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What are hollow cathodes?

Hollow cathodes are a critical component of most electric space thrusters in use today. Hollow cathodes are the electron source in Hall and ion thrusters for ionizing neutral propellant gas to create a plasma which is then directed to create thrust. They are also used to supply the electrons that neutralize the exhaust ion beam in ion thrusters. The operational lifetime, maximum power, and performance of these thrusters depend heavily on the capability of the hollow cathode.

Hollow cathodes typically are a conductive tube with an emitter material inside that has a low work function. This low work function allows the material to thermionically emit electrons at relatively low operating temperatures (\(1000-1700^{\circ}C\)) and at high current densities (e.g., \(20\) cm\(^2\)). A neutral gas, like xenon, is fed into the upstream side of the cathode cavity and emitted electrons ionize the gas as it travels downstream. This ionization leads to a dense plasma near the emitter which prevents space charge limiting and thus allows the emitter surface to operate at high current densities. A schematic side-cutaway view of a typical hollow cathode is shown below.

Hollow cathodes are used instead of solid filament cathodes (e.g. in vacuum tubes) due to their ability to supply high electron current densities at relatively low voltages and temperatures, requiring less power as well as providing much longer operational lifetimes than filament cathodes.

Why high current density? And why are novel concepts needed?

Hollow cathodes can limit both the operational lifetime and the maximum power for a given electric thruster. As space missions become more demanding and larger power sources become available (e.g., Solar Electric Propulsion and Nuclear Electric Propulsion missions operating in the range of 100-200 kW) hollow cathodes are quickly being pushed to their limits. Future hollow cathodes will need to be able to handle much higher discharge currents (up to 600-700 A!) and recently proposed missions require hollow cathode lifetimes to be about twice the longest life tests to date.

A strategy for accomplishing these goals involves creating and testing new hollow cathode concepts to lower the peak current density while maintaining the same discharge current. Much higher discharge currents can then be used while keeping the peak current density within desired thermal limitations which directly impacts the maximum power and the operational lifetime. One of the concepts conceived at EPPDyL to do this is the RF-Controlled Hollow Cathode.

RF-Controlled Hollow Cathode (RF-CHC)

This concept adds RF power to a large-diameter hollow cathode to ultimately lower the peak current density while maintaining the same discharge current. The RF-CHC concept was first presented at the 48th AIAA Joint Propulsion Conference in in 2012, and recent numerical modeling work on the concept was presented at the 49th AIAA Joint Propulsion Conference in July of 2013. These conference papers are under 'publications' on this website. We will also be presenting our initial experimental findings of the RF-CHC at the 33rd International Electric Propulsion Conference from October 6-10, 2013. The scope of this project is a proof-of-concept experiment, as well as theoretical and computer modeling verifying the promise of the RF-CHC concept. Other high-potential novel concepts will be explored over the next several years as well.