Collisionless “thermalization” in the sheath of an argon discharge

Coulette, David; Manfredi, Giovanni

doi:10.1063/1.4917239

Cited by 9 publications

(8 citation statements)

References 27 publications

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“…Each of these predictions was found to be consistent with the model of reference [15]. Recent Vlasov simulations have also pointed out that ion acceleration by the sheath electric field alone leads to the appearance of a "collisionless thermalization" effect that is akin to velocity bunching in charged particle beams [110]. They conclude that although wave-particle collisions are likely responsible for much of the experimentally observed thermalization, the collisionless mechanism also plays a role in understanding the observations.…”

Section: Plasmas With One Ion Speciessupporting

confidence: 54%

Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

Baalrud

Scheiner

Yee

et al. 2020

Plasma Sources Sci. Technol.

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Biased electrodes are common components of plasma sources and diagnostics. The plasma-electrode interaction is mediated by an intervening sheath structure that influences properties of the electrons and ions contacting the electrode surface, as well as how the electrode influences properties of the bulk plasma. A rich variety of sheath structures have been observed, including ion sheaths, electron sheaths, double sheaths, double layers, anode glow, and fireballs. These represent complex self-organized responses of the plasma that depend not only on the local influence of the electrode, but also on the global properties of the plasma and the other boundaries that it is in contact with. This review summarizes recent advances in understanding the conditions under which each type of sheath forms, what the basic stability criteria and steady-state properties of each are, and the ways in which each can influence plasma-boundary interactions and bulk plasma properties. These results may be of interest to a number of application areas where biased electrodes are used, including diagnostics, plasma modification of materials, plasma sources, electric propulsion, and the interaction of plasmas with objects in space.

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Section: Plasmas With One Ion Speciessupporting

confidence: 54%

Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

Baalrud

Scheiner

Yee

et al. 2020

Plasma Sources Sci. Technol.

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“…In the normal incidence case, for which only the DS exists, the temperature T xx (ie, the variance along v x ) decreases as the ion population is accelerated towards the wall by the electric field. This well-known "acceleration cooling" [18,19] persists in the magnetized case. More importantly, as the electric field profile is spread out with decreasing 0 20 40 60 80 100 120…”

Section: Comparison Between the Fluid Model And Kinetic Simula-tionsmentioning

confidence: 92%

Kinetic simulations of the Chodura and Debye sheaths for magnetic fields with grazing incidence

Coulette

Manfredi

2016

Plasma Phys. Control. Fusion

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When an unmagnetized plasma comes in contact with a material surface, the difference in mobility between the electrons and the ions creates a non-neutral layer known as the Debye sheath (DS).However, in magnetic fusion devices, the open magnetic field lines intersect the structural elements of the device with near grazing incidence angles. The magnetic field tends to align the particle flow along its own field lines, thus counteracting the mechanism that leads to the formation of the DS.Recent work using a fluid model [P. Stangeby, Nucl. Fusion 52, 083012 (2012)] showed that the DS disappears when the incidence angle is smaller than a critical value (around 5 • for ITER-like parameters). Here, we study this transition by means of numerical simulations of a kinetic model both in the collisionless and weakly collisional regimes. We show that the main features observed in the fluid model are preserved: for grazing incidence, the space charge density near the wall is reduced or suppressed, the ion flow velocity is subsonic, and the electric field and plasma density profiles are spread out over several ion Larmor radii instead of a few Debye lengths as in the unmagnetized case. As there is no singularity at the DS entrance in the kinetic model, this phenomenon depends smoothly on the magnetic field incidence angle and no particular critical angle arises. The simulation results and the predictions of the fluid model are in good agreement, although some discrepancies subsist, mainly due to the assumptions of isothermal closure and diagonality of the pressure tensor in the fluid model. * Electronic address: coulette@unistra.fr † Electronic address: manfredi@unistra.fr 1 arXiv:1509.04479v2 [physics.plasm-ph]

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“…Previous measurements made in the same device showed an increase in the ion density at the sheath entrance before the characteristic density drop of the sheath 19 which was not predicted by theoretical models. 20,21 This rise of density is again observed, both for the metallic and ceramic plates even at low laser power (Fig. 8), and combines with the LIF saturation effect.…”

Section: B Influence Of the Plasma Density On The Ivdfmentioning

confidence: 61%

“…It has been successfully applied to sheaths facing metal samples 11,19 and gave quantitatively good agreements with theory and simulations. 20,21 Some experiments have been performed in plasma sheaths with a wide variety of purposes. [22][23][24][25][26][27][28][29] Moreover, performing LIF in a sheath implies that a surface is present and reflects the laser beam, which may cause some undesired additional signals.…”

Section: Introductionmentioning

confidence: 99%

Laser-induced fluorescence saturation effects on ion velocity distribution functions in the vicinity of reflecting surfaces

et al. 2019

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Laser-induced fluorescence (LIF) measurements of Argon ions performed in the sheath/pre-sheath facing a floating metallic plate and a BNSiO 2 ceramic one immersed in a low temperature plasma exhibit unexpected features. It appears that a strong fluorescence signal which could be unduly attributed to ions moving backward in the sheath is detected, even though the floating potential is far below the plasma potential. Moreover, this signal may be stronger than the one corresponding to ions having a forward motion. It is demonstrated that this abnormal measurement is due to the optical pumping saturation of the incident laser beam, while this saturation does not exist for the scattering of the beam at the sample surface. The reflected signal is unambiguously identified using a theoretical beam scattering model. It is also shown that the presence of the sheath/presheath density gradient is able to trigger the LIF saturation effect, complicating ion density measurements.

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Collisionless “thermalization” in the sheath of an argon discharge

Cited by 9 publications

References 27 publications

Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

Kinetic simulations of the Chodura and Debye sheaths for magnetic fields with grazing incidence

Laser-induced fluorescence saturation effects on ion velocity distribution functions in the vicinity of reflecting surfaces

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