Spacecraft Potential Control Using Indium Ion Sources - Experience and Outlook Based on Six Years of Operation in Space

The conditions for measurements by particle instruments at low energies are significantly improved by stabilizing and reducing the high positive potentials sunlit spacecraft acquire when they are embedded in tenuous plasma. Among the many techniques employed for this purpose the emission of a beam of positive ions with energies from 5 to 8 keV is considered to be the most efficient approach. Its feasibility has been demonstrated by two instruments on board the spacecraft Geotail and Equator-S launched in July, 1992, and December, 1997, respectively. The ion emitter instruments onboard these spacecraft use liquid metal ion sources with indium as the charge material. Already small and lightweight due to carrying the charge material as a solid, these ion sources feature very high electrical and excellent mass efficiency. The reliability of these ion sources has been demonstrated in space by the flawless operation on board the Geotail spacecraft over six years. The second instrument on board Equator-S has been operating on a routine basis for four months throughout the active lifetime of the spacecraft. The efficiency of the method is shown by overviews and examples from the missions. It can also be demonstrated both in theory and experiment that this technique produces only few, if any, undesirable side effects. The outlook of this type of instrument is focused on the mission Cluster-II.