| Hall thrusters have been used widely in orbit correction and station-keeping of geostationary satellites for the advantage of small size, light weight, high specific impulse, and long life. Engineering applications have achieved since the seventies of last century. The mechanism of electron near wall conduction (NWC) is one of the most important physics problems for thruster performance. The electrons conduction mechanism determines the electric potential distribution in channel, characteristics of ion acceleration, and determines the thruster efficiency, lifetime and specific impulse at last. Sheath was found to be a very influential factor to electronic conduction behavior near the wall. It is produced by the interactions of plasma and insulating wall, the electric field distribution in sheath influence the behavior of electrons near the wall, and influence the near wall conduction further.Near wall conduction, sheath, and the relation between them are a hot topic to be concerned and discussed by Scholars. However, due to complexity physical processes of hall thruster, the contribution of near wall conduction of electronic to current is still without any definite conclusion. In this thesis, it is founded that the research is restricted by the understanding of characteristics of sheath and the near wall conduction mechanism, Based on this, this paper carried out research work related.First, Two-dimension particle model (Particle-In-Cell,PIC) was built to compute the sheath and near wall conduction. Because of the small size of sheath, the method of PIC with many advantages become the Research tools in this paper.Secondly, Simulation results shows that the dynamic characteristics of sheath are largely influenced by electron temperature. With a low electron temperature, secondary electron emission coefficient is small than 1, the sheath has the characteristics of one-dimension dynamic sheath. The secondary electron emission coefficient increase with the electron temperature and the sheath begin periodic oscillation with the frequency near that of the plasma oscillation, at this time the secondary electron emission coefficient is below 1. When the electron temperature continues to rise, and the secondary electron emission coefficient near 1, sheath oscillations come into the range of space oscillation. At this range, except the oscillation in time domain, the sheath electric potential present fluctuation in space domain, the oscillation size is at the order of wavelength of electrostatic wave. When the electrons temperature becomes higher, the secondary electron emission coefficient bigger than 1, sheath enters into space charge saturation region.Then, we found that the different conduction of sheath have important effect on near wall conduction. Before the sheath present space oscillation, with classic sheath and oscillation in time domain, the simulation results show the conduction is formed by secondary-electrons produced by the oscillation of electrons and wall. Near wall conduction current and classical conduction current (the current produced by oscillation of electrons and heavy particles) are same order of magnitude, the result is save with many researchers. The conduction is too little to explain the big current in experiments. However, when the sheath state enter the space oscillation, non-normal component of sheath electrons field come to play effect, the electrons that can not overcome the sheath barrier and reach wall have take part in the new wall conduction. Because of the reasonable quantity of electrons, the current of near wall conduction is bigger than classical current by an order of magnitude, the effect can explain the current in experiments.Finally, No matter in what state of the sheath, the near wall current is proportional to electric field strength, and inversely proportional to the square of the magnetic field strength. The computation results further verify the near wall conduction is taken as"collision conduction"for through the magnetic field. |
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