Review of Development History and Ongoing Development Status of a 100 A Discharge Current Reservoir Hollow Cathode for Electric Propulsion

The reservoir cathode was the first “dispenser” cathode predating the impregnated dispenser cathode in common usage today throughout the vacuum electron device community by nearly 5 years. Extremely long processing times necessitated by its use of barium oxide, incorporated. in the reservoir in the form of barium-strontium carbonate, as the source of barium adsorbates for work function lowering made it impractical for use in many applications. Upon introduction of the impregnated dispenser cathode the reservoir cathode was relegated to use only in a very few applications where its unique capabilities, primarily the ability to deliver high current densities and useful life times at operating temperatures substantially in excess of those of which the impregnated version was capable, justified its use. In 2004 the capabilities of the reservoir cathode led to it being recognized as a possible solution to meeting the lifetime and discharge current requirements for the NEXIS ion thruster being developed for use on the Jupiter Icy Moons Orbiter mission. This required development of an entirely different cathode structure and demonstration of the ability to operate as a gas discharge electron source, rather than a vacuum electron source. Development was quickly curtailed due to cancellation of the mission, but sufficient fabrication and testing experience was acquired to demonstrate the viability of the reservoir cathode in the hollow cathode configuration. Starting in 2012 e-beam initiated a program to further the development of the reservoir hollow cathode. The history of the development of the reservoir hollow cathode is reviewed in this paper and the status of current development efforts are reported. Reservoir hollow cathodes have been successfully operated at discharge currents as high as 100 A in low duty pulse operation and work is progressing on test vehicles capable of extending the capability to CW operation.