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Title: On the production of energetic neutrals in the cathode sheath of direct-current discharges

Abstract

Direct measurements of the energy distribution of energetic neutrals incident onto the cathode of a dc glow discharge are presented. The measurements are performed by time-of-flight analysis of neutrals escaping through a cathode orifice. The experimental results are found to be in excellent agreement with Monte Carlo simulations, although the forward angle of the neutrals considered is limited in the present experimental configuration. It is found that the commonly used theories for the production of energetic neutrals through charge exchange in the cathode sheath do not capture the neutral energy distribution over the range of discharge voltage studied.

Authors:
;  [1]
  1. Mechanical Engineering Department, Stanford University, Stanford, California 94305-3032 (United States)
Publication Date:
OSTI Identifier:
20971862
Resource Type:
Journal Article
Resource Relation:
Journal Name: Applied Physics Letters; Journal Volume: 90; Journal Issue: 10; Other Information: DOI: 10.1063/1.2711416; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; CATHODES; CHARGE EXCHANGE; COMPUTERIZED SIMULATION; DIRECT CURRENT; ELECTRIC POTENTIAL; ENERGY SPECTRA; GLOW DISCHARGES; MONTE CARLO METHOD; ORIFICES; PLASMA; PLASMA DIAGNOSTICS; PLASMA SHEATH; TIME-OF-FLIGHT METHOD

Citation Formats

Ito, Tsuyohito, and Cappelli, Mark A. On the production of energetic neutrals in the cathode sheath of direct-current discharges. United States: N. p., 2007. Web. doi:10.1063/1.2711416.
Ito, Tsuyohito, & Cappelli, Mark A. On the production of energetic neutrals in the cathode sheath of direct-current discharges. United States. doi:10.1063/1.2711416.
Ito, Tsuyohito, and Cappelli, Mark A. Mon . "On the production of energetic neutrals in the cathode sheath of direct-current discharges". United States. doi:10.1063/1.2711416.
@article{osti_20971862,
title = {On the production of energetic neutrals in the cathode sheath of direct-current discharges},
author = {Ito, Tsuyohito and Cappelli, Mark A.},
abstractNote = {Direct measurements of the energy distribution of energetic neutrals incident onto the cathode of a dc glow discharge are presented. The measurements are performed by time-of-flight analysis of neutrals escaping through a cathode orifice. The experimental results are found to be in excellent agreement with Monte Carlo simulations, although the forward angle of the neutrals considered is limited in the present experimental configuration. It is found that the commonly used theories for the production of energetic neutrals through charge exchange in the cathode sheath do not capture the neutral energy distribution over the range of discharge voltage studied.},
doi = {10.1063/1.2711416},
journal = {Applied Physics Letters},
number = 10,
volume = 90,
place = {United States},
year = {Mon Mar 05 00:00:00 EST 2007},
month = {Mon Mar 05 00:00:00 EST 2007}
}
  • We recently reported on the direct measurements of the energy distribution of energetic neutrals incident onto the cathode of an argon dc glow discharge [T. Ito and M. A. Cappelli, Appl. Phys. Lett. 90, 101503 (2007)]. In this paper, we provide a more extensive report of the experimental data, as well as direct comparisons to Monte Carlo simulations in which neutral-neutral and ion-neutral collision processes are modeled with differential scattering cross sections. The experimental results, which are limited to relatively small forward angle sampling in our present configuration, are found to be in excellent agreement with Monte Carlo simulations. Themore » simulations reveal that high energy neutrals are striking the cathode with a wide range of angles, while ions are more anisotropic. Therefore, it is difficult to predict the neutral energy distribution with commonly used simple analytical models that do not consider scattering of neutrals within the sheath. We propose the use of an extended analytical model in this paper, which seems to provide reasonable energy distributions over the range of discharge voltage studied.« less
  • It is experimentally demonstrated that energetic electrons originating from the cathode or produced in volumetric processes in the cathode region of a cold cathode direct current discharge can create a large potential drop in the near-wall sheath. This voltage drop may be much greater than kT{sub e}/e (where k is the Boltzmann constant, T{sub e} is the electron temperature, and e is the electron charge). Due to the large near-wall sheath voltage, slow and moderately energetic electrons cannot reach the wall and move toward the anode. Application of additional potentials to the wall can change the amount of energetic electronsmore » reaching the wall. This effect can be used for regulation of the near-cathode plasma and near-wall sheath properties and thus may be useful in technical applications.« less
  • We experimentally investigate the characteristics of dielectric barrier discharges in an electrode system with one-dimensional nanostructures of gap size at micrometer scale. Evidence of quasistationary direct current discharges in air has been observed under the applied voltage several times lower than the first ionization potential of O{sub 2}. The results qualitatively agree with the hypothesis on the ionization mechanism of stepwise inelastic collisions within a metastable pool, which is populated through field excitation and inelastic impact between the neutrals and the nanostructures.
  • A detailed one-dimensional simulation of the ion dynamics of the plasma sheath near a substrate (cathode) in the presence of fast monoenergetic electrons has been carried out in this article. The sheath evolution is investigated by using a fluid model assuming that the ions, plasma electrons and monoenergetic, fast electrons act as three fluids (fluid approach). The effect of the density of fast electrons on the ion density, ion velocity, and ion energy near the cathode and the evolution of the sheath boundary in front of the cathode are separately explored. Also, the variation of the ion velocity and ionmore » density at the vicinity of the cathode as a function of time is investigated in the absence and presence of the electron beam. Results indicate that the presence of fast electrons in the sheath causes significant change in the sheath thickness and therefore basically changes the ion velocity, ion density, and ion impact energy on the cathode compared to the absence of the electron beam case.« less
  • Ions with energies significantly in excess of the applied discharge voltage have been reported for many years in hollow cathode discharges. Models of dc potential hills downstream of the cathode and instabilities in postulated double layers in the cathode orifice have been proposed to explain this, but have not been substantiated. Measurements of the dc and rf plasma density and potential profiles near the exit of hollow cathodes by miniature fast-scanning probes suggests that turbulent ion acoustic fluctuations and ionization instabilities in the cathode plume significantly increase the energy of the ions that flow from this region. Increases in themore » discharge current and/or decreases in the cathode gas flow enhance the amplitude of the fluctuations and increase the number and energy of the energetic ions, which increases the erosion rate of the cathode electrodes. The transition from the quiescent 'spot mode' to the noisy 'plume mode' characteristic of these discharges is found to be a gradual transition of increasing fluctuation amplitudes.« less