Numerical Simulation Research About Sheath Characteristics And Lunar Dust Levitation In Terminator Region Of Lunar Surface

There are many troubles during the exploration of astrovehicle and astronaut on the moon due to the levitated lunar dust, such as clogging the mechanisms, abrading the key component, causing failure in the thermal control system, compromising seals, disabling the sensitive materials and so on. Thus it is very important to study mechanism of the lunar dust levitation around the lunar surface. Since the mechanism for the transport of minor size lunar dust particles is electrostatic transport and the intense electric field formed in lunar terminator, the sheath structure and the electric field in lunar terminator, which is the key point to explore the mechanism of lunar dust levitation on lunar surface, should be studied very carefully and deeply. It has been found that only few works have been carried out on the research of the sheath structure and the electric field on the terminator region.A finite element numerical simulation method which is based on Immersed Finite Element Particle in Cell (IFE-PIC) is developed in this dissertation to study the sheath structure and electric field around the lunar surface. According to the research on the single sheath, it can be found that the sheath near the lunar surface is oscillating and the physical mechanism for this phenomenon is analyzed. Then the sheath structure and electric field near the terminator region are simulated by solving Possion’s equation with numerical model. The movement of lunar dust in the terminator region near the lunar surface is simulated with IFE-PIC method to study the mechanism of the levitation process. The study includes the following aspects:Due to the phenomenon that the charge deposits on the lunar surface when the solar wind plasma interacts with the moon since the lunar surface is dielectric, a bilinear nonhomogeneous immersed finite element method which can capture the jump on the surface is developed. Based on this finite element method, we develop a numerical simulation code which is based on bilinear nonhomogeneous immersed finite element and particle in cell to study the sheath structure on the lunar surface.The boundary condition for IFE-PIC method simulating the movement of plasma particles is considered in this thesis, such as the infinite boundary condition and the periodic boundary condition. When the standard particle in cell algorithm is combined with the immersed finite element, the traditional deposit algorithm is modified to suit the IFE-PIC method. Also an iterative immersed finite element method for an electric potential interface problem based on given surface electric quantity is aevelopea to simulate the potential aistribution arouna the lunar surface when we can only obtain the total electric quantity on the lunar surface.The sheath structure around the lunar surface is simulated with the IFE-PIC method.Numerical simulations about the space charge saturated sheath and classical sheath are carried out. Through the simulations, we can obtain the potential distribution, charge density distribution, the density distribution of solar wind ions, the density distribution of solar wind electrons and the density distribution of photoelectron in the simulation domain. The characteristics of the lunar sheath oscillation are analyzed according to the simulation result and then we explain the formation mechanism of sheath oscillation.The effect of solar wind speed and the photoelectron density to the sheath structure of space charge saturated sheath and classical sheath is discussed in this part. According to the simulation results, the transformation process between different types of sheath around lunar surface and the effect on the characteristics of sheath oscillation are analyzed. Then numerical simulations about the sheath structure and electric field around the terminator region (region around the terminator line and local region) are carried out, and then the cause of intense electric field in the terminator region is analyzed. Also, the process that the sheath oscillation on dayside affects the sheath structure on the nightside is analyzed.A mathematical model is developed to simulate the charging process of lunar dust particles. Based on the numerical simulation results in the terminator region, the simulations about the levitation process of lunar dust particles in terminator region are carried out and the transport mechanism of dust particles in intense electrostatic field is analyzed to explain the observed Horizon Glow phenomenon in the terminator region around the terminator line.The results obtained in the dissertation improve the understanding of space plasma environment around the lunar surface and provide important theoretical support for the lunar dust problems encountered in the exploration on the moon to promote the development of lunar exploration program.