Examination of a 5 A-class cathode with a LaB6 flat disk emitter in the 2 A-20 A current range

Cathodes are electron sources used for gas ionization and ion beam neutralization in Hall thrusters. A laboratory model of a 5 A-class cathode has been experimentally studied in the so-called diode configuration with a flat metal anode. The cathode design is based on the direct heating of a flat disk lanthanum hexaboride (LaB6) insert to emit electrons. The paper presents the characterization of the LaB6 cathode operated at xenon mass flow rates between 0.2 and 0.6 mg.s-1 with discharge currents ranging from 2 A to 19 A. Electrical parameters and temperature of several parts, including the LaB6 insert, were collected and analyzed in order to investigate the possibility of operating the cathode in an extended range of discharge current with respect to its nominal condition (5 A at 0.4 mg.s-1). Particular attention was paid to the identification of the discharge mode (spot versus plume) through spectral analysis of discharge current waveforms. Temperature measurements performed with thermocouples reveal that the LaB6 insert is moderately heated during the cathode operation (about 1600 °C at 19 A), except for the lowest mass flow rate.