Numerical Study On Micro-newton Configuration Of Cylindrical Hall Thrusher

In recent years,the proposal of gravitational wave detection and other tasks has made the research on micro-Newton electric thrusters attract wide attention from scholars all over the world.Cylindrical Hall thrusters have significant potential in micro-Newton propulsion.However,due to too small geometric size of the discharge channel,it is difficult to measure the physical parameters inside the discharge channel by experiments,which greatly restricts the development and optimization of micro-Newton configuration of cylindrical Hall thrusters.The distribution characteristics of plasma parameters inside the discharge channel can be obtained by means of numerical simulation.Therefore,numerical simulation is used as the main research method in this paper to carry out relevant research on the key problems in the design of the micro-Newton configuration of cylindrical Hall thrusters.Firstly,the modeling process of PIC simulation method is described in detail,and the special problems involved in the simulation of cylindrical Hall thruster micro-Newton configuration are refined.In the cylindrical coordinate system,due to the existence of the singularity of the central axis,there will be significant particle number distortion near the axis,so the axis interpolation algorithm is modified.In order to study the effect of additional microwave feed on the thruster discharge process,the microwave electric field module is added to the PIC model in this thesis.Secondly,this paper proposes a new boundary module of non-propellant cathode,which can reflect the energy and direction of the emitted electrons more realistically.The accuracy of the model can be verified by comparing the experimental and simulated performance parameters under typical operating conditions.The magnetic field design of cylindrical Hall thrusters is particularly critical if the micro-Newton configuration is to be realized.Cylindrical Hall thrusters can be divided into two categories according to the difference of the magnetic field configuration,one is the magnetic mirror field thruster,and the other is the cusped field thruster.In this paper,the magnetic field of the magnetic mirror field thruster is optimized.The difference of the thruster discharge process under the permanent magnet ring excitation and the permanent magnet column excitation is compared.The research shows that the permanent magnet ring excitation mode can effectively bound the electrons and improve the thruster voltage regulation.At the same time,the influence of the magnetic cusp position and the matching of the magnetic field and the channel is investigated under the permanent magnet ring excitation mode.Finally,the design of the permanent magnet ring excitation combined with the straight channel configuration is determined as the optimal design scheme of the magnetic mirror field thruster.The physical mechanism and experimental performance parameters of the optimized magnetic mirror field thruster and the cusped field thruster are compared and studied.The research shows that the cusped field design can effectively improve the voltage regulation range and avoid the occurrence of anode overheating,effectively solve the ionization difficulty under the low propellant density,and achieve a larger thrust regulation range.Therefore,the cusped magnetic field configuration is finally established as the magnetic field design scheme of the micro-Newton thruster.In order to solve the difficult problem of micro-Newton discharge under dilute propellant,it is necessary to introduce additional microwave feed-in to enhance ionization.This paper first analyzes the influence of additional microwave feed-in on the discharge characteristics of thrusters under different flow rates.Studies have shown that additional microwave feed-in can significantly improve the performance of the thruster at low flow rates,but the introduction of additional microwave feed-in at high flow rates has no obvious effect on the performance improvement of the thruster.The reason is that the strong microwave electric field is located upstream of the channel,and most electrons participate in ionization on the channel downstream,so those electrons cannot be effectively heated by the microwave electric field.According to this phenomenon,this paper uses numerical simulation to optimize the channel length and the number of permanent magnets.The research shows that the design of two-stage magnet combined with short channel is more beneficial for electrons moving to the channel upstream strong electric field region,so that these electrons can be better heated by microwave to improve the performance of the thruster.This paper also analyzes the phenomenon of thruster performance degradation caused by antenna sputtering.After the wall is deposited,it is connected with the anode to form a high potential,which causes the acceleration region to be more concentrated at the exit of the channel,so that the electron energy of the plume is improved.The electron flux of the magnetic cusp wall at the exit of the channel increases,and the increase of the electron loss leads to the weakening of the ionization process,so the thruster performance is significantly attenuated.Finally,the coupled discharge process between non-propellant cathode and microwave feed-in enhanced cusped field thruster is simulated.Firstly,the distribution characteristics of typical plasma parameters in the coupled discharge process are analyzed.It is found that the electron temperature of the non-propellant cathode is significantly higher than that of the traditional hollow cathode,and there is a strong additional ionization near the cathode outlet in the plume region.In this paper,the influence of cathode position on the discharge process of thruster is studied.Too close cathode position will lead to serious electron loss of thruster outlet shell,and too far cathode position will lead to increased electron loss in plume region.In this paper,the selection of the voltage parameters of the extraction electrode is analyzed.The results show that the appropriate increase of the extraction electrode potential is beneficial to the electron extraction,but the excessive potential of the extraction electrode will lead to the stronger bounding ability of the extraction electrode to the electron,the ionization of the plume region is intensified,the ionization inside the channel is weakened,and the thruster performance is significantly reduced.Relevant research provides a reference for the design and optimization of the cathode.