(ed.). Gravitational waves (IOP, 2001)(422s).

130 LISA: A proposed joint ESA–NASA gravitational-wave missionFigure 10.8. Schematic drawing of cesium Field Emission Electric Propulsion (FEEP)thruster. The thrust can be controlled by changing the accelerator voltage smoothly. A setof six or more thrusters is used to control the spacecraft position and attitude.Research Centre Seibersdorf, and such devices have been flown on severalmissions. The geometry used consists of a sharpened tungsten needle with a tipa few micrometres in diameter mounted in the centre of a heated In reservoir.Capillary forces bring the In to the tip, and again an accelerator plate applies ahigh voltage. A single Taylor cone is formed, and the In ions are produced byfield emission from the sharp tip of the cone.If such devices are used as thrusters, there is a limit to the thrust achievableper tungsten needle. If too much current is drawn, small droplets form andgive much less thrust per unit mass than the ions do. However, it has beendemonstrated that a number of needles can be operated in parallel to meet thequite low thrust requirements of the LISA mission during its operational phase.The results of research on such thrusters have been reported by the Seibersdorfgroup [23]. The possibility of using In instead of Cs with the channel capillaryflow approach is now being investigated by Centrospazio, but no decision hasbeen made on the overall merits of the two materials with this approach.The advantages of FEEP thrusters, in addition to their high specific impulse,is that they can run continuously and be controlled easily by small changesin the acceleration voltage. Work on their further development and testing isbeing supported by ESA because of their potential for use on a number of futuremissions. If In ions are used, the thrusters have to run hotter because of the 156 ◦ Cmelting temperature for In. However, the power required is still low, and doesnot appear to be a problem. To control both attitude and translation and allowsufficient redundancy, probably between 12 and 24 individual thrusters pointingin different directions will be included per spacecraft.