【作者】 李鸿；

【导师】 于达仁；

【作者基本信息】 哈尔滨工业大学 ， 动力机械及工程， 2011， 博士

【摘要】 具有高效率、高比冲、长寿命等优点的霍尔推力器是目前航天领域中应用最多的电推进装置,是电推进技术中的研究热点。近壁传导是推力器放电通道内最重要的电子传导机制,是影响推力器性能的关键物理过程。在霍尔推力器的寿命周期内出现了能够对电子近壁传导特性产生影响的多种因素。由于当前关于寿命的研究焦点倾向于应用层面,因此目前还没有系统性地开展相关工作。站在推力器整个寿命周期的角度,本文研究了电子近壁传导特性变化的规律与机理。在推力器寿命周期内共有三种影响电子近壁传导特性的因素:存在于整个寿命周期内的鞘层振荡、寿命前期(减速侵蚀阶段)离子溅射壁面强度的变化以及寿命后期(反常侵蚀阶段)周向周期性壁面沟槽形貌的出现。首先,本文根据现有理论对振荡鞘层进行了建模,并利用Monte-Carlo模拟方法研究了振荡鞘层的特征参数影响近壁电导率的规律。结果表明,鞘层电势振荡的幅值越大,近壁电导率越大。结合实验中观测到的鞘层振荡强度的变化特性,振荡鞘层引起的近壁传导电流在整个寿命周期内是先增大再减小的。此外,针对近壁传导电流位形的实验测量结果与当前经典稳态鞘层框架下近壁传导理论之间的矛盾,本文通过解析分析与数值模拟的方法研究了振荡鞘层特征参数对近壁传导电流位形的影响。结果表明,振荡鞘层作用下的电流位形特点与实验位形特点相似,这为完善电子近壁传导理论提供了新的视角。其次,本文通过实验测量与Particle-in-Cell(PIC)模拟的方法研究了离子溅射壁面强度的变化对电子近壁传导特性的影响。通过设计不同磁场位形的方法在实验中模拟了不同的离子溅射强度,并测量得到了离子溅射强度影响电子近壁传导特性的实验规律。结果表明,离子溅射强度越大,近壁传导电流越大。在此基础上,利用PIC粒子模拟方法计算了不同离子溅射强度引起的近壁传导电流的大小。计算结果与实验规律吻合,从中获得了影响机理。由于在减速侵蚀阶段,随着时间的推移,离子溅射壁面的作用越来越弱,因此其引起的近壁传导电流在寿命周期内是越来越小的。再次,针对反常侵蚀阶段出现的周向周期性的壁面沟槽形貌,本文将其简化为三角沟槽形貌并开发了相适应的数值处理方法。考虑到这种反常侵蚀形貌在稳定之前要经历一个发展变大的过程,本文分别研究了大小两种周向尺寸的三角沟槽对近壁区等离子体参数分布以及近壁传导电流的影响,分析了造成不同影响规律的原因,并进一步研究了多沟槽条件下的电子近壁传导特性。结果表明,鞘层厚度是区分周向小尺寸形貌与周向大尺寸形貌的特征尺度。尺寸小于鞘层厚度的沟槽对电子近壁传导特性没有影响;只有大于鞘层厚度的沟槽才会增强电子的近壁传导。造成这种差异的本质原因是壁面形貌与鞘层的相互作用特性发生了变化。同时,研究发现相邻沟槽之间不存在耦合效应,多个沟槽对电子近壁传导特性的影响可以由单个沟槽的情况简单拓展得到。这些发现表明反常侵蚀壁面形貌对电子近壁传导的影响只在反常侵蚀阶段的后期才会体现出来。结合振荡鞘层以及离子溅射强度变化的影响,寿命周期内放电电流几乎不变的特性可以被定性地解释。此外,针对模拟中发现的近壁区等离子体参数周向波动传播的现象,本文分析了其产生的条件与相关物理性质,明确了这是一种至今在霍尔推力器研究领域没有被报道过的低频离子声速表面波。这种表面波会引起空间电势的波动从而诱导电子近壁传导。这一发现对完善电子近壁传导理论具有重要意义。最后,利用具有反常侵蚀特征的壁面形貌能够改变电子近壁传导特性的特点,本文在霍尔推力器的放电通道壁面上引入了矩形沟槽形貌,并通过实验手段研究了矩形沟槽对电子近壁传导特性的影响以及沟槽引起的局部近壁电导率的改变对推力器放电特性的影响。结果表明,加速区壁面形貌对电子近壁传导特性具有显著影响,而电离区壁面形貌的影响却很小。进一步的PIC模拟研究指出,这种巨大的影响差异在一定程度上取决于壁面形貌在不同放电区域参数作用下引起的电子近壁传导特性的差异;然而,更重要的决定因素是不同放电区域本身的电导率的相对大小。此外,PIC模拟发现了壁面材料二次电子发射特性几乎不会对壁面形貌效应下的近壁传导特性产生影响;壁面形貌周向尺寸的连续变化影响近壁传导的规律与形貌的形状特点有关。对电离过程及性能参数进行测量表明,增大电离区的局部电导率会使得电离区向通道出口移动,从而导致羽流发散角变大,推力及效率降低;增大加速区的局部电导率会使得通道内外出现两个电离区,并严重增大了加速电场的径向分量,使得羽流发散角显著增大,效率显著下降。上述研究结果表明具有反常侵蚀特征的壁面形貌能够作为一种深入理解推力器放电工作机制的有效手段。更多还原

【Abstract】 Hall thrusters, with the advantages of high efficiency, high specific impulse and longlifetime, are the most widely used electric propulsion (EP) device in aerospace field andalso the research hotspot among the EP technologies. Near wall transport is the mostimportant electron transport mechanism in thruster channel and also the pivotal physicalprocess that affect the thruster performance. Several factors, which can in?uence theelectron near wall transport, appear over whole lifetime of Hall thrusters. As the majoreffort on thruster lifetime is inclined to application at present, no relevant work has beendone systematically so far. In this thesis, from the viewpoint of the whole lifetime of Hallthrusters, the laws and principles of electron near wall transport are studied.There are three factors that affect the electron near wall transport over the thrusterlifetime. The first one is sheath oscillation existing in the whole thruster lifetime; thesecond one is variation of ion sputtering intensity emerging in earlier stage of lifetime (theso-called erosion rate reduction stage); the third one is azimuthal periodic wall groovesappearing in later stage of lifetime (the so-called anomalous erosion stage).First, the oscillating sheath is modeled according to current theory and its in?uenceon near wall conductivity is studied by varying the characteristic oscillation parameterswith Monte-Carlo simulation method. The results show that as oscillating amplitude ofsheath potential increases, the near wall conductivity increases. In view of the detectedvariation of sheath oscillation amplitude in experiments, the near wall transport currentinduced by oscillating sheath increases first and decreases later in thruster lifetime. Fur-thermore, as to the contradictory between near wall transport current profile measured inexperiments and that deduced from theories in the frame of classical steady sheath, theeffect of oscillating sheath on near wall transport current profile is studied through analyt-ical method and numerical simulation. The results show that the current profile inducedby oscillating sheath has the similar feature to the measured one. This finding providenew viewpoint for the further development of electron near wall transport theory.Second, the effect of ion sputtering intensity variation on electron near wall transportis studied with both the experimental measurement and a Particle-in-Cell (PIC) simula-tion. The different ion sputtering intensities are simulated in experiments by designing different magnetic field topologies; the experimental effect of ion sputtering intensity onelectron near wall transport is consequently obtained. The results show that as the ionsputtering intensity increases, the near wall transport current increases. On that basis, thenear wall transport currents are calculated in different ion sputtering intensity cases withthe PIC method. The numerical results are found to accord well with the experimentalones; the mechanism of ion sputtering intensity affecting electron near wall transport isthen obtained. As the ion sputtering effect becomes weaker and weaker as the erosion ratereduction stage processes, the corresponding near wall transport current becomes smallerand smaller.Third, as to the periodic wall grooves appeared in azimuthal direction in the anoma-lous erosion stage, they are simplified as triangle grooves and a specific numerical schemabased on PIC method is developed to deal with them. Taking into account that the anoma-lous erosion wall geometry undergoes a developing process before being steady, the effectof triangle groove with both the small dimension and large dimension on near wall plasmaparameter distributions and transport currents are studied. The reason for the differentvariations with different groove dimensions is then analyzed. The effect of groove num-ber on electron near wall transport is further researched. The results show that the sheaththickness is the characteristic length that classifies small dimension groove and large di-mension groove. The groove with dimension smaller than sheath thickness has no effecton electron near wall transport; only the groove with dimension larger than sheath thick-ness can enhance electron near wall transport. The substantial reason for that discrepancyis the change of interaction between wall geometry and sheath. Besides, it is found thatno coupling effect exists between adjacent grooves and the collective in?uence can beobtained by simply expanding the result of a single groove case. These findings indicatethat the wall geometry induced near wall transport current only appears in the later periodof anomalous erosion stage. Combing the effects of oscillating sheath and ion sputteringintensity variation, the changelessness of discharge current over the whole thruster life-time can be understood qualitatively. In addition, as to the simulated phenomenon thatplasma parameters oscillate and propagate in azimuthal direction, its emerging conditionand physical properties are analyzed. It is justified to be an ion-acoustic surface wavewith low frequency, which has not been reported in Hall thruster community before. Thiskind of surface wave can cause the ?uctuation of space potential and the consequent elec- tron near wall transport. This finding is significant for improving the electron near walltransport theory.Last, making use of the speciality that wall geometry with anomalous erosion fea-tures can change electron near wall transport, the azimuthal periodic rectangular groovesare introduced and manufactured on thruster channel wall. Their effect on electron nearwall transport is then studied in experiments; also is the effect of local near wall conduc-tivity change caused by grooves on thruster discharge features. The experimental resultsshow that the electron near wall transport is affected significantly by the grooves locatedin the acceleration region but merely by the grooves located in the ionization region. Aspointed out by further PIC simulation, this great discrepancy is determined partly by thedifferent groove effects on electron near wall transport under different discharge param-eters of different regions; however, a more important determinant factor is the relativemagnitude of electron mobility of different regions. Besides, it is found with the PICsimulation that the secondary electron emission of wall material has no effect on wall ge-ometry induced near wall transport current and the current variation with the continuouschange of azimuthal geometry dimension relates to the profile features of the geometry.Furthermore, the measurements of ionization process and thruster performance show thatthe increase of electron mobility in the ionization region makes the ionization region shifttowards channel exit, which further leads to the increase of plume divergence angle anddecrease of thrust and efficiency. The increase of electron mobility in the accelerationregion makes two ionization regions emerge; one is inside channel, the other is outside.This results in a notable increase of plume divergence angle and a remarkable decreaseof efficiency. Above findings indicate that wall geometry with anomalous erosion fea-tures can be an effective tool for deeply understanding the operating mechanisms of Hallthrusters.更多还原