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The concept of Field Emission Electric Propulsion is well known since the late 70's. First proposed by ESTEC, this thruster was investigated by several European research institutes during the past decade, with the aim of developing a NSSK system. While attractive under the mass consumption point of view, FEEP was eventually judged not competitive with respect to other electric propulsion concepts (ion thrusters and arcjets), mainly because of its high specific power (about 60 mW/µN), and was almost abandoned at the end of the 80's. At the beginning of the 90's, a renewed interest in this technology was prompted by the scientific community following the need for high-performance drag-free systems. FEEP was soon recognized as the only existing thruster capable of accurate thrust modulation at micronewton level. Research and development were therefore given a new impulse. Centrospazio is presently in charge of the development of FEEP under ESA and ASI funding, while LABEN (Milan, Italy) is currently developing the thruster power and control electronics.
In FEEP emitters, unlike most ion engines, ions are directly extracted from the liquid phase. The thruster can accelerate a large number of different liquid metals: cesium is usually selected for its high atomic weight, low ionization potential, low melting point (m.p.= 28.4 °C), and good wetting capabilities on the emitter substrate. Rubidium (m.p.= 39 °C) is a good alternative, especially in the 1-100 µN thrust range. Indium (m.p.= 156 °C) is also considered.
Thrust is produced by exhausting a beam of mainly singly-ionized cesium atoms, produced by field evaporation. The emitter module consists of two metallic plates with a small propellant reservoir. A sharp blade is accurately machined on one side of each plate. A thin layer of material is sputter-deposited on the other three sides of one of the plates, to act as a spacer; when the two emitter halves are tightly clamped together, a slit of about 1 µm is left between the blades. The propellant flows through this tiny channel, forming a free surface at the exit of the slit with a radius of curvature in the order of 1 µm. Under a strong electric field generated by the application of a voltage difference between the emitter and an accelerator electrode located directly in front of it, the free surface of the liquid metal approaches a condition of local instability, due to the combined effects of the electrostatic force and the surface tension. A series of protruding cusps, or "Taylor cones", are created. When the electric field reaches a value of about 109 V/m, the atoms at the tip spontaneously ionize and a thrust-producing ion jet is extracted by the electric field, while the electrons are rejected in the bulk of the liquid.
An external source of electrons (neutralizer) provides negative charges to maintain global electrical neutrality of the thruster assembly. Mass flow rate is extremely low and requires no control, as the particles extracted are replaced by the capillary actions from the propellant reservoir at a rate sufficient to maintain dynamic equilibrium at the emitter tip. When voltage is removed, the capillary force prevents the propellant from pouring out of the slit.
