Hollow Cathode with Electride Insert

Rand, Lauren P.; Williams, John D.; Hoyt, Robert P.

doi:10.2514/6.2011-5992

Cited by 6 publications

(5 citation statements)

References 8 publications

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“…C12A7:e − was fabricated through a one-step procedure in a vacuum furnace capable of reaching temperatures of 1800°C [13], [14]. The precursors (CaCO 3 and Al 2 O 3 ) were mixed in a 12:7 stoichiometric ratio and carefully ground with a mortar and pestle to minimize the crystal size and help facilitate a solid-state reaction.…”

Section: Experimental Setup and Cathode Fabricationmentioning

confidence: 99%

A Calcium Aluminate Electride Hollow Cathode

Rand

Williams

2015

IEEE Trans. Plasma Sci.

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This paper discusses the development and testing of a C12A7 electride hollow cathode. C12A7:e − is a crystalline ceramic in which electrons clathrated in subnanometer sized cages act as a conductive medium. C12A7:e − has a predicted work function of 0.6 eV and hence is of interest in electron emission devices. A 6-mm diameter hollow cathode was fabricated with a C12A7:e − insert and operated for over 50 h on xenon with no signs of degradation. It was successfully started at room temperature without a heater and steady state operating temperatures below 650°C were observed during some tests. In addition, the C12A7:e − cathode was operated on iodine for over 20 h at discharge currents up to 15 A.

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Section: Experimental Setup and Cathode Fabricationmentioning

confidence: 99%

A Calcium Aluminate Electride Hollow Cathode

Rand

Williams

2015

IEEE Trans. Plasma Sci.

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“…The C12A7 electride (C12A7: e − ) has attracted wide attention due to its low work function and chemical stability, despite the fact that the effective work function is demonstrated to be much higher than theoretical prediction [1][2][3]. In 2011, Rand first applied the C12A7 electride to hollow cathode emitter materials [4]. The C12A7 hollow cathode was ignited successfully and operated for several tens of minutes, while instability and frequent extinguishment of discharge existed [5].…”

Section: Introductionmentioning

confidence: 99%

“…In 2011, Rand first applied the C12A7 electride to hollow cathode emitter materials [4]. The C12A7 hollow cathode was ignited successfully and operated for several tens of minutes, while instability and frequent extinguishment of discharge existed [5]. Through the joint efforts of Rand, McDonald and Drobny et al, the stability and long duration of C12A7 hollow cathodes were achieved with modified cathode configurations and emitter shapes [6][7][8][9].…”

Section: Introductionmentioning

confidence: 99%

“…The results showed that the work function increased as the number of tests increased until no current was detected. They hypothesized that the development of a layer on the C12A7: e − surface after discharge might cause a decrease and the eventual cessation of electron emission [4]. McDonald et al reported degradation of a low-current C12A7 hollow cathode ignited with an external heater.…”

Section: Introductionmentioning

confidence: 99%

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An experimental study on the degradation of the C12A7 hollow cathode

Hua¹,

Wang²,

Luo³

et al. 2022

Plasma Sci. Technol.

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Emitter overheating is by far the largest problem limiting the performance of novel C12A7 hollow cathodes. To explore the failure operating point and degradation mechanism of the C12A7 hollow cathode, microscopic analyses of a degraded electride emitter after ten hours of thermal electron emission are presented in this paper. The morphology and composition variation of overheated electride emitters by SEM, EDS, and XRD indicate the melting and decomposition of electride of the surface layer. The monitored temperature of the electride emitter during the C12A7 hollow cathode operation shows that to avoid overheating the electride emitter, the average current density should be about 64 mA/mm2 allowed for the C12A7 hollow cathode in its current configuration. Experiment results of the heaterless C12A7 hollow cathode demonstrate that the Xe ions bombardment can remove the insulating layer and restore the thermionic emission capability for less degraded emitters. Based on experimental results and microscopic characterization, the depletion and degradation mechanism of electride emitters during the hollow cathode operation are discussed.

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“…When C12A7 : e − is exposed to an ambient atmosphere, the rigid structure of the lattice framework made of Ca − O and Al − O bonds prohibits the diffusion of H 2 O and O 2 molecules into a cage to react with the low-work-function electrons [143]. A high-intensity thermal field electron emission was observed during heating at 900 K to 1200 K, demonstrating thermal stability at high temperatures [62,110].…”

Section: Preliminary Model For Bare Thermionic Tethermentioning

confidence: 99%

Low Work-Function Thermionic Emission and Orbital-Motion-Limited Ion Collection at Bare-Tether Cathodic Contact

Chen¹

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An electrodynamic tether operates on electromagnetic principles and exchanges momentum through the planetary magnetosphere, by continuously interacting with the ionosphere. It is a reliable passive subsystem to deorbit spent rocket stages and satellites at its end of mission, mitigating the growth of orbital debris. A tether left bare of insulation collects electrons by its own uninsulated and positively biased segment with kilometer range, while electrons are emitted by a low-impedance active device at the cathodic end, such as a hollow cathode, to emit the full electron current.In the absence of an active cathodic device, the current flowing along an orbiting bare tether vanishes at both ends and the tether is said to be electrically floating. For negligible thermionic emission and orbital-motion-limited (OML) collection throughout the entire tether (electron/ion collection at anodic/cathodic segment, respectively), the anodic-to-cathodic length ratio is very small due to ions being much heavier, which results in low average current and Lorentz drag.The electride C12A7 : e − , which might present a possible work function as low as W = 0.6 eV and moderately high temperature stability, has been proposed as coating for floating bare tethers. Thermionic emission along a thus coated cathodic segment, under heating in space operation, can be more efficient than ion collection and, in the simplest drag mode, may eliminate the need for an active cathodic device and its corresponding gas-feed requirements and power subsystem, which would result in a truly "propellant-less" tether system.With this low-W coating, each elemental segment on the cathodic segment of a kilometers-long floating bare-tether would emit current as if it were part of a hot cylindrical probe uniformly polarized at the local tether bias, under 2D probe conditions that are also applied to the anodic-segment analysis. In the presence of emission, emitted electrons result in negative space charge, which decreases the electric field that accelerates them outwards, or even reverses it, decelerating electrons near the emitting probe. A double sheath would be established with electrons being emitted from the probe and ions coming from the ambient plasma. The thermionic current density, varying along the cathodic segment, might follow two distinct laws under different convii

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Hollow Cathode with Electride Insert

Cited by 6 publications

References 8 publications

A Calcium Aluminate Electride Hollow Cathode

A Calcium Aluminate Electride Hollow Cathode

An experimental study on the degradation of the C12A7 hollow cathode

Low Work-Function Thermionic Emission and Orbital-Motion-Limited Ion Collection at Bare-Tether Cathodic Contact

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