GRID EROSION MODELING OF THE NEXT ION THRUSTER OPTICS

Ion optics modeling is being conducted to aid in the design process of the NEXT thruster. Determining the life of an ion thruster experimentally is a difficult, costly, and lengthy process. Modeling can give order of magnitude or better life estimates in a relatively short time, and at a low cost. This has special relevance in the case of NEXT, as the consideration of two accelerator grid geometries makes experimental testing even more complex and expensive. Results from several different computational studies of the NEXT ion thruster optics are presented. A study of the effect of beam voltage on accelerator grid aperture wall erosion shows a non-monotonic, complex behavior. Comparison to experimental performance data indicates improvements in simulation of the accelerator grid current, as well as very good agreement with other quantities. Also examined is the effect of ion optics choice on the thruster life, showing that TAG optics provide better margin against electron backstreaming than NSTAR optics. The model is used to predict the change in performance with increasing accelerator grid voltage, showing that although the current collected on the accel grid downstream face increases, the erosion rate decreases. A study is presented for varying doubly-ionized xenon current fraction. The results show that computed performance data is not very sensitive to the current fraction. The focus of this paper is on modeling of the NEXT thruster ion optics, considering both NSTAR and TAG accelerator grids. Simulation data from various thruster operating points and with varying simulation parameters are analyzed at NEXT operating conditions. Life estimates and performance results are presented for the maximum operating condition of the NEXT thruster, considering differences between the use of progressive erosion rates and initial erosion rates. Performance changes with varying beam voltage are analyzed and compared to experimental data. The effects on the simulated results of varying accelerator grid voltage as well as varying the doubly-charged ion fraction are also presented.