contact ion thruster
Cesium ion engine developed by Electro-Optical Systems, Inc. as an experimental high-energy thruster for spacecraft. It engine was successfully tested twice in space in flights of Air Force Blue Scout missiles in October and December 1964.
A contact ion thruster is a form of electrostatic propulsion in which ions are produced on a heated surface and then accelerated in an electric field to produce a high-speed exhaust. Of the two types of ion propulsion that have been studied thoroughly over the past four decades – the other being the electron bombardment thruster – it has so far proved much the less useful for practical use in space.
The difficulty is that the only propellant that has been shown to work in the contact ion method is cesium, because only cesium atoms have an outer, valence electron that can be removed when the atoms are adsorbed onto the surface of a suitable metal, such as tungsten. However, cesium is so corrosive that it has been impractical to handle in devices that operate reliably over the long periods required for ion propulsion.