Study on the plasma plume structures in a cylindrical Hall thruster

The structures of two typical plume modes (divergent and convergent plume) and their respective formation mechanisms in a cylindrical Hall thruster are investigated. The non-monotonic distribution of the two-dimensional ion energy distribution function map is obtained to segment the plume into 5 levels with different energy interval. The test results of the ion current fraction indicate that the ion beams in Level III and IV were the main components of the two plume modes. For Level IV (enhanced significantly in the convergent mode), these ions with focusing structure could only originate from the upstream E × B field due to their similar high potential and identical variation tendency in the mode transition, where the electric field converges towards the axis. This is mainly responsible for the formation of convergent mode. On the other hand, for Level III (dominating the divergent mode), these ions with hollow structure and less acceleration voltage are speculated to originate from the downstream magnetic mirror field, where the electric field manifests a defocusing characteristic along the diverging magnetic field lines. This is mainly responsible for the formation of divergent mode. Through the thrust equivalent calculation method, it is revealed that the upstream acceleration associated with the E × B field is the dominant contributor to the enhancement of thrust in the convergent mode. This study on the complex plume structure would facilitate the understanding of the CHT acceleration mechanism and provide useful ideas for the optimization of electric field structure and enhancement of plume efficiency.