An improvement has been made to the design of the hollow cathode geometry that was created for the rare-earth electron emitter described in “Compact Rare Earth Emitter Hollow Cathode” (NPO-44923), NASA Tech Briefs, Vol. 34, No. 3 (March 2010), p. 52. The original interior assembly was made entirely of graphite in order to be compatible with the LaB6 material, which cannot be touched by metals during operation due to boron diffusion causing embrittlement issues in high-temperature refractory materials. Also, the graphite tube was difficult to machine and was subject to vibration-induced fracturing.
This innovation replaces the graphite tube with one made out of refractory metal that is relatively easy to manufacture. The cathode support tube is made of molybdenum or molybdenum-rhenium. This material is easily gun-bored to near the tolerances required, and finish machined with steps at each end that capture the orifice plate and the mounting flange. This provides the manufacturability and robustness needed for flight applications, and eliminates the need for expensive e-beam welding used in prior cathodes. The LaB6 insert is protected from direct contact with the refractory metal tube by thin, graphite sleeves in a cup-arrangement around the ends of the insert. The sleeves, insert, and orifice plate are held in place by a ceramic spacer and tungsten spring inserted inside the tube.
To heat the cathode, an insulating tube is slipped around the refractory metal hollow tube, which can be made of high-temperature materials like boron nitride or aluminum nitride. A screw-shaped slot, or series of slots, is machined in the outside of the ceramic tube to constrain a refractory metal wire wound inside the slot that is used as the heater. The screw slot can hold a single heater wire that is then connected to the front of the cathode tube by tackwelding to complete the electrical circuit, or it can be a double slot that takes a bifilar wound heater with both leads coming out the back. This configuration replaces theprevioussheathedheaterdesignthat limited the cycling-life of the cathode.
This work was done by Dan M. Goebel of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Mechanics/Machinery category. NPO-46782
This Brief includes a Technical Support Package (TSP).
Improved Rare-Earth Emitter Hollow Cathode
(reference NPO-46782) is currently available for download from the TSP library.
Don't have an account?
Overview
The document is a technical support package from NASA's Jet Propulsion Laboratory (JPL) detailing advancements in hollow cathode technology for electric thrusters, specifically focusing on the LaB6 (lanthanum hexaboride) hollow cathodes. These cathodes are designed for use in center-fed Hall thrusters, which are critical components in electric propulsion systems for spacecraft.
The document outlines the design and performance features of the new hollow cathode, which includes a co-axial geometry compatible with flight applications. It is capable of operating with discharge currents ranging from 5 to 60 A, demonstrating stable performance across the desired flow range during testing. The cathode's construction includes a LaB6 electron emitter, which is known for its high efficiency and stability.
Additionally, the document discusses a high current LaB6 hollow cathode designed for high power electric thrusters. This version features a 1.5-cm outer diameter cathode tube with orifices tested between 2.5 to 4 mm, achieving discharge currents of over 100 A. The design incorporates a LaB6 insert with a "pusher" spring, a high-temperature Ta-alumina sheathed heater, and a graphite keeper, which enhances its operational capabilities.
The document emphasizes the significance of these advancements in hollow cathode technology, as they are expected to have broader technological, scientific, and commercial applications beyond aerospace. The improvements in cathode performance are crucial for the development of more efficient electric propulsion systems, which are increasingly important for space exploration and satellite operations.
Overall, the technical support package serves as a comprehensive overview of the research and development efforts at JPL regarding hollow cathodes, highlighting their potential impact on future space missions and the broader field of electric propulsion technology. For further inquiries or detailed information, the document provides contact details for the Innovative Technology Assets Management at JPL.
