Ion acoustic waves and double-layers in electronegative expanding plasmas
- Laboratoire de Physique Laboratoire de Physique, Ecole Normale Superieure de Lyon, CNRS, Universite de Lyon, 46 allee d'Italie, 69364 Lyon Cedex (France)
- Laboratoire de Physique des Plasmas, Ecole Polytechnique, UPMC, Universite Paris Sud-11, CNRS, 91128 Palaiseau Cedex (France)
Ion acoustic waves and double-layers are observed in expanding plasmas in electronegative gases, i.e., plasmas containing an appreciable fraction of negative ions. The reported experiments are performed in argon gas with a variable amount of SF{sub 6}. When varying the amount of SF{sub 6}, the negative ion fraction increases and three main regimes were identified previously: (i) the plasma smoothly expands at low negative ion fraction, (ii) a static double-layer (associated with an abrupt potential drop and ion acceleration) forms at intermediate negative ion fraction, (iii) double-layers periodically form and propagate (in the plasma expansion direction) at high negative ion fraction. In this paper, we show that transition phases exist in between these regimes, where fluctuations are observed. These fluctuations are unstable slow ion acoustic waves, propagating in the direction opposite to the plasma expansion. These fluctuations are excited by the most unstable eigenmodes and display turbulent features. It is suggested that the static double layer forms when the ion acoustic fluctuations become non-linearly unstable: the double layer regime being a bifurcated state of the smoothly expanding regime. For the highest negative ion fraction, a coexistence of (upstream propagating) slow ion acoustic fluctuations and (downstream) propagating double layers was observed.
- OSTI ID:
- 22043377
- Journal Information:
- Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 8 Vol. 18; ISSN PHPAEN; ISSN 1070-664X
- Country of Publication:
- United States
- Language:
- English
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