Ion Impingement Limits of Sub-Scale Ion Optics: Comparison of Simulation and Experiment

The behavior of the accelerator grid current defines the operational envelope of an ion optics system. Under normal operating conditions, the ions in the discharge plasma upstream of the screen grid should form focused ion beamlets when extracted and accelerated through the optics. However, if the discharge plasma density is too low, the ion beamlet may become over-focused by the upstream sheath, resulting in cross-over ion impingement on the acceleration grid. On the other hand, if the discharge plasma density is too high, the ion beamlet may become under-focused, resulting in direct interception of beam ions by the acceleration grid (perveance limit). Thruster operation that results in direct impingement will cause excessive sputter erosion of the accelerator grid. Some of the sputtered material will build up into thick films on the screen grid, and one ion thruster failure mode involves shedding of these sputter deposited films. Accurate predictions of the cross-over limit and the perveance limit are needed to ensure long-term, trouble-free operation. Most existing ion optics models are either axi-symmetric models or three-dimensional models constructed for a cross section of one twelfth of a single aperture (a 30 degree by 60 degree right-triangle cross section) using the six-fold hexagonal symmetry of the aperture array. Because of the inherent symmetric boundary conditions, existing models are suitable for local simulations of a single ion beamlet for an aperture located near the center of the optics grid. Existing modelling predictions of important ion optics parameters, such as the cross-over limit, perveance limit, and backstreaming limit, are all based on extrapolations of single beamlet simulations, where one uses different upstream plasma conditions to represent apertures at different locations on the grid surface. Previously, numerical simulations based on a single aperture model were carried to model the impingement limit for the Carbon Based Ion Optics (CBIO) gridlet. While the simulation gave good predictions of the perveance and backstreaming limits, it did not indicate a crossover limit for much of the operating voltage range for the CBIO geometry. Both the disagreement between simulation and experiment and the results from ongoing experimental studies at Colorado State University (CSU) suggest that the effects of geometric asymmetry and plasma sheath interaction between adjacent holes, may have a significant influence on the cross-over limit. Such effects could explain why early numerical modeling programs in the electric propulsion community typically predict a crossover limit value that can be 50% lower than the experimentally measured value. A new optics model was developed recently at Virginia Tech (VT) for three-dimensional global simulations of plasma flow in an entire sub-scale ion optics. This model explicitly includes apertures located near the ∗Associate Professor, Department of Aerospace and Ocean Engineering, AIAA Associate Fellow. jowangvt.edu †Postdoctoral Fellow, Department of Aerospace and Ocean Engineering ‡Graduate Research Assistant, Department of Mechanical Engineering §Assistant Professor, Department of Mechanical Engineering