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The instruments for active spacecraft potential control to be launched with the NASA Magnetospheric Multiscale (MMS) mission in 2014 have recently been qualified, and the first two out of a total of eight flight models have completed their acceptance test campaign. At the heart of these instruments are liquid metal ion sources with indium, developed at Forschungsund Technologietransfer GmbH. Earlier versions have been flown in the Equator-S, Cluster, and Double Star missions. The redesign efforts to comply with the stringent requirements of the MMS mission are described in detail, as well as the qualification program for the emitters and the instruments. Starting with mechanical and thermal analyses and tests, continuing with performance and life testing, and finally by analysis of the overall performance in the mission application, it can be demonstrated that the new generation of ion emitters beats the performance of previous versions in all aspects. The description of verification efforts is completed by the results from tests at instrument level of the qualification model and the first flight models. [ABSTRACT FROM PUBLISHER]

The NASA Magnetospheric Multiscale (MMS) mission to be launched in 2014 is going to study events of magnetic reconnection in the Earth's magnetosphere by means of four observatories in close formation on near-equatorial orbits. The required highly accurate and undisturbed plasma and electric field measurements will be supported by an actively controlled spacecraft potential provided by Active Spacecraft Potential Control (ASPOC) instruments. ASPOC uses capillary-type liquid metal ion sources with indium as the source material to generate ion beams. Two instruments per spacecraft will generate beams in antiparallel direction. The instruments for MMS will be based on heritage from the ESA Cluster and Double Star missions, but several significant changes and improvements to accommodate the requirements of the new mission have been implemented. [ABSTRACT FROM PUBLISHER]

Abstract-Emitters based on the liquid metal ion source principle have been operating on the Cluster spacecraft between 2000 and 2004, in order to control the spacecraft potential. The resulting reduction of positive spacecraft potential reduces perturbations to the plasma measurements on board. Ion currents up to 40 μA have been applied, which reduced the energy band in which photoelectrons disturb the plasma electron measurements to values close to the lower detection limit of the instrument. The experience with this method, meanwhile, covers both the maximum and minimum of the present solar cycle and allows one to study the variations of photoemission and other long-term trends and their effects on the measurements. A long-term trend of the controlled spacecraft potential is indeed observed. In addition, it appears that reconstruction of the uncontrolled spacecraft potential from the controlled one is possible if certain conditions are fulfilled. Spacecraft potential control can thereby improve the plasma measurements while still allowing one to estimate the total plasma density from the residual potential. [ABSTRACT FROM AUTHOR]

The ion emitter instrument ‘active spacecraft potential control’ (ASPOC) has been used successfully in several magnetospheric missions including the European Space Agency Cluster Project. An improved version has been developed for the equatorial spacecraft of the Chinese-European Double Star mission (TC-1) launched in December 2003. The modifications include a new design of the ion emitter modules. As a result, higher currents than in previous missions can be achieved. The main objective of the investigation is the reduction of positive spacecraft potential in order to minimize perturbations to the plasma measurements onboard, in particular to the plasma electron instrument PEACE. These data show an almost complete suppression of photoelectrons when ASPOC is emitting at 30- to 50-μA beam current. The angular distribution of the electrons in the presence of the ion beam is investigated in detail. The measurement of ambient electron distributions is highly improved. [ABSTRACT FROM AUTHOR]