Field Emission Electric Propulsion

Field Emission Electric Propulsion (FEEP) is an advanced electrostatic propulsion concept that uses liquid metal[?] (usually either caesium or indium) as a propellant. A FEEP device consists of an emitter and an accelerator electrode. A potential difference on the order of 10 kV is applied between the two, which generates a strong electric field at the tip of the metal surface. The field extracts ions, which then are accelerated to high velocities, typically more than 100 km/s. A separate electron source is required to keep the spacecraft electrically neutral.

Due to its very low thrust (in the micro - milli newton range), FEEPs are primarily used for micro-radian, micro-Newton attitude control on spacecraft.