Field-emission electric propulsion

Field-emission electric propulsion (FEEP) is an advanced electrostatic space propulsion concept, a form of ion thruster, that uses liquid metal (usually either caesium, indium or mercury) as a propellant. A FEEP device consists of an emitter and an accelerator electrode. A potential difference of the order of 10 kV is applied between the two, which generates a strong electric field at the tip of the metal surface. The interplay of electric force and surface tension generates surface instabilities which give rise to Taylor cones on the liquid surface. At sufficiently high values of the applied field, ions are extracted from the cone tip by field evaporation or similar mechanisms, which then are accelerated to high velocities (typically 100 km/s or more).

A separate electron source is required to keep the spacecraft electrically neutral. Due to its very low thrust (in the micronewton to millinewton range), FEEP thrusters are primarily used for microradian, micronewton attitude control on spacecraft, such as in the ESA/NASA LISA Pathfinder scientific spacecraft.

Field Emission Electric Propulsion (FEEP) is an electrostatic propulsion concept based on field ionization of a liquid metal and subsequent acceleration of the ions by a strong electric field. FEEP is currently the object of interest in the scientific community, due to its unique features: sub-μN to mN thrust range, near instantaneous switch on/switch off capability, and high-resolution throttleability (better than one part in 10⁴), which enables accurate thrust modulation in both continuous and pulsed modes. Presently baseline for scientific missions onboard drag-free satellites, this propulsion system has also been proposed for attitude control and orbit maintenance on commercial small satellites and constellations.

This type of thruster can accelerate a large number of different liquid metals or alloys. The best performance (in terms of thrust efficiency and power-to-thrust ratio) can be obtained using high atomic weight alkali metals, such as cesium and rubidium (133 amu for Cs, 85.5 amu for Rb). These propellants have a low ionization potential (3.87 eV for Cs and 4.16 eV for Rb), low melting point (28.7 oC for Cs and 38.9 °C for Rb) and very good wetting capabilities. These features lead to low power losses due to ionization and heating and the capability to use capillary forces for feeding purposes (i.e. no pressurised tanks nor valves are required). Moreover, alkali metals have the lowest attitude to form ionized droplets or multiply-charged ions, thus leading to the best attainable mass efficiency. The actual thrust is produced by exhausting a beam of mainly singly-ionized cesium or rubidium atoms, produced by field evaporation at the tip of the emitter.

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