Study On Physics Processes In Coupling Area Between Hall Thruster And Its Hollow Cathode

Hall thruster is a kind of plasma propulsion device that has been extensively used for satellite station-keeping and deep space explorations.Coupling area is an intermediate region that links the acceleration channel and hollow cathode.This area injects electrons into the acceleration channel and into the ejected ion beam,therefore has obvious influences on thruster efficiency,ignition reliability,discharge modes and component lifetime.However,due to some ideological misunderstanding,there were few studies on coupling area,and the general physics processes remain unclear.At the same time,this area is also unique in its asymmetric,highly non-equilibrium,highly localized and multicomponent-beam-flow features that distinguish itself from the acceleration channel and the hollow cathode,which encourages re-investigation on its working principles,leading mechanisms and main conflicts.As for such,this thesis studied the main processes of the coupling area by further dividing it spatially into several subdivisions,namely the electron extraction process on cathode exit,the neutralization between ion beam and primary electrons in exterior magnetic field and the plume diverging process.Hoperfully this generalized mapping of key physics effects can provide step stones for futuristic researches of specific dedications.In the electron extraction process,we found that the heatedly-discussed highamplitude discharge oscillations on cathode exit actually originated from a unique instability mechanism.The oscillations were initially triggered by irreversible loss of beam electrons on the keeper electrode and powered by the feedback of external power supplies.This triggering mechanism was denotated as ‘robbing stimulation'.The oscillations could easily develop into instabilities(dual component instability)in the potential and density distributions on cathode exit that were similar to a positive sheath.Then the instability gave rise to higher extraction voltage,power fluctuations and ion sputtering.The parallel magnetic field can shrink the electron beam to be narrower than the keeper orifice and can avoid the robbing stimulation from the first place.As a result,the oscillation amplitude reduced by ?50% and extraction voltage ?30%.This provided instructions on how to determine cathode location in thruster magnetic field.In the neutralization process,we found that the coupling voltage is a result of initial charge separation between ion beam and primary electrons.The magnetic field line threading cathode orifice captured the high density primary electrons,forming a virtual cathode along the field line.As an important plasma structure,the virtual cathode determines electron neutralization pathways and the distance of transverse-magnetic-field transport.We derived an analytical expression for the transport voltage between the ion beam and virtual cathode in the form of generalized Ohm's law,using the magnetic field distribution of a magnetic dipole.The expression followed by sensitivity analysis gives the orientation for decreasing the coupling voltage,or realizing certain coupling voltage value by tuning cathode location.In experiment,the transport voltage was reduced by?40%.The sum of extraction voltage and transport voltage is the coupling voltage,which is a sensitive parameter for voltage unitilization efficiency of the thruster.Then the cathode location's effect on thruster voltage efficiency is explained.In the plume diverging process,we found that the potential distribution in the coupling area formed a deflecting field that radially diverges the ion beam.Therefore,the plume divergence is acutally ‘field-driven divergence',the contribution of previously adopted thermal diffusion scenario in the ‘source-flow approximation' is one magnitude lower.By tuning cathode location,the plume divergence half angle was reduced by ?36%to 18.6°on the length of 0.55 m.The field-driven divergence can co-function with certain microscopic processes(e.g.ion transit-time instability)and lead to anomalously energetic ion flux towards the flanking area of the thruster,accelerating the keeper sputtering by1.5 ? 5.7 times.Under such circumstance,the determining factor of cathode lifetime is cathode location,instead of its operation parameters.Meanwhile,the cathode standalone life test should also take into account the influence of accelerated erosion in coupling discharge,the duration of the test is then given.In some cases,there will be conflict between thruster efficiency and cathode lifetime and we have to sacrifice one of them.For such cases,we proposed a ‘regenerationcompensation' design based on exterior electron emission of hollow cathode.The main purpose was to suppress the robbing stimulation from the first place,to disengage the cathode's dependence on magnetic field environment,and to give the cathode more options on locations.Results showed that such design could significantly suppress ionization oscillations by ?55%,lower plume ion energies by 40% ?50% and broaden self-sustain margin by one magnitude from 1.3A to 0.2A,that allowed cathode to coupling with low power Hall thruster without the cost of increasing cathode flow rate or maintaining keeper current.However,the extraction voltage remained unaffected.This was due to a discontinuous potential distribution,the ‘step region',in cathode plume.The statistics of the step region were highly related to the two-stream instability,which is hardly avoidable in direct current discharge.Therefore,the applicability of regenerationcompensation design should be discussed upon specific purposes.Although the focus of this thesis is on mapping physics processes,the captured physics effects still provided a series of formulas for the design of key component dimensions and the following test setups.Moreover,the proposed physics models were highly extendable in the sense of nonlinearity and chaos,providing references for further topics of multi-scale effects,discharge consistency,et al.