Life Evaluation of a Lanthanum Hexaboride Hollow Cathode for High-Power Hall Thruster

Lanthanum hexaboride (LaB6) hollow cathodes have been in development in the United States for electric thruster applications for nearly 15 years. The life of the lanthanum-hexaboride insert in these hollow cathodes is projected to range from 10 kh to over 100 kh based on insert evaporation calculations. A detailed investigation of the life of a 1.5 cm outside-diameter lanthanum-hexaboride hollow cathode has been made. This cathode can provide discharge currents between 5 and 100 A and has an estimated lifetime in excess of 50 kh depending on discharge current and mass flow rate. The insert evaporation rate is determined from a 4000 h discharge wear test of a laboratory model cathode, where insert weight-loss measurements were made at the start, middle, and end of the test. Detailed insert temperature and plasma measurements are used in a one-dimensional disk evaporation model that accommodates the insert temperature profiles found with different discharge currents, flow rates, and cathode orifice diameters, to predict the insert life. The results suggest that insert outgassing is significant early in life, and lanthanum redeposition reduces the net evaporation rate and extends the cathode life.