Ion Optics’ Thermally-Induced Gap and Aperture Misalignment Reduction by Selective Laser Melting

Abstract

High operation temperatures in ion thrusters with metallic ion optics lead to unequal thermal expansion of screen and accelerator grids. Larger normal and tangential displacements of the screen grid result in grid-to-grid gap change and aperture misalignment, which affects the perveance of the optics and modifies ion trajectories. To mitigate this problem, a solution that consists of more uniform thermal expansion of the optics is proposed. By rendering accelerator grid more elastic (i.e., reducing the elastic modulus) to make it easier to expand and increasing the coefficient of thermal expansion (CTE), grid-to-grid gap change and aperture center misalignment are not as pronounced as when both optics have the same properties. Selective laser melting with different fabrication parameters was used to produce molybdenum and titanium samples with different CTE and Young moduli, which were introduced in a thermoelastic model applied to a set of 50 cm domed ion optics. It has been showed that a 20% CTE increase and 25% elastic modulus reduction of the accelerator grid with respect to screen grid produced a 65% grid-to-grid gap change reduction and a 38% aperture misalignment reduction, compared with typical optics’ sets. However, the selective laser melting fabrication and testing of grids with feature control have not been accomplished yet.