Sensitivity Analysis of the Effects of Doubly Charged Ions on Ion Acceleration Grid Erosion

Abstract

A sensitivity analysis was performed using a three-dimensional code to understand the effect of doubly charged ions on the erosion of ion acceleration grids. A preliminary analysis showed that the neutral mass flow rate, estimated by assuming all the ions are singly charged, contained significant error when the doubly charged ion fraction was nonnegligible. Calculations were conducted for the "10 EM1 ion acceleration grid system with different doubly charged ion fractions. For fractions of 0.1 and 0.2, which were similar to the experimental conditions, the simulation results for the acceleration grid current and grid mass loss were in good agreement with the experimental data. Calculations showed that when the doubly charged ion fraction was reduced from 0.1 to 0, the acceleration grid current and grid mass loss changed by À14% and À18%, respectively. Further, when the fraction was increased from 0.1 to 0.2, the acceleration grid current and grid mass loss changed by þ9.5% and þ32%, respectively. Electron backstreaming was also found to vary with the doubly charged ion fraction: it occurred 14% slower when the fraction was decreased from 0.1 to 0 and 12% faster when the fraction was increased from 0.1 to 0.2. The structural failure of the deceleration grid was less sensitive: the deceleration grid eroded 8.3% slower when the doubly charged ion fraction was reduced from 0.1 to 0 and 8.6% faster when the fraction was increased from 0.1 to 0.2. n inlet : plasma density at the inlet boundary r s , r a , r d : aperture radii of the screen grid, acceleration grid and deceleration grid, respectively t s , t a , t d : thicknesses of the screen grid, acceleration grid and deceleration grid, respectively T e : electron temperature T k : stay time of the k-th flux tube inside an element v: velocity vector V B : Bohm velocity V d : discharge voltage V element : element volume

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