• cathode life model
• cathode test
• cathode thermal model
• electric propulsion
• hollow cathodes

Lanthanum Hexaboride (LaB6) hollow cathodes have been found in electric propulsion applications since the 1970’s. They demonstrated high reliability at discharge current level 1 - 10 A, in particular as main discharge providers in Hall effect thrusters.
The next generation of high power electric propulsion thrusters requires higher level of discharge current, up to 300 A, and longer life, to be selected for the future manned deep space missions.
This thesis project focused on LaB6 hollow cathodes, capable of maintaining discharge current in the range 5 - 300 A. The main goal is the experimental characterization of the behavior of plasma from the inside of the cathode varying the operative conditions of discharge current, propellant flow rate and orifice geometry. From the property of the plasma we can assess the thermal behavior of the cathode, in particular of the emitting surface, and use the thermal analysis to evaluate the cathode operative life. Experiments were conducted using laboratory model of the cathodes designed for the Hall Effect Rocket with Magnetic Shielding (HERMeS) thruster and the nested Hall thruster XR-100. An extensive characterization of the internal of the cathode was made using internal scanning Langmuir probes, which provide on-axis behavior of the plasma parameters, in particular electron temperature, plasma potential and plasma density. These measurements were used as input of a coupled thermal/plasma model, in order to evaluate the temperature profiles of the cathode, at several conditions. This temperature was then necessary to predict quantitatively the expected life of the cathode. Results of thermal and life model were compared with previous experiments done on the cathodes, in particular with a 4000 hours wear test effectuated on the smaller of the cathodes analyzed. Comparison of the theoretical prediction with the experimental results suggests that lanthanum redeposition reduces the net evaporation rate and extends the cathode life. The predicted life of this type of cathodes is found to be more than 30000 hours, thus it qualifies the LaB6 hollow cathode for long deep space missions.