Characteristics of a Sheath with Secondary Electron Emission in the Double Walls of a Hall Thruster

In order to investigate the effects of secondary electrons, which are emitted from the wall, on the performance of a thruster, a one-dimensional fluid model of the plasma sheath in double walls is applied to study the characteristics of a magnetized sheath. The effects of secondary electron emission （SEE） coefficients and trapping coefficients, as well as magnetic field, on the structure of the plasma sheath are investigated. The results show that sheath potential and wall potential rise with the increment of SEE coefficient and trapping coefficient which results in a reduced sheath thickness. In addition, magnetic field strength will influence the sheath potential distributions.

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 （At, Kr, and Xe）, as the ion mass increases the sheath potentials and the secondary electron emission coefficients reduce in sequence.

Study on the characteristics of non-Maxwellian magnetized sheath in Hall thruster acceleration region

The secondary electron emission(SEE) and inclined magnetic field are typical features at the channel wall of the Hall thruster acceleration region(AR), and the characteristics of the magnetized sheath have a significant effect on the radial potential distribution, ion radial acceleration and wall erosion. In this work, the magnetohydrodynamics model is used to study the characteristics of the magnetized sheath with SEE in the AR of Hall thruster. The electrons are assumed to obey non-extensive distribution, the ions and secondary electrons are magnetized.Based on the Sagdeev potential, the modified Bohm criterion is derived, and the influences of the non-extensive parameter and magnetic field on the AR sheath structure and parameters are discussed. Results show that, with the decrease of the parameter q, the high-energy electron leads to an increase of the potential drop in the sheath, and the sheath thickness expands accordingly,the kinetic energy rises when ions reach the wall, which can aggravate the wall erosion.Increasing the magnetic field inclination angle in the AR of the Hall thruster, the Lorenz force along the x direction acting as a resistance decelerating ions becomes larger which can reduce the wall erosion, while the strength of magnetic field in the AR has little effect on Bohm criterion and wall potential. The propellant type also has a certain effect on the values of wall potential,secondary electron number density and sheath thickness.

Effect of Secondary Electron Emission on the Sheath in SPT Chamber

A one-dimensional slab model of the plasma sheath in the stationary plasma thruster （SPT） chamber is developed in this study. It is considered that secondary electrons emitted from ceramic walls are partially trapped by the bulk plasma in the SPT chamber; some secondary electrons drift across the sheath where they are generated and the bulk and move towards the opposite sheath. Thus both the secondary electron emission （SEE） from one sheath and the partially trapped secondary electrons from the opposite sheath contribute to this sheath. The results indicate that both the SEE coefficient and trapping coefficient have a significant impact not only on the distributions of both electrons and ions of the SPT sheath but also on the energy flux loss to the SPT wall. When the trapping coefficient increases, the energy flux of electrons deposited to the walls will increase whereas that of ions will decrease. Besides, the critical electron temperature will decrease greatly with the increase of the trapping coefficient.

Numerical study of the effect of aft-loaded magnetic field on multiple ionizations in Hall thruster

It is assumed that the shift of a strong magnetic field region with a positive gradient from exit plane to outside, namely the transit from a normal loaded magnetic field to an aft-loaded one, enhances the multiple ionization process in the magnetically shielded Hall thruster. To confirm this conjecture, a comparative study is carried out numerically with a particle-in-cell method. The simulation results prove that compared with the normal loaded magnetic field, the application of aft-loaded magnetic field enhances the multiple ionization process. This study further analyzes the ionization characteristics of the transition from low-charged ions to high-charged ions under two magnetic field conditions and the influence of the magnetic strength of aft-loaded magnetic field on the multiple ionization characteristics. The study described herein is useful for understanding the discharge characteristics of Hall thruster with an aft-loaded magnetic field.

A method to measure the in situ magnetic field in a Hall thruster based on the Faraday rotation effect

This paper presents a method to measure the in situ magnetic field in a Hall thruster by optical non-invasive means, based on the optical Faraday rotation effect.This method does not affect the discharge of the thruster.Furthermore, its time resolution depends on the speed of the photodetector, and measurement at a MHz scale can be achieved.

Effects of Magnetic Field and Ion Velocity on SPT Plasma Sheath Characteristics

The distribution of magnetic field in Hall thruster channel has significant effect on its discharge process and wall plasma sheath characteristics. By creating physical models for the wall sheath region and adopting two-dimensional particle in cell simulation method, this work aims to investigate the effects of magnitude and direction of magnetic field and ion velocity on the plasma sheath characteristics. The simulation results show that magnetic field magnitudes have small impact on the sheath potential and the secondary electron emission coefficient, magnetic azimuth between the magnetic field direction and the channel radial direction is proportional to the absolute value of the sheath potential, but inversely proportional to the secondary electron emission coefficient. With the increase of the ion incident velocity, secondary electron emission coefficient is enhanced, however, electron density number, sheath potential and radial electric field are decreased. When the boundary condition is determined, with an increase of the sinmlation area radial scale, the sheath potential oscillation is aggravated, and the stability of the sheath is reduced.

氮化硼复合陶瓷表面二次电子发射抑制

二次电子产额δ是影响霍尔推进器鞘层的关键参数之一。为了降低推进器通道壁面常用材料BN−SiO_(2)复合陶瓷的表面δ,通过激光刻蚀工艺和表面镀层技术对表面δ进行了调控。使用脉冲红外光纤激光器在材料表面构造微阵列结构,使用激光扫描显微镜对表面结构表征,表面δ测试方法采用收集极法。δ测试结果表明:激光功率10 W且扫描周期50时,样品表面形成的周期性微阵列结构δ抑制效果较为理想,两种不同质量配比的复合陶瓷表面δ峰值分别由2.62和2.38降低至1.55和1.46。使用磁控溅射在上述微结构表面沉积一层TiN薄膜,δ得到进一步抑制。使用扫描电子显微镜对薄膜表面表征,表面颗粒呈现三棱锥结构。结果表明:溅射功率100 W且时长90 min时,TiN膜厚约为246 nm,镀覆该厚度TiN薄膜的两组微结构样品δ峰值分别由1.55和1.46降低至0.82和0.76,相比原始表面大幅降低。该研究通过表面刻蚀和薄膜沉积工艺,大幅降低了BN−SiO_(2)复合陶瓷表面的δ,研究工作对于特定工作场景中开展低δ表面处理工艺研究具有工程应用价值。

霍尔推进器等离子体鞘层特性的Particle-in-Cell模拟

霍尔(Hall)等离子体推进器鞘层特性对推进器的性能具有重要影响。针对Hall推进器鞘层区域建立物理模型,采用粒子模拟方法(PIC),通过求解泊松方程得到粒子位置产生的电场,确定边界条件,研究了不同磁感应强度和方向、不同推进工质和粒子权重对推进器鞘层电势及壁面二次电子发射系数的影响规律。结果表明:当磁感应强度为0.04T时,随着磁场方位角的增大,鞘层电势绝对值升高,壁面二次电子发射系数降低,变化量在10-3量级;磁场大小对鞘层电势及壁面二次电子发射系数影响较小;对于氩、氪和氙3种工质,鞘层电势和二次电子发射系数依次降低;而当粒子权重大于106时,等离子体鞘层振荡明显,推进器的稳定性降低。

霍尔推力器等离子体磁鞘特性

为进一步研究霍尔推力器壁面二次电子发射对发动机性能的影响,建立流体模型数值模拟了霍尔推力器磁化二次电子等离子体的鞘层特性,采用Sagdeev势的方法得到了鞘层的玻姆判据,以这个判据为鞘层边界条件数值讨论了磁场、二次电子发射以及不同等离子体对推力器等离子体鞘层结构的影响。结果表明,磁场和二次电子发射系数的增加都将使鞘层中粒子密度增加,鞘层电势升高,鞘层厚度减小;对于氩和氙二种等离子体,氙等离子体产生的鞘层势垒高,粒子密度大,鞘层厚度增加。这些结果直接影响推力器的电子传导机制及性能。

伴有二次电子发射的磁化等离子体鞘层结构特性

建立了包括电子、离子以及器壁发射二次电子的磁化等离子体鞘层流体模型,采用四阶龙格库塔法数值研究了伴有二次电子发射的磁鞘结构特性。模拟结果显示二次电子发射对于弱磁等离子体鞘层中的离子密度影响较大,而对于磁场较强的等离子体鞘层,鞘层中离子密度分布主要由磁场来决定。磁场的存在可以促进器壁电子的发射,磁场的增加或二次电子发射系数的增加都将使得鞘层厚度的减小,同时将导致沉积到器壁的离子动能流发生变化,从而直接影响器壁材料的性能。

二次电子发射对稳态等离子体推进器加速通道鞘层的影响

稳态等离子体推进器(Stationary Plasma Thruster,SPT)工作时产生的高密度等离子体遇到其加速通道陶瓷器壁时,在陶瓷器壁与等离子体之间形成鞘层。离子会在鞘层电场作用下到达SPT加速通道器壁表面进而复合,而等离子体中的电子由于具有高能可跃过鞘层电场轰击器壁表面,从而产生二次电子发射效应。从器壁表面发射出的二次电子由于受到鞘层电场的排斥,导致其向等离子体源区移动,进而影响等离子体鞘层的特性。建立了考虑二次电子发射效应的无碰撞等离子体鞘层的一维流体模型,研究了二次电子发射对SPT加速通道鞘层特性的影响。计算结果显示,随二次电子发射系数增加,鞘层电势、离子密度、电子密度和二次电子密度增加,而离子速度降低,鞘层中离子密度始终大于电子密度。鞘层中二次电子绝大多数集中在器壁附近,随二次电子穿越鞘层厚度的增加,二次电子密度快速下降。

离子速度对霍尔推力器壁面鞘层特性的影响

为进一步探索霍尔推力器通道内等离子体鞘层的物理机制,针对霍尔推力器等离子体鞘层区域建立二维物理模型,采用二维粒子模拟方法研究了二次电子发射系数、鞘层电子数密度、鞘层电势以及电场随离子入射速度的变化规律,分析了模拟区域径向尺度大小对鞘层稳定性的影响。结果表明:壁面二次电子发射系数随离子入射速度的增大有少许增大,变化在10-3数量级;随着离子入射速度的增大,电子数密度、鞘层电势降及径向电场都减小,而轴向电场几乎不变;在相同的边界条件下,模拟区域径向尺度的增大会导致壁面电势随时间的振荡加剧,鞘层稳定性降低。

霍尔推进器磁屏蔽对磁场的优化效应研究

为研究磁屏蔽对霍尔推进器磁场位形分布的影响,以轴对称环形霍尔推进器为研究对象,采用FEMM软件对各种磁屏蔽情况下的磁场分布进行仿真;结合流体模型,利用四阶龙格库塔方法对放电通道内各粒子的输运性质进行研究。结果表明:内外磁屏蔽材料高度变化时磁场位形存在最优分布;电子温度在放电通道出口附近达到最大值,约64eV。离子化频率和电子轴向有效散射频率峰值也出现在通道出口附近。

欠匹配型磁绝缘感应电压叠加器次级阻抗优化方法

磁绝缘感应电压叠加器(MIVA)次级阻抗对脉冲功率驱动源和负载之间的功率耦合具有重要影响.基于稳态磁绝缘Creedon层流理论和鞘层电子流再俘获(re-trapping)理论,建立了负载欠匹配型MIVA电路分析方法,数值分析获得了MIVA输出参数(输出电压、阴/阳极电流和电功率)随负载欠匹配程度的变化规律.考虑阴极传导电流作为闪光X射线照相二极管的有效电流,建立了以MIVA末端X射线剂量率最大为目标的次级阻抗优化方法.获得了欠匹配型MIVA次级优化阻抗Z_(op)~*的变化规律:随着X射线剂量率对电压依赖程度提高,欠匹配型MIVA次级优化阻抗Z_(op)~*呈指数降低;负载阻抗越大,Z_(op)~*越大.

霍尔推力器变电压下电子能量平衡机制

在磁场不变的情况下,随着霍尔(Hall)推力器放电电压的提高,其通道内的最大电子温度会在一定的电压区间内出现"饱和"现象。为进一步理解这一现象,在完成了变电压不变磁场PIC(Particle in cell)模拟的基础上,首先分析了电子能量的平衡机制的构成要素和最大电子温度的影响因素,进而对各个影响因素在变电压下的变化趋势进行了研究。结果显示:最大电子温度点上游区域的电场加热效应是最大电子温度变化的主导因素,而上游区域的电子与壁面碰撞效应对最大电子温度的变化起到一定的调节作用。在高电压下,由于磁场无法有效束缚电子,上游区域的电子数密度急剧降低,导致电子壁面碰撞能量损失大幅降低,使得碰撞损失对最大电子温度的影响变得较为微弱。进一步指出了磁场在霍尔推力器变电压运行中的核心地位,并提出了高电压放电优化的2个方向:增大放电磁场以及更换二次电子发射系数更高的陶瓷壁面。

电子非麦氏分布的二次电子发射磁化鞘层特性

采用空间一维速度三维的磁流体模型研究了电子的非麦克斯韦分布对具有二次电子发射的磁化等离子体鞘层特性的影响.假设鞘层中电子速度服从非广延分布,离子在具有一定倾斜角度的磁场中被磁化.通过建立自洽的磁流体方程,研究了电子非广延分布参数q及磁场强度和角度对等离子体鞘层玻姆判据、壁面悬浮电势、鞘边二次电子数密度、鞘层厚度、离子速度等的影响.研究表明,当电子速度分布偏离麦克斯韦分布时,非广延参数q值越大,玻姆判据的值越小,壁面电势越高,鞘边二次电子数密度增大,鞘层厚度减小,鞘层区域离子、电子数密度下降加快,且壁面附近离子数密度较高,离子3个方向的速度均降低.此外,随着磁场强度增大,鞘层厚度减小,鞘层区域离子、电子数密度下降加快;磁场角度越大,参数q值对壁面电势、鞘层厚度的影响程度越显著,在超广延、亚广延分布情况下壁面附近离子x方向速度随磁场角度变化呈相反趋势.

ECR中和器鞘层特性数值计算

建立了带有外加磁场和离子溅射二次电子的离子推力器中和器壁面鞘层模型,通过采用四阶龙格库塔方法,研究了带有二次电子发射条件下的磁鞘结构特性。计算结果显示,磁场大小和角度的变化都会对壁面鞘层产生显著影响。二次电子的引入使得计算结果曲线更加光滑,二次电子对电子的计算结果影响显著而对离子影响甚微。此外,壁面外加电势的大小是引发鞘层厚度大小变化的主要原因,从而很大程度上影响了壁面参数的变化。

霍尔推进器中振荡鞘层对电子与壁面碰撞频率的影响研究

利用二维粒子模拟方法研究振荡鞘层对近壁电导的影响.研究结果表明,当二次电子发射系数大于1时,鞘层处于振荡状态.在振荡鞘层状态下,电子与壁面的碰撞通量沿平行与壁面方向剧烈的周期性振荡,振荡的波长为电子静电波波长量级,电子与壁面的碰撞频率高出经典鞘层状态下电子与壁面碰撞频率1—2个数量级,此时的碰撞频率对通道中电流的贡献不可忽略.振荡鞘层相对与经典鞘层增大了电子与壁面的碰撞频率,但是振荡鞘层的存在,仍然会使一部分慢电子无法穿越鞘层的势垒而打到壁面.

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。