The First ‘Hot-Gap’ Ion Thruster Grid-Plate Separation Measurement Under Operating Conditions Using Frequency Scanning Interferometry

Abstract

The Joint ESA/JAXA BepiColombo mission to the planet Mercury is using ion thrusters designed and built by QinetiQ, UK. The ion thruster generates thrust by accelerating ionised gas through a strong electric field produced using a gridded plat. The grid-plate has thousands of holes that allow ions to pass through generating a thrust in accordance to Newton’s second law. The grid-plate assembly, comprising the holed screen and acceleration grids, must be maintained at the design separation profile in order to meet the thruster performance requirements for the mission.

Prior to this work, no one has successfully measured unobtrusively the separation between the two grid plates while the thruster is operating, due to the presence of hot plasma and the vacuum environment required for thruster operation.

In this presentation, the National Physical Laboratory (NPL) will present the design of a measurement system they developed for this purpose. It uses frequency-scanning interferometry to directly measure the screen grid to acceleration grid separation at several points across the plates, thus allowing the separation profile – the ‘hot-gap’ to be determined under operating conditions. During ground-based testing, the ion engine is located within a large vacuum chamber at cryogenic temperatures. The harsh environment means that the instrument was located outside the vacuum chamber with line-of-sight access to the thruster though a pre-existing small window in the chamber. The novel optical design required to obtain the measurement data will be described along with the data processing algorithms and (normalised) measurement results.