The LISA disturbance reduction system needs a quiet, continuously variable thruster to precisely control the attitude and position of the spacecraft. The thrust range is set by the solar radiation pressure, which gives a force of ~ 25 microNewtons, plus requirements for control recovery from separation from the propulsion stage.
The FEEP thrusters work by warming a reservoir of cesium to the liquid state, conveying the liquid metal by capillary action to a narrow (1 micron) slit and accelerating metal ions off the slit. A separate filament is used to eject electrons to prevent charge accumulation on the spacecraft. The figure shows a single thruster with a 1cm long slit in a 6 cm diameter housing.
Figure shows the micronewton thrusters developed by Centrospazio (Italy) under contract from the European Space Agency.