Grid-current levels were obtained at various operating points of a T5 ion thruster using ight-model extraction grids. Because of the improved grid design and precision fabrication, better analysis of the dependence of grid current onoperatingpointcouldbeperformed. The decelerator current can be interpreted in terms of impingement by charge exchange ions, although the dependence on operating point is not simple. The accelerator grid current was found to have a major component that appears to be independent of charge exchange. A direct simulation Monte Carlo–particle-in-cell model was constructed that includes the effects of momentum transfer, ion-neutral charge exchange, coulomb collisions, and double-charge ions. Through the use of existing far- eld data on the energy distribution of beam ions, good relative and absolute agreement between theory and experiment has been achieved. The results suggest a new erosion mechanism that depends on ion radial velocity distribution during extraction. Ions with high radial velocity have relatively high probability of impinging on the accelerator grid and will do so with larger sputter yield than charge exchange ions. Double-charge ions have a lower impingement probability.The results have implicationsfor the lifetimeandcontaminationeffects of ion thrusters using extraction grids.
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Crofton et al. (2001) studied this question.
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