Development of Next Generation Active Spacecraft Potential Control (ASPOC-NG) Instrument for Science Missions

Fundamental plasma processes in space are studied in the Earth's magnetosphere or interplanetary space, for example. Interactions between spacecraft, space plasmas and solar UV photons generally result in electrostatic charging of spacecraft external surfaces. The active spacecraft potential control (ASPOC) system developed in the 1990s emits positive ions to neutralise the satellite potential, such as used in several missions like Geotail, Equator-S, Cluster, Double Star and MMS (Magnetospheric Multiscale Mission). With the experience gained, the next generation of the active spacecraft potential control (ASPOC-NG) instrument has been developed over the last three years. This includes two emitter modules, one for ion and one for electron emission developed by FOTEC, whereas the electronics control unit was developed by IWF. The main objective was the reduction of mass and power consumption to comply with the requirements of future scientific missions. The previously used capillary-based emitter was replaced by porous tungsten needle multi-emitter. This allows a higher mass efficiency, while the liquid metal propellant feeding is still protected from surface contamination. The main power consumption of the test module was caused by the heater for liquefying the propellant. Therefore, the propellant was changed from pure indium to an indium-gallium alloy with a lower melting point. This significantly reduces the required heating power. In addition, the mixture has improved wetting properties which facilitates stronger bonding of the propellant to the capillary feeding system. Both ASPOC-NG instruments for positive and negative charge compensation and their performance values are presented. These values qualify ASPOC-NG as low resource spacecraft potential control device candidate for future missions under various charging conditions.