Pre-Phase A Report - Lisa - Nasa

156 Chapter 7 Spacecraft Design
• Centrospazio is currently investigating the possibility of using indium as propellant
for the Centrospazio FEEP thruster in collaboration with the National Physics
Laboratory (United Kingdom).
Activities at the Austrian Research Centre Seibersdorf.
The standard Indium Liquid Metal Ion Source (In-LMIS) developed at the Austrian Research
Centre Seibersdorf is of the central needle type, in which a sharpened tungsten
needle of tip radius of a few µm is mounted in the centre of a heated indium reservoir.
Foroperation, a potentialoftypically5–7kV isappliedbetweentheneedleandanac celerating electrode. If the needle is well wetted by the liquified indium, the electrostatic
stress of the applied field pulls the indium film towards the electrode. This stress is
balanced by surface-tension forces which leads to the establishment of a characteristic
equilibrium configuration, the so called Taylor cone (see Figure 7.6). Once the electric
Figure 7.6 Schematic of the operation of a needle type liquid-metal ion source.
field at the apex of this cone gets to the order of volts per nanometer, the most protruding
surface atoms are field evaporated, ionized and accelerated towards the electrode. Via
hydrodynamic flow the atoms leaving the tip area are constantly replenished and a stable
emission regime can thus be maintained. Figure 7.7 shows an image of the space qualified
indium LMIS.
Originally developed for spacecraft potential control instruments these indium LMIS so
far have been flown on four missions (GEOTAIL, EQUATOR-S and two times on MIR) and
are currently scheduled to fly on CLUSTER-II, ROSETTA and again to the MIR station
in the next future. A total of 35 ion sources will be used in these three projects. The
reliability of the indium LMIS design up to now has been proven in more than 800 hours
3-3-1999 9:33 Corrected version 2.08