ACTIVE SPACECRAFT POTENTIAL CONTROL

Charging of the outer surface or of the entire structure of a spacecra ft in orbit can have a severe impact on the scientific output of the i nstruments. Typical floating potentials for magnetospheric satellites (from +1 to several tens of volts in sunlight) make it practically imp ossible to measure the cold (several eV) component of the ambient plas ma. Effects of spacecraft charging are reduced by an entirely conducti ve surface of the spacecraft and by active charge neutralisation, whic h in the case of Cluster only deals with a positive potential. The Clu ster spacecraft are instrumented with ion emitters of the liquid-metal ion-source type, which will produce indium ions at 5 to 8 keV energy. The operating principle is field evaporation of indium in the apex fi eld of a needle. The advantages are low power consumption, compactness and high mass efficiency. The ion current will be adjusted in a feedb ack loop with instruments measuring the spacecraft potential (EFW and PEACE). A stand alone mode is also foreseen as a back-up. The design a nd principles of the operation of the active spacecraft potential cont rol instrument (ASPOC) are presented in detail. Flight experience with a similar instrument on the Geotail spacecraft is outlined.