This thesis work deals with theoretical and experimental studies carried out at
Alta-Centrospazio on cathodes materials for electric propulsion. Hollow cathodes with barium calcium aluminate impregnated tungsten emitters for thermionic emission are widely used in electric propulsion. These cathodes are employed in ion thrusters, Hall thrusters and are being considered for MPD thrusters too.
After a brief introduction on the current emission processes from a metallic surface, we study the chemistry and the physics of cathodes activation and operation. The understanding of chemistry of impregnate materials is important in order to increase cathodes reliability and then in order to extend commercialization and application of electric thrusters in space missions.
In this thesis work for the first time we formulated a numerical model illustrating chemical and physical characteristics of the cathode impregnants during activity. Then it is able to predict cathodes activation parameters and end of life. The model accounts for production, transport and evaporation of barium. We pay special attention to model versatility, in fact the model can describe the operation of many cathode impregnate components. The model has been implemented in Matlab software.
Experimental study concerned a hollow cathode using a 5:3:2-type barium calcium aluminate impregnated tungsten emitter. Unfortunately, cathode activation has been unsuccessful, maybe because of the long inactivity time before making experiment. In fact, when the cathode is not operating, air exposure or imperfect storage can result in a severe surface poisoning and contamination, leading to performance degradation and pores clogging, that will results in failure cathode.
Literature data and available experimental results at past activities at Alta-Centrospazio have been used to validate the model.