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

Description of the LISA mission 129engineering extrapolation from tests at this level plus some in-flight tests withintentionally increased disturbances will provide a sound basis for proceedingwith the LISA mission.10.1.5 Micronewton thrustersThe main non-gravitational force on a LISA spacecraft is expected to be fromthe solar radiation pressure and to have a magnitude of about 20 µN. If notcompensated for, it would cause an acceleration of the spacecraft of roughly10 −7 ms −2 . Since the test masses are shielded from this source of acceleration, itis necessary to apply force to the spacecraft to keep it from moving with respectto them. The spectral amplitude of the fractional fluctuations in the solar pressureforce [22] over the 0.1–10 mHz frequency range is approximately1.3 × 10 −3 ( f/1 mHz) −1/3 .Comparable fluctuations in force but with a more reddened spectrum and amuch lower dc level will be present from the solar wind.The type of thrusters that are planned for use on the LISA mission are fieldemission electric propulsion (FEEP) thrusters. They operate by accelerating ionsthrough a potential drop of 5–10 kV and ejecting them to provide thrust. Theejection velocity is roughly 60–100 km s −1 , corresponding to a specific impulseof 6000 to 10 000 s. In view of this high specific impulse and the low thrust levelneeded, the fuel required per thruster for a ten year extended mission lifetime isonly a few grams.Historically, most of the development of FEEP thrusters has been basedon the use of Cs ions. This work has been done mainly at the EuropeanSpace Research and Technology Centre in Noordwijk, The Netherlands and atCentrospazio in Pisa, Italy (see section 7.3 of [6]).A schematic drawing of a thruster is shown in figure 10.8. In each thruster,liquid Cs metal at a temperature somewhat above the melting point of 29 ◦ Cis contained in a small reservoir. It is drawn by capillary forces through anarrow channel between two polished metal plates spaced 1 or 2 µm apart. Theaccelerating voltage is applied by a plate with a slot in it, located at the outer edgeof the channel.The high field at the surface of the Cs metal causes an instability, and Taylorcones roughly a micrometre in diameter and a few micrometres apart form on thesurface. The tips of these cones are very sharp, and the high field around the tipscauses Cs ions to be drawn out by field emission and accelerated away in a beamperhaps 30 ◦ wide. The one substantial drawback to the use of Cs ions is thatany water vapour that is present will react with the Cs to form CsOH. Thus, thethrusters are kept in vacuum containers roughly 5 cm in dimensions until in spaceand ready for use. A spring-loaded cover with an O-ring seal is then released.An alternative to Cs is the use of In. Field emission of In ions for theneutralization of positive charge on spacecraft has been pioneered by the Austrian