Theoretical And Experimental Investigation Of The Low-Power Magnetically Shielded Hall Effect Thruster

The low-power Hall effect thruster has taken on a substantial prospect in the application of propulsion technology for micro-satellite platform.However,the lifetime of the discharge chamber of the Hall thruster has become one of the bottlenecks restricting the application of the Hall thruster in micro-satellite platform.With the development of Hall thruster's life extension technology,the magnetically shielded technology,has entered the field of vision of researchers,which can reduce the erosion rate of the discharge chamber wall and increase the lifetime.However,during the process of the magnetic shielding applied to low-power Hall thrusters,there is a serious problem of the performance decline.Therefore,based on the above problems,the present study will investigate the influence of magnetic shielding on the performance and lifetime of low-power Hall thrusters.The purpose of this dissertation is to obtain an optimal design method for magnetic shielding.1)A numerical approach based on the particle in cell model(PIC)/plasma chemical dynamics model(PCD)/direct Monte-Carlo collision model(DSMC)has been established to solve the various distribution parameters of the plasma flow field inside the hall thruster.Without the particle acceleration strategy,this model set with two time steps for electrons and ions intended to accurately solve the sheath potential,the particle-to-wall collision process,and the space potential.The solution of plasma flow field was to obtain the propulsion performance parameters(thrust,impulse and anode efficiency)and provide the input conditions for the solution of the wall surface erosion.On this basis,the lattice-lapse model was established to calculate the evolution process of erosion profile of discharge chamber walls.Under the condition of particle-to-wall collision input,the sputtering yield of the chamber wall could be solved.The lattice-lapse model was used to simulate the evolution of discharge chamber contour to estimate the lifetime of discharge chamber wall.2)The numerical model was used to calculate the relevant parameters of the cutting process under 12 cases with different conditions of magnetic shielding design,and the influence mechanisms of different magnetic shielding cases on both the wall lifetime and the propulsion performance were obtained.3)In order to examine the correctness of the numerical model,the discharge tests of HETUS-350 and HETMS-350 thrusters(two 350 class Hall prototype)were conducted in the vacuum chamber,and the parameters of propulsion performance(ion current density,ion energy distribution,electron temperature,atomic excitation line and thrust)were measured by Faraday probe,RPA probe,spectrometer and thrust frame,and compared with the calculated values,it was found that there was a high error in judging the collision between high energy electron and atom,After the geometric mean of the collision section is corrected,the error is eliminated to a great extent.In addition,the sputtering quality and erosion profile of the discharge chamber wall were verified by high precision electronic balance and optical microscope.After the causing reason of errors being found,the carbon deposition rate model was introduced to correct the calculation error of the sputtering yield,and several empirical parameters were introduced to correct the error induced by"sputtering excess".4)Based on the double influence law of magnetic shielding on lifetime and performance,the design parameter L_mof adjusting the magnetic shielding effect was established(L_mrepresents the distance between the farthest point of wall magnetic force line in the discharge chamber and the point of the magnetic induction intensity maximum in the middle line),and the optimization strategy of L_min different propulsion tasks was obtained.Meanwhile,the new scheme(HETMS-350-2),is improved by the optimization strategy,and its design thrust is 20.05 mN and the design lifetime is about6600 h,which is superior to the original HET-350 series.The main conclusions of this dissertation are as follows:(1)The space potential and wall sheath potential are the direct factors of magnetic shielding to reduce wall erosion.Further,the net residual charge in space and the electron temperature near the wall are the parameters of a deeper reason.However,the mechanism of both changes depends on the extending internally of the wall magnetic field line and the magnetic field line configuration design.(2)The mechanism of the effect of magnetic shielding on propulsion performance lies in the downstream extension and the convergence of the main ionization region.The frequency of electron-atomic collision decreases,and the frequency of ion-atomic charge exchange collision increases.As a result,the number of ions and energy decreases.This mechanism still depends on the magnetic route configuration of the magnetic shielding.(3)After a series of corrections of the numerical model,the thrust calculation accuracy of the non-magnetic shielding condition and the strongest magnetic shielding condition can be controlled within 4%,the absolute error of radial displacement in cutting profile calculation is within 0.162mm,the error range of other 10 intermediate operating cases can be thought to be controlled.(4)The magnetic shielding effect should be strengthen reasonably.If the parameter L_mcan be used to characterize the internal extension of the wall magnetic force line and the effect of the magnetic mirror,the L_m/L_c(L_cis the depth of discharge chamber)is generally considered to be a more reasonable option within 0.34?0.39 on the basis of ensuring the lifetime requirements of mission indicators,so that the lifetime and the performance could be given consideration to the greatest extent.