【作者】 张凤奎；

【导师】 于达仁； 刘金远；

【作者基本信息】 哈尔滨工业大学 ， 动力机械及工程， 2009， 博士

【摘要】 霍尔推力器以其体积小,重量轻,比冲高,寿命长等优点广泛应用于小卫星的姿态控制和轨道保持等方面。自从上个世纪七十年代诞生以来,霍尔推力器已经取得了很好的工程化应用。电子近壁传导是影响霍尔推力器工作性能的关键物理过程之一;电子的近壁传导机制决定了通道的电势分布特性,离子的加速特性,并最终影响到推力器的效率、寿命、比冲等性能。与电子的近壁传导行为密切相关的因素是鞘层。鞘层是推力器通道内放电产生的等离子体与绝缘壁面相互作用的自洽产物,鞘层内的电场分布影响电子在近壁面的运动行为,从而影响电子的近壁传导。近壁传导、鞘层以及两者之间的关系一直是各国学者关注和讨论的热点问题。然而,由于霍尔推力器工作物理过程的复杂性,当前关于电子近壁传导对电子电流的贡献作用的评估依然没有定论。本文发现这受限于当前对鞘层特性以及近壁传导机制的认识。基于此,本文开展了相关研究工作。首先,本文建立了计算鞘层和电子近壁传导行为的二维粒子模拟(Particel-In-Cell, PIC)模型。由于鞘层的小尺度特性,具有众多优势的PIC方法成为本文问题的研究工具。其次,通过模拟本文发现了电子温度对鞘层的二维动态特性的影响规律。当电子温度较低时,二次电子发射系数小于1,鞘层具有一维稳态经典鞘层的特性。随着电子温度的升高,二次电子发射系数增大,鞘层开始在时间上周期性振荡,振荡频率为电子等离子体频率量级,然而此时二次电子发射系数仍然小于1。当电子温度继续升高使得二次电子发射系数在1附近时,鞘层开始在空间上出现振荡;此时,鞘层不仅保持着原有的在时域上的振荡,而且鞘层电势在空间上出现起伏,空间周期尺度为电子静电波波长量级。继续提升电子温度使得二次电子发射系数大于1之后,鞘层进入空间电荷饱和状态。继而,本文发现鞘层的不同状态对电子的近壁传导有着重要影响。当鞘层还未进入空间振荡状态前,即在经典状态和时域振荡的状态下,模拟发现电子只能通过与壁面碰撞发射二次电子产生传导,这种壁面发射的传导电流大小与古典传导(即电子与重粒子的碰撞传导)电流在同一数量级;因此和众多研究者得到的结论一样:近壁传导率太低,不足以解释实验中发现的大电子电流。然而,当鞘层进入空间振荡状态之后,鞘层电场出现非法向分量,此时不能克服鞘层势垒打到壁面参与传导的电子在鞘层的非法向反射作用下参与了近壁传导。由于这部分电子的数目相当可观,因此近壁传导电流较之古典传导电流增大了一个数量级,这一效应足以解释实验测量的电子电流。目前,鞘层影响近壁传导行为的这一特性还没有被他人提出。最后,本文发现无论鞘层处于何种状态,近壁传导电流的大小都与电场强度成正比,与磁场强度的平方成反比,从而进一步验证了近壁传导作为穿越磁场的“碰撞传导”理论的正确性。更多还原

【Abstract】 Hall thrusters have been used widely in orbit correction and station-keeping of geostationary satellites for the advantage of small size, light weight, high specific impulse, and long life. Engineering applications have achieved since the seventies of last century. The mechanism of electron near wall conduction (NWC) is one of the most important physics problems for thruster performance. The electrons conduction mechanism determines the electric potential distribution in channel, characteristics of ion acceleration, and determines the thruster efficiency, lifetime and specific impulse at last. Sheath was found to be a very influential factor to electronic conduction behavior near the wall. It is produced by the interactions of plasma and insulating wall, the electric field distribution in sheath influence the behavior of electrons near the wall, and influence the near wall conduction further.Near wall conduction, sheath, and the relation between them are a hot topic to be concerned and discussed by Scholars. However, due to complexity physical processes of hall thruster, the contribution of near wall conduction of electronic to current is still without any definite conclusion. In this thesis, it is founded that the research is restricted by the understanding of characteristics of sheath and the near wall conduction mechanism, Based on this, this paper carried out research work related.First, Two-dimension particle model (Particle-In-Cell,PIC) was built to compute the sheath and near wall conduction. Because of the small size of sheath, the method of PIC with many advantages become the Research tools in this paper.Secondly, Simulation results shows that the dynamic characteristics of sheath are largely influenced by electron temperature. With a low electron temperature, secondary electron emission coefficient is small than 1, the sheath has the characteristics of one-dimension dynamic sheath. The secondary electron emission coefficient increase with the electron temperature and the sheath begin periodic oscillation with the frequency near that of the plasma oscillation, at this time the secondary electron emission coefficient is below 1. When the electron temperature continues to rise, and the secondary electron emission coefficient near 1, sheath oscillations come into the range of space oscillation. At this range, except the oscillation in time domain, the sheath electric potential present fluctuation in space domain, the oscillation size is at the order of wavelength of electrostatic wave. When the electrons temperature becomes higher, the secondary electron emission coefficient bigger than 1, sheath enters into space charge saturation region.Then, we found that the different conduction of sheath have important effect on near wall conduction. Before the sheath present space oscillation, with classic sheath and oscillation in time domain, the simulation results show the conduction is formed by secondary-electrons produced by the oscillation of electrons and wall. Near wall conduction current and classical conduction current (the current produced by oscillation of electrons and heavy particles) are same order of magnitude, the result is save with many researchers. The conduction is too little to explain the big current in experiments. However, when the sheath state enter the space oscillation, non-normal component of sheath electrons field come to play effect, the electrons that can not overcome the sheath barrier and reach wall have take part in the new wall conduction. Because of the reasonable quantity of electrons, the current of near wall conduction is bigger than classical current by an order of magnitude, the effect can explain the current in experiments.Finally, No matter in what state of the sheath, the near wall current is proportional to electric field strength, and inversely proportional to the square of the magnetic field strength. The computation results further verify the near wall conduction is taken as“collision conduction”for through the magnetic field.更多还原