A NOVEL MEDIUM-ENERGY ION EMITTER FOR ACTIVE SPACECRAFT POTENTIAL CONTROL

An ion emission instrument has been developed to actively control the electrostatic surface potential of a spacecraft in orbit. Spacecraft s urfaces tend to charge positive when their conductive surfaces are par tially sunlit, and photoelectrons emitted, and the spacecraft is posit ioned outside of the very dense plasma region which is referred to as plasmasphere. A small, lightweight instrument with a lifetime of more than 10(4) h at a nominal emission current of 10 muA was the design go al. The ion emitter is based on the liquid metal ion source principle; a constant stream of liquified indium is evaporated from the tip of a needle and ionized; the ions are then extracted by a strong electric field. The size of the instrument is about 18 by 15 by 23 cm (length X width X height) and the entire instrument weighs about 1.9 kg and con sumes 2.4 W of electrical power in the nominal emission mode. An instr ument of this type is already operational on the Japanese Geotail spac ecraft, while five more instruments are at the stage of flight model a ssembly for integration on the Russian Interball satellite and the ESA /NASA four-spacecraft Cluster mission. The Geotail instrument is funct ioning as expected. Preliminary results show that the ion emitter is a ble to reduce surface potentials of +70 V (highest value observed up t o now) down to 2-4 V.