Effects of translation misalignment on ion optics with slit apertures

Abstract

For ion thrusters, the deflection of the ion beam caused by the relative translation of the grids is one of the primary factors limiting the erosion lifetime of the ion optical system. A two-dimensional (2D) simulation package of the ion optic system is developed to investigate the ion sputtering corrosion due to grid translation misalignment. For benchmark cases, the 2D simulation package shows a reasonable consistency compared to the experimental method in ion beam deflection angle and drain-to-beam current ratio, indicating the effectiveness of the simulation package. The deviation between the simulated deflection angle and the experimental value is within 8.86%. Furthermore, this 2D package is employed to analyze the variation patterns of ion beam, ion collection, and the distribution of ion sputtering rates caused by grid translation. The simulation results indicate that the ion beam deflection occurs in the direction opposite to grid translation. The number of ions collected at different positions on the acceleration grid shows different tendencies. Only the upstream surface $S_{\mathrm{y-}}$ and the aperture surface $S_{\mathrm{x+}}$ will be subjected to energetic ion impingement. The sputtering of energetic ions on these two surfaces becomes the dominant factor limiting the grid’s ion corrosion lifetime when the grid translation is significant. The sputtering rate of energetic ions can exceed that of charge exchange (CEX) ions by more than 10 times. The proportion of the region where energetic ions contribute to sputtering expands with increasing grid misalignment, reaching up to 52.5% on surface $S_{\mathrm{x+}}$. Additionally, the downstream surface $S_{\mathrm{y+}}$ and the aperture surface $S_{\mathrm{x-}}$ are only subjected to CEX ion sputtering regardless of grid translation. Moreover, the CEX ion sputtering regions on surfaces $S_{\mathrm{x-}}$ and $S_{\mathrm{x+}}$ expand as the grid misalignment distance increases. The peak in the ion sputtering rate distribution profile on surface $S_{\mathrm{y+}}$ becomes more prominent due to grid translation, with the sputtering rate at the peak position reaching approximately 1.65 times that of the surrounding lower-rate regions. The analysis of the electric field indicates that local electric field variations caused by grid misalignment are the underlying reason for the ion erosion characteristics.