FEEP Thruster Performance at High Background Pressure

In FEEP thrusters, field ionization of cesium occurs on the liquid metal surface at the outlet of a 1 μm slit. The main concern with this ionization mechanism is the possible clogging of the emitter slit due to oxidation of cesium. As an alkali metal, cesium reacts very rapidly with water, forming cesium hydroxide (CsOH), and with oxygen, forming cesium oxide (Cs2O). These compounds melt or decompose at 272 °C and 400 °C, respectively. At the normal operating temperature of FEEP (about 35 °C), contamination by oxides may lead to partial obstruction of the slit by solid particles and to poor ion emission. This detrimental situation could take place in two different ways: first, because of interaction of the free liquid metal surface with the environment atmosphere; second, because of contact of liquid cesium with water vapour absorbed on the stainless steel internal surfaces of the emitter. As a consequence, special care must be devoted to the pre-flight emitter preparation procedure, and the operation of FEEP should be restricted to orbits at a sufficiently high altitude, where environment pressure and water content are below a certain limit. However, recent tests show that this requirements have to be met only during the very first wetting of the emitter blades by the propellant, while much more unfavourable conditions may be tolerated after a new emitter has been successfully fired1. Since then, several other tests were carried out to investigate the effect of prolonged switchoff periods, and enhanced confidence in the use of FEEP at high pressure was gained. Centrospazio is preparing for ESA a flight demonstration of the FEEP system, which will fly in 1999 on a Get Away Special canister onboard the Space Shuttle; hence, it is mandatory to demonstrate that FEEP is impervious to contamination from such substances as water, carbon dioxide, molecular and atomic oxygen, which are normally found surrounding the Shuttle, due to the spacecraft own outgassing and to the natural LEO environment. The aim of this work is to address the problem of cesium contamination in FEEP and to summarize some recent, related experimental results. Abstract