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Micropropulsion Technologies 241

with chemical or other electric propulsion systems, that is, continuous thrust
throttling, small impulse bit, instantaneous switch-on/switch-off capability, mech-
anical and electrical simplicity and thruster clustering.

11.2.3.1 Principle of Operation
The basic element of a FEEP thruster is a liquid metal ion source. In a liquid metal
ion source, the ions are generated directly from the surface of the liquid metal by a
high electric field applied between the LMIS (anode) and an extractor geometry
(cathode). When the surface of a liquid metal is subjected to a high electric field, it
is distorted into a cone or a series of cones as depicted in Figure 11.8. The radius of
such a cone is determined by the applied electric field E and the surface tension of
the liquid metal, g, given by:

4g
r ¼ (11:11)
« 0 E 2
With increasing applied voltage, the radius of curvature of these cones decreases,
thereby, further enhancing the local electric field. When the local field reaches
9
values of the order 10 V/m, atoms of the metal tip are ionized either by field
evaporation or field ionization. Subsequently, the ions are accelerated and expelled
from the emitter by the same electric field that has ionized them. The charged
particles leaving the liquid metal surface as an ion beam are replenished by the
hydrodynamical flow of the liquid metal. The liquid metal is converted directly into
an ion beam without the transitional vapor phase, which is common in the technol-
ogy of other ion sources; therefore ionization operates with high power efficiency.

Ion beam

Extractor electrode
−

Jet

Taylor cone

+ Liquid metal pool
Needle
FIGURE 11.8 FEEP principle of ion current production.

© 2006 by Taylor & Francis Group, LLC