Experimental investigation of a large-diameter cathode

Large-diameter thermionic hollow cathodes allow for higher-current operation than smaller cathodes but present new design and operational challenges. Significantly higher heater powers are required for ignition, and heater-insulating ceramics typically used can chemically interact with refractory metal heaters at high temperatures. In order to meet the mission requirements for the next generation of Hall thrusters, hollow cathodes will have to produce 300 to 700 A of discharge current while operating for tens of thousands of hours. A large-diameter hollow cathode intended to combine both long-life and highcurrent operation has been constructed as part of the development of the RF-Controlled Hollow Cathode. This concept proposes the use of RF power to control the dense-plasma attachment area, reducing peak current density while maintaining high current output. The ignition behavior of this hollow cathode was found to depend heavily on gas species, with krypton allowing for plasma ignition at much lower voltages than argon for comparable flow rates. Steady-state operation was achieved for discharge currents from 20 to 225 A using krypton for which current-voltage traces are presented. While operating at 20 A, for both argon and krypton, cathode temperatures were found to decrease with increasing mass flow, contrasting with the behavior of prior large cathodes. Heater failures due to arcing were mitigated with an appropriate ignition procedure and electrical design. Chemical reactions due to high heater temperatures were identified as a source of frequent heater failures and using a graphite standoff is proposed.