The Influence Of Magnetic Field Topology On Electron Motion In Hall Thrusters

A Hall thruster, with its high-efficiency and high specific impulse, is one of the most important electric propulsion devices widely used for spacecraft propulsion such as attitude control, orbital transfer and deep space exploration missions in the future. The magnetic field in Hall thruster plays a significant role in confining magnetized electrons in stable closed-loop Hall drift, and it also affects electron axial conductivity which determines the distribution of plasma parameters in Hall thrusters. This paper mainly focuses on the influence of magnetic field topology on electron motion in Hall thrusters.The main principles of the particle-in-cell algorithm for plasma in Hall thrusters are introduced, involving basic parts of particle-in-cell. Then we develop a test particle method combining Monte Carlo method to research on electron motion, making simulating electron motion feasible.With the concept of magnetic len applied for the magnetic design of Hall thruster, electron motion in such curved magnetic fields has changed compared to that in mainly straight magnetic fields. Adopting test particle method, we simulate electron motion in magnetic mirror confinement and puts forward magnetic mirror as another factor for the asymmetry of radial near wall conductivity current profile. Besides, the magnetic mirror effect will further affect plasma parameters in Hall thruster through impacting electron conductivity.The axial gradient of magnetic filed along the channel serves as another significant factor for the magnetic design of Hall thrusters. With it enhanced, the length of the accelerating region becomes short, which reduces the length and strength of erosion. However, when the characteristic length approaches to Larmor radius, the stable Hall drift will be destroyed. The influence of the axial gradient of the magnetic field on electron motion is examined. Electron dynamics in the accelerating region are modeled using collisionless guiding center theory to predict the effect of the axial gradient of the magnetic field on azimuthal drift velocities and resulting particle trajectories, so does electron cross-field mobility. At limit, the condition that electron motion in stable Hall drift destroyed is obtained. Finally, the important function of magnetic field parameters in Hall thruster is summarized based on the influence of the magnetic field on electron motion. In the case of real magnetic fields in a Hall thruster, the fundamental plasma parameter in the channel is attempted to be presented, using the full particle-in-cell simulation of a Hall thruster. It, as an effective tool, is of great aid to enhance the understanding of the role of the magnetic field on the performance of a Hall thruster.