RESULTS FROM ACTIVE SPACECRAFT POTENTIAL CONTROL ON THE GEOTAIL SPACECRAFT

A low and actively controlled electrostatic potential on the outer sur faces of a scientific spacecraft is very important for accurate measur ements of cold plasma electrons and ions and the DC to low-frequency e lectric field. The Japanese/NASA Geotail spacecraft carries as part of its scientific payload a novel ion emitter for active control of the electrostatic potential on the surface of the spacecraft. The aim of t he ion emitter is to reduce the positive surface potential which is no rmally encountered in the outer magnetosphere when the spacecraft is s unlit. Ion emission clamps the surface potential to near the ambient p lasma potential. Without emission control, Geotail has encountered pla sma conditions in the lobes of the magnetotail which resulted in surfa ce potentials of up to about +70 V, The ion emitter proves to be able to discharge the outer surfaces of the spacecraft and is capable of ke eping the surface potential stable at about +2 V. This potential is me asured with respect to one of the electric field probes which are curr ent biased and thus kept at a potential slightly above the ambient pla sma potential. The instrument uses the liquid metal field ion emission principle to emit indium ions. The ion beam energy is about 6 keV and the typical total emission current amounts to about 15 mu A. Neither variations in the ambient plasma conditions nor operation of two elect ron emitters on Geotail produce significant variations of the controll ed surface potential as long as the resulting electron emission curren ts remain much smaller than the ion emission current. Typical results of the active potential control are shown, demonstrating the surface p otential reduction and its stability over time.