An optical technique to measure ion engine grid distortion due to differential thermal expansion

Abstract

An optical technique to measure small differential grid displacements due to thermal expansion of an ion thruster accelerator system has been developed. Emphasis was placed on an optical technique because of the wide variety of applications, simple operation and easy set-up. There is a great deal of interest in malung measurements of small displacements with the purpose of applying them to robotic position-sensing in machine vision systems and also to measure the shape and size of machined pm with a noncontacting sensor. For this work, several techniques were investigated, including interferometric techniques, however, a new, recently developed optical technique based on type I1 microscope measurement techniques, was selected. Experiments were performed to demonstrate the applicability of this technique for measuring small displacements of ion engine screen and accelerator grids at a distance of several meters. Several different optical components were tested for their ability to discern displacements of a 30-cm meter, two-grid ion accelerator system which was separated from the thruster body and mounted outside of the vacuum system. Differential thermal expansion of the grids was induced through the use of a 500 W heat lamp placed adjacent to the screen gnd. An analysis of these experimental results and a suggestion for future experiments to be performed with the gnds inside the vacuum chamber are also presented.