An anomalous phenomenon was observed in the retarding potential analyzer （RPA） measurements of the energy of the ion beam from an 8 cm argon ion source. The current-voltage （Ⅰ- Ⅴ） curve, which should theoretically descend, went up as the ion retarding potential was increased. Various explanations, such as the Townsend discharge theory and secondary electron emission etc. were proposed but denied by the theory application condition or the experiment results. An angle of about -10° was found between the axes of the ion beam and the RPA according to the contours of the ion beam density and direction. The particle simulation and experiment of the sum of the collector and wall current were conducted at different incident ion angles. The trends of the Ⅰ- Ⅴ curve in simulation results conformed with the experimental results in most cases. The ion trajectories were simulated at different retarding potentials with an incident angle of -10°. According ：to these results, the reason for the anomalous phenomenon is that when there is a specific angle between the axes of the ion beam and the RPA, more ions are repelled from the vicinity of the ion retarding grid to avoid striking on the grid as the ion retarding potential increases. These redundant ions reach the plate and thus lead to the formation of an ascending Ⅰ- Ⅴ curve.
Based on the three-dimensional particle-in-cell （PIC） method and Compute Unified Device Architecture （CUDA）, a parallel particle simulation code combined with a graphic processor unit （GPU） has been developed for the simulation of charge-exchange （CEX） xenon ions in the plume of an ion thruster. Using the proposed technique, the potential and CEX plasma distribution are calculated for the ion thruster plume surrounding the DS1 spacecraft at different thrust levels. The simulation results are in good agreement with measured CEX ion parameters reported in literature, and the CPU＇s results are equal to a CPU＇s. Compared with a single CPU Intel Core 2 E6300, 16-processor GPU NVIDIA GeForce 9400 GT indicates a speedup factor of 3.6 when the total macro particle number is 1.1 × 10^6. The simulation results also reveal how the back flow CEX plasma affects the spacecraft floating potential, which indicates that the plume of the ion thruster is indeed able to alleviate the extreme negative floating potentials of spacecraft in geosynchronous orbit.
Based on the particle-in-cell （PIC） method, a two-dimensional numerical scheme was developed to investigate the ion beam extraction phenomena through the ion thruster optics. According to the calculated results, the plasma sheath upstream of the screen grid, the electric field in the calculation domain, and the ion and electron spatial distributions are obtained for different accelerator grid voltages. The results indicate that the accelerator grid voltage affects the plasma sheath upstream of the screen grid significantly. It is found that a moderate accelerator grid voltage results in an ion optical performance better than either a higher or lower voltage, from a point of ion extraction from the discharge chamber and erosion mitigation of the accelerator grid due to the direct ion impingement.
