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Title: Influence of neutral pressure on instability enhanced friction and ion velocities at the sheath edge of two-ion-species plasmas

Authors:
 [1];  [1];  [2]
  1. Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242, USA
  2. Laboratoire de Physique des Plasmas, CNRS, Sorbonne Universites, UPMC Univ Paris 06, Univ Paris-Sud, Ecole Polytechnique, 91128 Palaiseau, France
Publication Date:
Sponsoring Org.:
USDOE
OSTI Identifier:
1412617
Grant/Contract Number:
SC0016473
Resource Type:
Journal Article: Publisher's Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 24; Journal Issue: 12; Related Information: CHORUS Timestamp: 2017-12-11 10:51:55; Journal ID: ISSN 1070-664X
Publisher:
American Institute of Physics
Country of Publication:
United States
Language:
English

Citation Formats

Adrian, P. J., Baalrud, S. D., and Lafleur, T. Influence of neutral pressure on instability enhanced friction and ion velocities at the sheath edge of two-ion-species plasmas. United States: N. p., 2017. Web. doi:10.1063/1.4986239.
Adrian, P. J., Baalrud, S. D., & Lafleur, T. Influence of neutral pressure on instability enhanced friction and ion velocities at the sheath edge of two-ion-species plasmas. United States. doi:10.1063/1.4986239.
Adrian, P. J., Baalrud, S. D., and Lafleur, T. 2017. "Influence of neutral pressure on instability enhanced friction and ion velocities at the sheath edge of two-ion-species plasmas". United States. doi:10.1063/1.4986239.
@article{osti_1412617,
title = {Influence of neutral pressure on instability enhanced friction and ion velocities at the sheath edge of two-ion-species plasmas},
author = {Adrian, P. J. and Baalrud, S. D. and Lafleur, T.},
abstractNote = {},
doi = {10.1063/1.4986239},
journal = {Physics of Plasmas},
number = 12,
volume = 24,
place = {United States},
year = 2017,
month =
}

Journal Article:
Free Publicly Available Full Text
This content will become publicly available on December 11, 2018
Publisher's Accepted Manuscript

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  • A generalized Lenard-Balescu theory that accounts for instability-enhanced collective responses is used to calculate the collisional friction between ion species in the plasma-boundary transition region (presheath). Ion-ion streaming instabilities are shown to cause such a strong frictional force that the relative flow speed between ion species cannot significantly exceed the critical threshold value (DELTAV{sub c}) at which instability onset occurs. When combined with the Bohm criterion, this condition uniquely determines the flow speed of each ion species at the plasma-sheath boundary. For cold ions, DELTAV{sub c}->0 and each ion species leaves the plasma at a common system sound speed c{submore » s}.« less
  • Recent experiments have shown that ions in weakly collisional plasmas containing two ion species of comparable densities nearly reach a common velocity at the sheath edge. A new theory suggests that collisional friction between the two ion species enhanced by two stream instability reduces the drift velocity of each ion species relative to each other near the sheath edge and finds that the difference in velocities at the sheath edge depends on the relative concentrations of the species. It is small when the concentrations are comparable and is large, with each species reaching its own Bohm velocity, when the relativemore » concentration differences are large. To test these findings, ion drift velocities were measured with laser-induced fluorescence in argon-xenon plasmas. We show that the predictions are in excellent agreement with the first experimental tests of the new model.« less
  • Recent experiments have shown that ions in weakly collisional plasmas containing two ion species of comparable densities approximately reach a common velocity at the sheath edge equal to the bulk plasma ion sound velocity. A recent theory [S. D. Baalrud, C. C. Hegna, and J. D. Callen, Phys. Rev. Lett. 103, 205002 (2009)] suggests that this is a consequence of collisional friction between the two ion species enhanced by the two stream instability. The theory finds that the difference in velocities at the sheath edge depends on the relative concentrations of the two ions. The difference in velocities is small,more » with both species approaching to the bulk sound velocity, when the concentrations are comparable, and is large, with each species reaching its own Bohm velocity, when the relative concentration differences are large. To test these findings, drift velocities of Ar and Xe ions were measured with laser-induced fluorescence in Ar-Xe and He-Xe plasmas and combined with ion acoustic wave and plasma potential data. In addition, electron temperature was varied by a Maxwell demon [K. R. MacKenzie et al., App. Phys. Lett. 18, 529 (1971)]. The predictions were found to be in excellent agreement with the experimental data. The generalized Bohm criterion in two ion species plasmas is also verified in a wider variety of relative ion concentrations.« less
  • The pre-sheath density drop along the magnetic field in field-aligned, radially propagating plasma blobs is investigated in the TORPEX toroidal experiment [Fasoli et al., Plasma Phys. Controlled Fusion 52, 124020 (2010)]. Using Langmuir probes precisely aligned along the magnetic field, we measure the density n{sub se} at a poloidal limiter, where blobs are connected, and the upstream density n{sub 0} at a location half way to the other end of the blobs. The pre-sheath density drop n{sub se}/n{sub 0} is then computed and its dependence upon the neutral background gas pressure is studied. At low neutral gas pressures, the pre-sheathmore » density drop is ≈0.4, close to the value of 0.5 expected in the collisionless case. In qualitative agreement with a simple model, this value decreases with increasing gas pressure. No significant dependence of the density drop upon the radial distance into the limiter shadow is observed. The effect of reduced blob density near the limiter on the blob radial velocity is measured and compared with predictions from a blob speed-versus-size scaling law [Theiler et al., Phys. Rev. Lett. 103, 065001 (2009)].« less