A study of the influence of different grid structures on plasma characteristics in the discharge chamber of an ion thruster

The grid structure has significant effects on the discharge characteristics of an ion thruster. The discharge performances of a 30 cm diameter ion thruster with flat, convex and concave grids are studied. The analysis results show that the discharge chamber with a convex grid has a larger 'magnetic-field free area' than the others, and the parallelism of the magnetic-field isopotential lines and anode is generally the same in the three models. Plasma densities of the three structures at the grid outlet are in the range of 3.1 × 10¹⁶–6.9 × 10¹⁷ m⁻³. Along the thruster axis direction, the electron temperature in the chamber with the convex and concave grids is in the range of 3.3–3.5 eV, while that with a flat grid is lower, in the range of 3.1–3.5 eV. In addition, the convex and the concave grids have better uniform distribution of electron temperature. Moreover, the collision frequency ratios show that the axial degree of ionization of the three models is the highest, and the flat grid has the highest discharge efficiency, followed by the convex grid and the concave grid is the least efficient. The test and simulation results of the 30 cm diameter ion thruster with the convex grid show that the measurement and calculation results are 3.67 A and 3.44 A, respectively, and the error above mainly comes from the ignorance of the doubly charged ions and parameter settings in the model. The comparison error between the simulation and measurement of beam current density is mainly caused by the actual thermal deformation of the grids during the discharge process, which leads to the change in electric potential distribution and variation of the focusing characteristics of the grids. Upon consideration of discharge performance and the thermal grid gap variation, it can be concluded that the flat and concave grids are more suitable for small-diameter ion thrusters, while the convex grid is a more reasonable choice for the higher-power and larger-diameter thrusters.