Heaterless 300 A lanthanum hexaboride hollow cathode

Abstract

Hollow cathodes used in electric propulsion typically have an external heater to raise the thermionic electron emitter to emission temperatures. Heaterless hollow cathodes that are heated by a Paschen discharge have been historically limited to low discharge currents (<5 A) due to arcing and inefficient heating. A new heaterless technology was previously developed for cathodes up to 50 A, utilizing a refractory metal tube to extend the gas feed line partway into the thermionic insert region. A high voltage (>700 V) Paschen discharge is ignited between the keeper and the tube, which quickly transitions to a lower voltage (<80 V) thermionic discharge from the inner tube surface and heats the thermionic insert by radiation. This “tube-radiator” configuration eliminates arcing and inhibits the long-path-length discharge between the keeper and gas feed tube upstream of the cathode insert that caused inefficient heating in prior designs. This paper describes extending this technology developed for a 50 A cathode to one that is capable of 300 A. The larger cathode uses a 5-mm diameter tantalum tube-radiator and a 6-A, 5-min ignition sequence. Ignition was challenging because the high heating power required (≥300 W) is difficult to maintain with the low voltage (<20 V) keeper discharge that exists prior to igniting the thruster discharge. To achieve self-heating from the lower voltage keeper discharge, the keeper current is raised to 10 A once the LaB6 insert starts emitting. This work shows that the novel tube-radiator heater is scalable to large cathodes capable of tens of thousands of ignitions.