morozov secondary electron emission model to investigate the influence of the characteristic of a space-charge-saturated sheath near the insulated wall of the Hall thruster on the near-wall conductivity, by the method of two-dimensional (2D) particle simulation (2D+3V). The results show that due to the sharp increase of collision frequency between the electrons and the wall under the space-charge-saturated sheath, the near-wall transport current under this sheath is remarkably higher than that under a classical sheath, and equals the near-wall transport current under a spatially oscillating sheath in order of magnitude. However, the transport currents under a space-charge-saturated sheath and a spatially oscillating sheath are different in mechanism, causing different current density distributions under the above two sheaths, and a great influence of channel width on the near-wall transport current under a space-charge-saturated sheath.