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.

A promising new hollow cathode concept for high-power, long-lifetime electric thruster applications is explored using finite-element analysis to inform an experiment presently being constructed. This RF-Controlled Hollow Cathode concept adds radio-frequency power to a large-diameter cathode. To explore the concept's feasibility and behavior, numerical simulations were performed with a simplified two-dimensional model which captures the plasma, the thermionic emission, and the added RF power. It was found that by increasing the RF power, the centerline plasma density profile increased and its peak shifted upstream, resulting in enhanced thermionic emission from a greater emitter area, and thus supporting the promise of the concept for high-power, long-lifetime applications. It was also observed that a pronounced "jump" occurs in the plasma density and other parameters at a critical RF power, and its occurrence is strongly dependent on gas pressure. This beneficial jump behavior was attributed to a cavity resonance effect caused by the RF waves constructively interfering to sharply increase the electric field amplitude at a critical RF electric field phase and RF-reflecting plasma density depth.

Exploration of a novel RF-Controlled HollowCathode concept is presented using finite element analysis. Commercial software is used to model the extent of RF power absorption in one configuration of such a cathode in order to describe whether the RF power is localized as in a "stinger" concept or is projected downstream to lower the emission current density while maintaining a constant discharge current. Plasma conductivity along the major axis is calculated from a baseline high current density lanthanum hexaboride hollow cathode and is used in the modeling of RF power absorption by the internal plasma. It was found that within a maximum axial distance from the orifice, a direct coaxial-cathode mating can lead to high percentages (>96%) of localized microwave power absorption. The configuration analyzed acted more as a stinger, as approximately 62% of the RF power was absorbed within 2 mm of the inner coaxial conductor tip. Xenon gas breakdown using RF waves was explored and deemed practical for the cathode parameters studied. RF heating of the emitter prior to plasma ignition was also examined, but low RF power absorption (<5%) without a lossy plasma suggested poor feasibility of this potential function.