Theoretical And Experimental Research On Plasma Beam Focalizati On In A Hall Thruster Using Krypton Propellant

Hall thrusters,with the advantages of high efficiency,high specific impulse and high reliability,have been widely used in various practical missions and become a spot among the electric propulsion device research all over the world.As the reserves of xenon gas becomes fewer and fewer,the research of Hall thrusters working with krypton gas as the propellant has been concerned about more and more extensively.However,a number of technique bottlenecks are encountered on the course of performance optimization of this kind of thruster.Recently,some institutes in Russia,America and Europe have studied the performance of Hall thrusters working with krypton propellant.They have noticed the key role of plasma beam focusing on the thruster performance;however,they have not realized a good focusing state of the plasma beam yet.Therefore,to solve this essential problem,the thesis here presents an attempt to obtain the basic principle and the mechanism on achieving a well plasma beam focusing state with both the theoretical analysis and the experimental study.To investigate the krypton propellant focalization during the ionization and acceleration processes,we have improved the existing experimental diagnoses.Both the probe and spectrum measurements are used in the near field of the plasma plume and the ionization region of the channel.The accuracy of the measured data is ensured,providing a foundation for the study of issues related.By optimizing the structure and adjusting the parameters of HET-P70,a designed Hall thruster working with krypton propellant,we have obtained a good focusing state of plasma beam and a well small plume divergence of 11.5 degree,which has been justified a heading level in the world.This achievement provides a good platform for our further study.Consequently,we have quantified the magnetic field configuration,analyzed its regulation with the work condition,obtained its features for the optimized focusing of plasma beam,and found out the internal connection between it and the plume shape.Moreover,by analyzing the ion dynamics in the channel,we have studied the effect of propellant ionization position on the plasma beam focusing deeply and illustrated the corresponding physics mechanism.We have further explained the effects of anode mass flow rate,low-frequency oscillation and discharge voltage on the focusing characteristics of the krypton plasma beam.By using the Monte-Carlo simulation method,we then studied the effects of ionization region width,the thermalized potential and the ion initial energy on the state of the plasma beam focusing.Furthermore,for the ionization of the krypton propellant,we have determined its uppermost factor,interpreted the relationship between electron energy and ionization region position,and expounded the effect of magnetic field intensity,magnetic field gradient,and mass flow rate on the position and width of the ionization region.To solve the decrease of propellant utilization and the extension of ionization region under the condition of low krypton gas flow rate,we have applied an adjustable channel cross section method to control the density distribution and the flow of the krypton propellant.This method is justified to be effective experimentally.Finally,we have studied the effect of hollow cathode heat power on the focusing characteristics of plasma beam and the thruster performance parameters,such as thrust,specific impulse and efficiency.The mechanism is then explained with the electron emission theory and the voltage change in the coupling region.Moreover,it is found that the worse of the thruster performance when the hollow cathode is over heated is related with the extension of the ionization region and the increase of the plume divergence along with the enhancement of the low-frequency oscillation.A brief analysis is then presented on this issue.