Pre-Phase A Report - Lisa - Nasa

152 Chapter 7 Spacecraft Design
There are mainly two designs being pursued by Centrospazio (Italy) and at the Austrian
Research Centre Seibersdorf (ARC). The FEEPs originally developped by Centrospazio
were designed for thrusts in the milli-Newton regime, as required for communication
sattelites. For LISA, they had to be scaled-down by a factor of a thousand from the
original design. The development at ARC, the Indium Liquid-Metal Ion Sources (In-LMIS)
have originally been designed for spacecraft charge control and microanalysis instruments.
They have already been flown on various missions and proven their reliability in space
during more than 800 hours of operations.
The description in the following subsections mainly refer to the Centrospazio design. We
come back to the ARC design in subsection 7.3.5 .
7.3.1 History of FEEP development
Research on Field Emission Electric Propulsion (FEEP) using liquid metals has been
carried out by ESA since 1972. The thruster has evolved from a single-pin emitter configuration
in 1972, through linear arrays of stacked needles in 1975, to the high-efficiency
solid-slit emitter in 1979. During the past phases of the development program, all the
components of the system have been designed and tested, and the emitter manufacture
problems have been solved. At that time, development had entered the industrialization
phase, and research was devoted to the testing of the whole system in view of an application
in the milli-Newton range, mainly for North-South station-keeping operations in
geostationary telecommunication satellites. The high power-to-thrust ratio of FEEPs, in
comparison with other electric propulsion systems now fully developed, made FEEPs lose
competitiveness for this specific application. However, a rising interest of the international
scientific community in missions requiring micro-Newton thrust levels for ultra-fine
pointing operations (such as in LISA) sparked the present reorientation of FEEP activities
to this new thrust range.
7.3.2 The Field Emission Electric Propulsion System
In the FEEP system, ions are created directly from the surface of a liquid metal exposed
to vacuum, by means of a strong electric field resulting from suitable voltages applied to
an emitting unit.
When the surface of a liquid metal is subjected to a strong electric field, it distorts itself
into a series of cusps that protrude more and more from the surface plane: the radius
of curvature of the cusp tips becomes smaller and smaller, thus in turn increasing the
strength of the local electric field. When the field reaches values of the order of 109 V/m,
the atoms of the metal tip spontaneously ionize, and these ions are accelerated away by
the electric field that has created them [123, 124].
Cesium, whose melting point is about 29 ◦C, has been chosen as propellant because of its
low work function, high atomic mass and good properties of wetting on steel surfaces.
The basic FEEP system comprises: a single solid emitter/accelerator system or a cluster
of them, a storage and feed system for the Cs propellant, a power conditioning and control
unit, and a neutralizer (Figure 7.5).
3-3-1999 9:33 Corrected version 2.08