Effect Of Magnetic Field On Discharge Characteristics In Channel Of Hall Thrusters

Hall Thruster is a kind of propulsion devices providing micro thrust for the on-orbit operation of spacecrafts. Due to its high efficiency, high specific impulse and high reliability, Hall thruster has been widely applied in many propulsion missions including orbit transfer, station keeping, and attitude control etc. In recent years, with the development of space technology such as large satellite platform, micro-satellite net and deep space exploration, the performance of the Hall Thrusters is left open to be improved as well.Strong axial electric field is built through applied magnetic field to prevent electrons transport along the channel of Hall thrusters, and then the ion flux can be accelerated effectively, so that magnetic field not only plays a great role in affecting the discharge characteristics and performance of Hall thrusters, but also acts as an important DOF (degree of freedom) for optimization design. In this paper, researches mainly focus on the influences of magnetic field affects the discharge characteristics of Hall thrusters, and base on this work, the control rules of magnetic field on electrons transport has been investigated, and thereby providing fundamental theories for designing optimized magnetic field.The main work in this paper is divided into following five parts:1. According to the physical mechanisms that magnetic field affect the operation of Hall thrusters, requirements of the magnetic field intensity and magnetic field gradient for Hall thrusters were systematically summarized. The distribution of magnetic field in discharge channel of Hall thrusters can come down to two quantitative parameters: magnetic field intensity in exit plane and axial magnetic field gradient in strong field zone, thus a research thought that analyzing the effect of magnetic field on discharge characteristics and electrons transport processes put forward, and thereby, optimized range of magnetic field matched to actual working conditions can be acquired.2. Experimental setups were designed according to the physical requirements of magnetic field intensity and axial gradient for Hall thrusters. The effects of magnetic field intensity and axial gradient on discharge characteristics of Hall thrusters were investigated respectively by using spectrometer, multi-grid probe, real-time spectrum analyzer and other measuring instruments, and started with analyzing ionization of propellants, acceleration of ion flux and plasma oscillation in strong filed zone, the optimized range of magnetic field strength and axial gradient were summarized.3. Electron conductive current in exit plane at different working conditions which refers to different discharge voltage or propellant mass flow was calculated by using magneto-ampere character and voltage-ampere character of Hall thruster and multi-grid probe respectively, and then the dominant electron conductive mechanism was analyzed according to the relations between electron current and local magnetic field intensity, finally the significant analysis was conducted by F test method.4. Electron energy dissipation and heating mechanisms in channel of Hall thruster were analyzed. The dominant electron heating mechanism was investigated through analyzing the relations between electron conductive current and electron temperature under different magnetic field intensity. Moreover, during the discharge experiments of Hall thruster, an anomalous electron heating phenomenon in particular range of magnetic field intensity was firstly discovered, and research work has been done on the changing of the particle proportions with different valences.5. Since the choosing of optimized magnetic field depends largely on external experiment conditions, the effects of vacuum backpressure on choosing magnetic field were analyzed. Firstly, the mechanisms of vacuum backpressure on the operation of Hall thrusters including plume effect, discharge effect, and surface deposition effect were discussed. Secondly, experimental research was conducted to investigate the discharge characteristics of Hall thruster under different backpressures, and then the influences of backpressure on ionization and acceleration processes of propellants were analyzed. Finally, the upper and lower limit of vacuum backpressure which matched actual working conditions of thrusters and the dimensions of vacuum tank were given.