Numerical Study On Erosion Mechanisms In Hall Thruster

Due to the high specific impulse, high efficiency and compact structure, Hall thrusters have become the preferred electric propulsion for the deep space exploration. The deep space exploration mission requires the long life of Hall thruster which is mainly limited by the channel wall erosion. Thus, the erosion problems become the hotspot among the life problems of Hall thruster. The wall erosion includes classic erosion and anomalous erosion. The studies of the classic erosion usually make a lot of simplifications on the plasma dynamic and do not consider the wall evolution. In addition, the effects of laboratory and working conditions on the classic erosion are still unacknowledged. On the other hand, the anomalous erosion which is the main failure cause of the later stage of lifetime is also unacknowledged. In order to gain further insight into the wall erosion mechanism and provide the theory basis for optimizing design of Hall thruster, this thesis includes the following aspects:First, based on the requirement of a more actual model for the coupling of wall surface evolution and plasma dynamic, the dynamic interface algorithm based on the immersed finite element and full PIC methods is developed. The boundary types which corresponds to the different wall conditions are also introduced into the above method, as well as the collision module, the acceleration module and the dynamic interface module. Thus, this method can model most phenomenons in Hall thruster completely and actually.Later, the distributions and evolutions of physical characteristics in main flow region are studied during the classic erosion evolution process. The effects of charge-exchange collision on the physical characteristic distributions and the wall erosion are investigated. The results show that the ion density decreases and the wall erosion alleviates after taking the charge-exchange collision into consideration. The effects of preionization on the physical characteristic distributions and the wall erosion are also investigated. The results indicate that the preionization donot affect the main physical characteristic in the discharge channel. While the wall erosion will be aggravated by the extra sputter plasma which is introduced by the preionization process.Subsequently, the exhaustive studies of the sheath are made in the main beam plane based on the above results. The results show that the sheath potential is greatly affected by the electron temperature and the vertical ion drift velocity to the wall, but is weakly affected by the plasma density and the parallel ion drift velocity to the wall. The axis low oscillation electric field intensity can cause the instability of sheath, while the instability will be eliminated as the electron temperature reaches 20 e V. The radial magnetic field intensity and the secondary electron emission have little effect on the sheath within the scope of this paper.Last, according to the deep analysis of the morphology and exit region of the anomalous erosion, this paper proposes that the anomalous erosion phenomenon is caused by its self-excitation mechanism. The self-excitation mechanism is then verified by both theoretical analysis and emulation study. A azimuthal sheath model is developed based on the dynamic interface immersed finite element PIC method, and the formation of the excitation source which is formed by the cumulative effect of ion sputter is well studied. Then the self-excitation mechanism is verified by the evolution simulation of the azimuthal wall on which the excitation source is already exist. The results show that the distribution of the sheath will change after the formation of the excitation source, a lower potential will form at the side of the wall along the wave propagation direction, then more ions will incident this side and cause more serious erosion. Finally, the azimuthal interval of the azimuthal wall will keep the same as the electron lamor radius and the morphology of the azimuthal wall will evolve to macth the equipotential line of the local sheath. Beside the increasing of the radial gap size, the altitude of the wall produces a nearly constant descent along the radial direction. The constant descent velocity does not decrease with the time increases or the erosion extent.On the other hand, the anlysis results of the impact factors on excitation source show that the electron temperature can significant enhance the excitation source, the axis low oscillation electric field intensity slight enhance the excitation source, while the vertical ion drift velocity to the wall almost have no effect on the excitation source. When there are pits which are formed by the classic erosion on the initial wall surface, the azimuthal erosion will smooth the pits and form the anomalous erosion morphology according to the self-excitation process.