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Title: Alternative model of space-charge-limited thermionic current flow through a plasma

It is widely assumed that thermionic current flow through a plasma is limited by a “space-charge-limited” (SCL) cathode sheath that consumes the hot cathode's negative bias and accelerates upstream ions into the cathode. In this paper, we formulate a fundamentally different current-limited mode. In the “inverse” mode, the potentials of both electrodes are above the plasma potential, so that the plasma ions are confined. The bias is consumed by the anode sheath. There is no potential gradient in the neutral plasma region from resistivity or presheath. The inverse cathode sheath pulls some thermoelectrons back to the cathode, thereby limiting the circuit current. Thermoelectrons entering the zero-field plasma region that undergo collisions may also be sent back to the cathode, further attenuating the circuit current. In planar geometry, the plasma density is shown to vary linearly across the electrode gap. A continuum kinetic planar plasma diode simulation model is set up to compare the properties of current modes with classical, conventional SCL, and inverse cathode sheaths. SCL modes can exist only if charge-exchange collisions are turned off in the potential well of the virtual cathode to prevent ion trapping. With the collisions, the current-limited equilibrium must be inverse. Inverse operating modesmore » should therefore be present or possible in many plasma devices that rely on hot cathodes. Evidence from past experiments is discussed. Finally, the inverse mode may offer opportunities to minimize sputtering and power consumption that were not previously explored due to the common assumption of SCL sheaths.« less
Authors:
 [1]
  1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Publication Date:
Report Number(s):
LLNL-JRNL-740249
Journal ID: ISSN 2470-0045
Grant/Contract Number:
AC52-07NA27344
Type:
Accepted Manuscript
Journal Name:
Physical Review E
Additional Journal Information:
Journal Volume: 97; Journal Issue: 4; Journal ID: ISSN 2470-0045
Publisher:
American Physical Society (APS)
Research Org:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; plasma instabilities; plasma sources; thermionic emission; plasma sheaths & surfaces
OSTI Identifier:
1438688
Alternate Identifier(s):
OSTI ID: 1433765

Campanell, M. D.. Alternative model of space-charge-limited thermionic current flow through a plasma. United States: N. p., Web. doi:10.1103/PhysRevE.97.043207.
Campanell, M. D.. Alternative model of space-charge-limited thermionic current flow through a plasma. United States. doi:10.1103/PhysRevE.97.043207.
Campanell, M. D.. 2018. "Alternative model of space-charge-limited thermionic current flow through a plasma". United States. doi:10.1103/PhysRevE.97.043207.
@article{osti_1438688,
title = {Alternative model of space-charge-limited thermionic current flow through a plasma},
author = {Campanell, M. D.},
abstractNote = {It is widely assumed that thermionic current flow through a plasma is limited by a “space-charge-limited” (SCL) cathode sheath that consumes the hot cathode's negative bias and accelerates upstream ions into the cathode. In this paper, we formulate a fundamentally different current-limited mode. In the “inverse” mode, the potentials of both electrodes are above the plasma potential, so that the plasma ions are confined. The bias is consumed by the anode sheath. There is no potential gradient in the neutral plasma region from resistivity or presheath. The inverse cathode sheath pulls some thermoelectrons back to the cathode, thereby limiting the circuit current. Thermoelectrons entering the zero-field plasma region that undergo collisions may also be sent back to the cathode, further attenuating the circuit current. In planar geometry, the plasma density is shown to vary linearly across the electrode gap. A continuum kinetic planar plasma diode simulation model is set up to compare the properties of current modes with classical, conventional SCL, and inverse cathode sheaths. SCL modes can exist only if charge-exchange collisions are turned off in the potential well of the virtual cathode to prevent ion trapping. With the collisions, the current-limited equilibrium must be inverse. Inverse operating modes should therefore be present or possible in many plasma devices that rely on hot cathodes. Evidence from past experiments is discussed. Finally, the inverse mode may offer opportunities to minimize sputtering and power consumption that were not previously explored due to the common assumption of SCL sheaths.},
doi = {10.1103/PhysRevE.97.043207},
journal = {Physical Review E},
number = 4,
volume = 97,
place = {United States},
year = {2018},
month = {4}
}