Measurements of Current Distribution on a Two-Grid-Ion-Extraction-System Gridlet

Abstract

Numerical models of ion extraction used in the design of ion thrusters need to be validated by comparison with experimental data; in particular, the magnitudes of ion-impingement currents as functions of screen and discharge voltages should be quantified. This paper provides measurements which may be used to test numerical models of ion extraction using two multiple-hole grids. A grid set, in which an isolated gridlet was embedded in an accelerator grid, was built to measure ion-impingement current distribution. The gridlet was composed of barrel and downstream electrodes, electrically isolated from each other and the rest of the accelerator grid. The obtained measurements of ion-impingement currents on the electrodes can be directly compared with numerical predictions. The barrel-electrode current has been found not to be proportional to the downstream-electrode current, a fact which cannot be shown by measurement of the total accelerator-grid currenL This current was also observed to significantly depend not only on screen voltage but also on discharge voltage, whereas the downstream-electrode current was found to be insensitive to both voltages. The total accelerator-grid current was influenced by both of these.