FEEP Thruster Performance at High Background Pressure (1997)

S INCEthe beginningofFEEPresearch, it was generally believed that the operational field of this thruster should be limited to high altitude orbits due to possible emitter contamination resulting from propellant interaction with the residual atmosphere. In fact, cesium reacts with water and oxygen forming compounds which may obstruct the emitter slit, causing performance degradation and eventually preventing ion emission. This concern is receiving much attention in view of the forthcoming FEEP flight demonstration on the Space Shuttle, whose gaseous environment is rich in contaminants. This paper reports on the results of a series of high background pressure tests carried out at Centrospazio using an emitter with a 5 mm long, 1.4 l.un high slit, and draws general conclusions on the suitability of PEEP emitters for LEO satellite applications. Experiments included tests at total pressure values as high as 10“ mbar, tests with a significant HZ0 partial pressure (up to 10’ mbar), and tests carried out to investigate the effect of prolonged thruster shut-off periods (up to 15 hours). As a result of this work, enhanced confidence in the use of PEEP at relatively high ambient pressures has been gained. It was concluded that the behaviour of the thruster is not as influenced by the background atmospheric environment as previously believed, even at relatively high water pressures, or at restart after prolonged inactivity. Of course this applies to proper thruster conditions, that can only be achieved if propellant filling, first wetting of the emitter internal surfaces and first firing are performed under the best possible vacuum conditions. + Research engineer, Centrospazio. * Project manages Centrospazio. $ Undergraduate student, University of Pisa. h Managing Director, Centrospazio; Professog Aerospace Dept., University of Pisa; memberA%% EPTechnical Committee. Copyright 1997 by the Electric Rocket Propulsion Society. All rights reserved. Introduction In PEEP thrusters, field ionization of cesium occurs on the liquid metal surface at the outlet of a 1 p 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 (CO 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 F’EEP 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 fired’. Since then, several other tests were carried out to investigate the effect of prolonged switchoff periods, and enhanced confidence in the use of PEEP at high pressure was gained. Centrospazio is preparing for ESA a flight demonstration of the FEEP system, which will fly in 1999 on aGet Away Special canister onboardthe Space Shuttle; hence, it is mandatory to demonstrate that PEEP 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 PEEP and to summarize some recent, related experimental results.