Study On Electron Near Wall Transport Over The Whole Lifetime Of Hall Thrusters

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.