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Title: Nonlocal effects in a bounded low-temperature plasma with fast electrons

Abstract

Effects associated with nonlocality of the electron energy distribution function (EEDF) in a bounded, low-temperature plasma containing fast electrons, can lead to a significant increase in the near-wall potential drop, leading to self-trapping of fast electrons in the plasma volume, even if the density of this group is only a small fraction ({approx}0.001%) of the total electron density. If self-trapping occurs, the fast electrons can substantially increase the rate of stepwise excitation, supply additional heating to slow electrons, and reduce their rate of diffusion cooling. Altering the source terms of these fast electrons will, therefore, alter the near-wall sheath and, through modification of the EEDF, a number of plasma parameters. Self-trapping of fast electrons is important in a variety of plasmas, including hollow-cathode discharges and capacitive rf discharges, and is especially pronounced in an afterglow plasma, which is a key phase of any pulse-modulated discharge. In the afterglow, the electron temperature is less than a few tenths of an electron volt, and the fast electrons will have energies typically greater than an electron volt. It is shown that in the afterglow plasma of noble gases, fast electrons, arising from Penning ionization of metastable atoms, can lead to the above conditionmore » and significantly change the plasma and sheath properties. Similar effects can be important in technologically relevant electronegative gas plasmas, where fast electrons can arise due to electron detachment in collisions of negative ions with atomic species. Both experimental and modeling results are presented to illustrate these effects.« less

Authors:
; ;  [1];  [2];  [3]
  1. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433 (United States)
  2. (United States)
  3. (Russian Federation)
Publication Date:
OSTI Identifier:
20975088
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2436470; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; AFTERGLOW; BEAM-PLASMA SYSTEMS; BOUNDARY LAYERS; ELECTRON BEAMS; ELECTRON DENSITY; ELECTRON DETACHMENT; ELECTRON TEMPERATURE; ELECTRONS; ENERGY SPECTRA; GLOW DISCHARGES; HIGH-FREQUENCY DISCHARGES; HOLLOW CATHODES; ION TEMPERATURE; PLASMA; PLASMA DENSITY; PLASMA SHEATH; RARE GASES; SOURCE TERMS; WALL EFFECTS

Citation Formats

DeJoseph, C. A. Jr., Demidov, V. I., Kudryavtsev, A. A., Department of Physics, West Virginia University, Morgantown, West Virginia 26506, and Department of Optics and Spectroscopy, St. Petersburg State University, 195904 St. Petersburg. Nonlocal effects in a bounded low-temperature plasma with fast electrons. United States: N. p., 2007. Web. doi:10.1063/1.2436470.
DeJoseph, C. A. Jr., Demidov, V. I., Kudryavtsev, A. A., Department of Physics, West Virginia University, Morgantown, West Virginia 26506, & Department of Optics and Spectroscopy, St. Petersburg State University, 195904 St. Petersburg. Nonlocal effects in a bounded low-temperature plasma with fast electrons. United States. doi:10.1063/1.2436470.
DeJoseph, C. A. Jr., Demidov, V. I., Kudryavtsev, A. A., Department of Physics, West Virginia University, Morgantown, West Virginia 26506, and Department of Optics and Spectroscopy, St. Petersburg State University, 195904 St. Petersburg. Tue . "Nonlocal effects in a bounded low-temperature plasma with fast electrons". United States. doi:10.1063/1.2436470.
@article{osti_20975088,
title = {Nonlocal effects in a bounded low-temperature plasma with fast electrons},
author = {DeJoseph, C. A. Jr. and Demidov, V. I. and Kudryavtsev, A. A. and Department of Physics, West Virginia University, Morgantown, West Virginia 26506 and Department of Optics and Spectroscopy, St. Petersburg State University, 195904 St. Petersburg},
abstractNote = {Effects associated with nonlocality of the electron energy distribution function (EEDF) in a bounded, low-temperature plasma containing fast electrons, can lead to a significant increase in the near-wall potential drop, leading to self-trapping of fast electrons in the plasma volume, even if the density of this group is only a small fraction ({approx}0.001%) of the total electron density. If self-trapping occurs, the fast electrons can substantially increase the rate of stepwise excitation, supply additional heating to slow electrons, and reduce their rate of diffusion cooling. Altering the source terms of these fast electrons will, therefore, alter the near-wall sheath and, through modification of the EEDF, a number of plasma parameters. Self-trapping of fast electrons is important in a variety of plasmas, including hollow-cathode discharges and capacitive rf discharges, and is especially pronounced in an afterglow plasma, which is a key phase of any pulse-modulated discharge. In the afterglow, the electron temperature is less than a few tenths of an electron volt, and the fast electrons will have energies typically greater than an electron volt. It is shown that in the afterglow plasma of noble gases, fast electrons, arising from Penning ionization of metastable atoms, can lead to the above condition and significantly change the plasma and sheath properties. Similar effects can be important in technologically relevant electronegative gas plasmas, where fast electrons can arise due to electron detachment in collisions of negative ions with atomic species. Both experimental and modeling results are presented to illustrate these effects.},
doi = {10.1063/1.2436470},
journal = {Physics of Plasmas},
number = 5,
volume = 14,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • It is demonstrated for the first time that the presence of a small number of fast, nonlocal electrons can dramatically change the thickness of and electric field in the near-wall sheath. Even if the density of the nonlocal fast group, n{sub f}, is much less than the density of the bulk electrons, n{sub b} (n{sub f}{approx}10{sup -5}n{sub b}), the near-wall potential can increase dramatically resulting in a comparable increase in the sheath thickness. Because of this low fractional density, the average energy (electron temperature T{sub e}) of all electrons is little changed from that of the bulk, yet the near-wallmore » potential drop can increase to tens of T{sub e}/e. More importantly, due to the nonlocal nature of this group of electrons, the near-wall sheath potential is found to be independent of T{sub e} and is determined only by the energy of the fast group.« less
  • Low-pressure pulsed plasmas are widely used in various technological applications. Understanding of the phenomena taking place in afterglow phase of the discharge makes possible the optimization of the operation conditions and improvement of the technical parameters. At low pressure the electron component of the plasma determines the main features of the discharge since its behavior dominates all other plasma properties. We study the electron kinetics in a low-pressure afterglow plasma of an inductively coupled discharge by means of a self-consistent model which uses the nonlocal kinetic approach. The main features of the model are given. Special attention is paid tomore » determination of the steady state of the discharge from which the decay of the plasma begins. Emphasis is also put on the description of the collisional interaction between the electrons and gas. Results of theoretical investigations for argon at a pressure of 2-4 Pa are presented. Calculated temporal evolutions of the isotropic part of the electron velocity distribution function, electron density, mean electron energy, and wall potential are discussed in comparison with experimental data.« less
  • This paper presents experimental results on plasma transport across the magnetic field (B) in magnetized low-temperature plasma sources. Due to the presence of chamber walls, this transport can be complex even in a non-turbulent regime. In particular, in configurations without cylindrical symmetry, the magnetic drifts tend to be bounded by the chamber walls, thereby inducing plasma asymmetry and reducing magnetic confinement. In this work, we measure electron and ion current densities at metal chamber walls bounding a rectangular magnetic filter and demonstrate that these current densities are asymmetrically nonuniform. We also provide an experimental confirmation of model predictions of increasedmore » cross-field electron transport in such filter configuration, scaling as 1/B rather than the classical 1/B{sup 2} scaling.« less
  • Using a simple model of a plasma between two boundaries at which temperature is fixed, we investigate the thermal transport in the collisionless limit, relaxing the usual local neutrality assumption. For physically relevant external parameters, an analytical solution is found where the heat flux is reduced with respect to its value when local electroneutrality is assumed.
  • It is demonstrated experimentally, in a pulsed discharge, that it is possible to modify the 'tail' of a nonlocal electron energy distribution (EED) without significantly changing the electron density and temperature (mean energy). The EED tail is modified by changing the potential of a small portion of the plasma boundary and/or by changing the volume creation rate of electrons with energies in the range of the tail of the EED. The discussed effects are a direct result of the nonlocal nature of the EED and have applications to a number of basic research issues associated with discharges under nonequilibrium conditions.more » As an example, we discuss the possibility of utilizing these methods to measure electron impact excitation cross sections from the metastable states of atoms, which are difficult to measure by other means. The experiments have been conducted in an argon and argon-nitrogen pulsed rf inductively coupled plasma discharge.« less