Optimized design of the ion optics based on the over-perveance characteristics of the beam current

Abstract

To illustrate the erosion mechanism of the edge aperture by high-energy ions, a three-dimension simulation model was established, and a particle-in-cell method was applied in tracking the hitting processes as well as a Monte Carlo collision method was used to deal with the particles’ collision. Numerical results showed that the potentials interacted among the sheaths of adjacent holes and the beam ions were over-focused in the accelerator gap, causing some ions to be intercepted on the downstream wall of the decelerator grid. This was a typical under-perveance characteristic of the beam current, operating at significantly less than the optimal perveance and corresponding to lower beam currents than optimal combination, which pushed the sheath to the left farther into the plasma. The farther the grid was, the more serious erosion patterns developed. In addition, the charge exchange ions were attracted and bombarded on the decelerator and the accelerator grids, respectively. The ion optics of the 30 cm ion thruster was optimized from four-zone to three-zone, where the plasma density upstream of the screen grid was increased by 1.5% and the perveance characteristics of the beam ions were improved such that many ions struck the edge hole. The background pressure was reduced from 3.0 × 10−3 to 8.0 × 10−4 Pa to decrease the erosion depths of the grids caused by the charge exchange ions, about 9.09% and two orders of magnitude, respectively, for the accelerator and decelerator grids.