Hall thrusters utilize identical hollow cathode technology as ion thrusters, yet must operate at much higher mass flow rates in order to efficiently couple to the bulk plasma discharge. Higher flow rates are necessary in order to provide enough neutral collisions to transport electrons across magnetic fields so that they can reach the discharge. This higher flow rate, however, has potential life-limiting implications for the operation of the cathode.
A solution to the problem involves splitting the mass flow into the hollow cathode into two streams, the internal and external flows. The internal flow is fixed and set such that the neutral pressure in the cathode allows for a high utilization of the emitter surface area. The external flow is variable depending on the flow rate through the anode of the Hall thruster, but also has a minimum in order to suppress high-energy ion generation.
In the co-flow hollow cathode, the cathode assembly is mounted on thruster centerline, inside the inner magnetic core of the thruster. An annular gas plenum is placed at the base of the cathode and propellant is fed throughout to produce an azimuthally symmetric flow of gas that evenly expands around the cathode keeper. This configuration maximizes propellant utilization and is not subject to erosion processes.
External gas feeds have been considered in the past for ion thruster applications, but usually in the context of eliminating high energy ion production. This approach is adapted specifically for the Hall thruster and exploits the geometry of a Hall thruster to feed and focus the external flow without introducing significant new complexity to the thruster design.
This work was done by Richard R. Hofer and Dan M. Goebel of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47901
This Brief includes a Technical Support Package (TSP).
Co-Flow Hollow Cathode Technology
(reference NPO-47901) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing Co-Flow Hollow Cathode Technology, which is significant for its applications in aerospace propulsion systems. It discusses the design and operation of hollow cathodes, which are critical components in electric propulsion systems, particularly Hall thrusters.
The document includes various figures and schematics that illustrate the conventional hollow cathode assembly and the innovative Co-Flow Hollow Cathode design. The conventional design features a gas feed and a graphite keeper electrode, while the Co-Flow design incorporates an external gas feed that enhances propellant utilization. This external feed is directed through an annular plenum, allowing for a more uniform gas distribution around the cathode keeper, which is crucial for efficient operation.
Key findings presented in the document include the performance differences between conventional and Co-Flow Hollow Cathodes. For instance, measurements of ion energy show that conventional cathodes produce ions with energies exceeding 30 V, which can lead to significant sputtering of the cathode electrodes. In contrast, the Co-Flow design suppresses these energetic ions, thereby reducing wear and improving longevity.
The document also discusses the total thrust efficiency of Hall thrusters operating with different cathode configurations. It highlights that the Co-Flow Hollow Cathode design allows for higher propellant utilization compared to external gas feeds, which is essential for optimizing the performance of electric propulsion systems.
Overall, the Technical Support Package emphasizes the advancements in hollow cathode technology that can lead to more efficient and durable propulsion systems for space exploration. It serves as a resource for understanding the implications of these technologies in aerospace applications and encourages further exploration and development in this field. The document is part of NASA's efforts to disseminate aerospace-related developments that have broader technological and commercial applications, reflecting the agency's commitment to innovation and collaboration in space technology.
