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Title: Plasma processes inside dispenser hollow cathodes

Abstract

A two-dimensional fluid model of the plasma and neutral gas inside dispenser orificed hollow cathodes has been developed to quantify plasma processes that ultimately determine the life of the porous emitters inserted in these devices. The model self-consistently accounts for electron emission from the insert as well as for electron and ion flux losses from the plasma. Two cathodes, which are distinctively different in size and operating conditions, have been simulated numerically. It is found that the larger cathode, with outer tube diameter of 1.5 cm and orifice diameter of 0.3 cm, establishes an effective emission zone that spans approximately the full length of the emitter when operated at a discharge current of 25 A and a flow rate of 5.5 sccm. The net heating of the emitter is caused by ions that are produced by ionization of the neutral gas inside the tube and are then accelerated by the sheath along the emitter. The smaller cathode, with an outer diameter of 0.635 cm and an orifice diameter of 0.1 cm, does not exhibit the same operational characteristics. At a flow rate of 4.25 sccm and discharge current of 12 A, the smaller cathode requires 4.5 times the current densitymore » near the orifice and operates with more than 6 times the neutral particle density compared to the large cathode. As a result, the plasma particle density is almost one order of magnitude higher compared to the large cathode. The plasma density in this small cathode is high enough such that the Debye length is sufficiently small to allow 'sheath funneling' into the pores of the emitter. By accessing areas deeper into the insert material, it is postulated that the overall emission of electrons is significantly enhanced. The maximum emission current density is found to be about 1 A/mm{sup 2} in the small cathode, which is about one order of magnitude higher than attained in the large cathode. The effective emission zone in the small cathode extends to about 15% of the emitter length only, and the power deposited at the emitter surface by returning electrons is found to be twice that deposited by ions. A previous study suggested that the computed particle flux and energy of ions to the emitter of the 1.5 cm cathode were not high enough to change the barium evaporation rate compared to thermally induced evaporation. The same suggestion is made here for the 0.635 cm cathode. The peak ion flux to the emitter is found to be 1.2 A/cm{sup 2} (7.6x10{sup 18}/s cm{sup 2}), and the corresponding peak sheath drop is 2.9 V. Consequently, once the emitter operating temperature is known it is possible to determine directly the barium depletion-limited life of these cathodes using existing vacuum-cathode data.« less

Authors:
; ; ; ;  [1]
  1. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 (United States)
Publication Date:
OSTI Identifier:
20787398
Resource Type:
Journal Article
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 13; Journal Issue: 6; Other Information: DOI: 10.1063/1.2208292; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 1070-664X
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; BARIUM; CURRENT DENSITY; DEBYE LENGTH; ELECTRON EMISSION; ELECTRONS; EVAPORATION; FLOW RATE; GLOW DISCHARGES; HOLLOW CATHODES; IONIZATION; IONS; PARTICLE LOSSES; PLASMA; PLASMA DENSITY; PLASMA FLUID EQUATIONS; PLASMA SHEATH; PLASMA SIMULATION; POROUS MATERIALS; TWO-DIMENSIONAL CALCULATIONS

Citation Formats

Mikellides, Ioannis G, Katz, Ira, Goebel, Dan M, Polk, James E, and Jameson, Kristina K. Plasma processes inside dispenser hollow cathodes. United States: N. p., 2006. Web. doi:10.1063/1.2208292.
Mikellides, Ioannis G, Katz, Ira, Goebel, Dan M, Polk, James E, & Jameson, Kristina K. Plasma processes inside dispenser hollow cathodes. United States. doi:10.1063/1.2208292.
Mikellides, Ioannis G, Katz, Ira, Goebel, Dan M, Polk, James E, and Jameson, Kristina K. Thu . "Plasma processes inside dispenser hollow cathodes". United States. doi:10.1063/1.2208292.
@article{osti_20787398,
title = {Plasma processes inside dispenser hollow cathodes},
author = {Mikellides, Ioannis G and Katz, Ira and Goebel, Dan M and Polk, James E and Jameson, Kristina K},
abstractNote = {A two-dimensional fluid model of the plasma and neutral gas inside dispenser orificed hollow cathodes has been developed to quantify plasma processes that ultimately determine the life of the porous emitters inserted in these devices. The model self-consistently accounts for electron emission from the insert as well as for electron and ion flux losses from the plasma. Two cathodes, which are distinctively different in size and operating conditions, have been simulated numerically. It is found that the larger cathode, with outer tube diameter of 1.5 cm and orifice diameter of 0.3 cm, establishes an effective emission zone that spans approximately the full length of the emitter when operated at a discharge current of 25 A and a flow rate of 5.5 sccm. The net heating of the emitter is caused by ions that are produced by ionization of the neutral gas inside the tube and are then accelerated by the sheath along the emitter. The smaller cathode, with an outer diameter of 0.635 cm and an orifice diameter of 0.1 cm, does not exhibit the same operational characteristics. At a flow rate of 4.25 sccm and discharge current of 12 A, the smaller cathode requires 4.5 times the current density near the orifice and operates with more than 6 times the neutral particle density compared to the large cathode. As a result, the plasma particle density is almost one order of magnitude higher compared to the large cathode. The plasma density in this small cathode is high enough such that the Debye length is sufficiently small to allow 'sheath funneling' into the pores of the emitter. By accessing areas deeper into the insert material, it is postulated that the overall emission of electrons is significantly enhanced. The maximum emission current density is found to be about 1 A/mm{sup 2} in the small cathode, which is about one order of magnitude higher than attained in the large cathode. The effective emission zone in the small cathode extends to about 15% of the emitter length only, and the power deposited at the emitter surface by returning electrons is found to be twice that deposited by ions. A previous study suggested that the computed particle flux and energy of ions to the emitter of the 1.5 cm cathode were not high enough to change the barium evaporation rate compared to thermally induced evaporation. The same suggestion is made here for the 0.635 cm cathode. The peak ion flux to the emitter is found to be 1.2 A/cm{sup 2} (7.6x10{sup 18}/s cm{sup 2}), and the corresponding peak sheath drop is 2.9 V. Consequently, once the emitter operating temperature is known it is possible to determine directly the barium depletion-limited life of these cathodes using existing vacuum-cathode data.},
doi = {10.1063/1.2208292},
journal = {Physics of Plasmas},
issn = {1070-664X},
number = 6,
volume = 13,
place = {United States},
year = {2006},
month = {6}
}