Ion acoustic wave studies near the presheath/sheath boundary in a weakly collisional argon/xenon plasma

The location of the plasma sheath edge, where quasineutrality is broken, is rigorously derived by using a kinetic description of the plasma. It is shown that sheaths can exist with arbitrarily small ion velocity at the sheath edge, thus violating the Bohm criterion, V i = c s at the sheath edge. Bohm's criterion is recovered in the case of large enough ion current through the wall, and it is found to be a reasonable approximation in floating potential conditions. However, in the case of a predominant electron current through the wall, Bohm's criterion is not able to describe the sheath-edge transition. The analytical results are supported by numerical simulations performed with a fully kinetic particle-in-cell code modeling a source-driven, weakly collisional plasma, bound between two absorbing walls.

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“…Since the function κ → ∞ as η se → 0, the scaling analysis of Eq. (11) shows that L sh → 0 in this limit, as confirmed by Fig. 1.…”

Section: Discussionsupporting

confidence: 76%

“…[5][6][7][8][9]). Only recently has their detailed experimental investigation become possible [10][11][12]. Sheaths are present in the edge of magnetically confined fusion plasmas, at the interface between spacecrafts and space plasmas, in the fabrication of semiconductor devices, and wherever a plasma interacts with a solid surface.…”

Section: Introductionmentioning

confidence: 99%

Existence of subsonic plasma sheaths

2011

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“…Langmuir probe biased at À67 V to collect ion saturation current. A Boxcar averager 19 was employed to do time sampling and averaging. A family of spatial profiles of the signal amplitude versus launcher-detector separation were created by systematically incrementing the trigger delay with the result that the phase velocity of the IAWs could be easily measured.…”

Section: Ion Acoustic Wave Diagnostics (Continuous Wave)mentioning

confidence: 99%

Experimental test of instability enhanced collisional friction for determining ion loss in two ion species plasmas

2011

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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, 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.

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“…In experiments by Saxena et al, 6,13 a rarefactive pulse is found to fission into several pulses. Recently Oksuz et al 14 have measured the phase velocity of ion acoustic wave near the presheath and sheath boundaries in a weakly collisional plasma. Okutsu and Nakamura 15 have carried out a numerical and experimental study of the temporal development of weakly nonlinear, broad ion acoustic pulses.…”

Section: Introductionmentioning

confidence: 99%

Excitation of ion rarefaction waves in a low pressure plasma by applying a short high negative voltage pulse

2011

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The ion rarefaction response to a high negative voltage pulse (U 0 >> kT e /e) applied to a metal plate immersed in a low pressure argon plasma, for time duration lower than ion plasma period, is experimentally examined. In the present experiment the pulse duration is kept intermediate between the ion and electron plasma response times. Such a pulse duration is chosen so that ions are collectively undisturbed and, according to general understanding, no force is given to ions. Hence no ion rarefaction wave should be excited. But contrary to the general understanding, excitation of a rarefaction wave is observed. The results indicate that the speed of the rarefaction waves for various conditions (like plasma density, applied pulse magnitude, and pulse duration) is supersonic. After a distance from the exciter (biased plate), typically three-fourth of the exciter diameter, the rarefaction waves are turned into ion acoustic waves. The experimental results indicate that even though the bias durations are shorter than the ion plasma period, if the bias magnitude is large enough, some collective plasma behavior can still be excited.

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Ion acoustic wave studies near the presheath/sheath boundary in a weakly collisional argon/xenon plasma

Cited by 38 publications

References 21 publications

Existence of subsonic plasma sheaths

Existence of subsonic plasma sheaths

Experimental test of instability enhanced collisional friction for determining ion loss in two ion species plasmas

Excitation of ion rarefaction waves in a low pressure plasma by applying a short high negative voltage pulse

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