Design of High Efficiency Grid System for Water Propellant Miniature Ion Thrusters

The two-grid ion optics of a water propellant miniature ion propulsion system (MIPS) was numerically studied by full-aperture ion-optics simulation considering an inhomogeneous plasma source in the discharge chamber. Three dominant ion species (H2O+, OH+, and H+) of water vapor plasma were included in the model. Their trajectories were tracked considering charge-exchange and elastic collisions between ions and neutral water molecules. The model was validated by experiments using the MIPS with two grids having 295 apertures each. The calculated accelerator impingement current agreed reasonably well with those obtained from the experiment. To increase its low propellant utilization efficiency, a new grid design method was introduced. This successfully improved neutral confinement by decreasing the diameter of the accelerator grid apertures without causing direct impingement of ions by optimizing the grid thickness distribution using the Particle In Cell-Monte Carlo Collision (PIC-MCC) calculation results.