Thermodynamics of Yukawa fluids near the one-component-plasma limit

Thermodynamics of weakly screened (near the one-component-plasma limit) Yukawa fluids in two and three dimensions is analyzed in detail. It is shown that the thermal component of the excess internal energy of these fluids, when expressed in terms of the properly normalized coupling strength, exhibits the scaling pertinent to the corresponding one-component-plasma limit (the scalings differ considerably between the two- and three-dimensional situations). This provides us with a simple and accurate practical tool to estimate thermodynamic properties of weakly screened Yukawa fluids. Particular attention is paid to the two-dimensional fluids, for which several important thermodynamic quantities are calculated to illustrate the application of the approach.Comment: Submitted to Phys. Plasma

Formation et dynamique de nanoparticules dans un plasma complexe (poussiéreux) : de l'allumage du plasma à la phase post-décharge.

Complex (dusty) plasmas are a subject of growing interest. They areionized gases containing charged dust particles. In capacitively-coupled RF discharges, dust growth can occur naturally and two methods can be used to grow dust particles: chemically active plasmas or sputtering. The growth of dust particles in argon discharges by RF sputtering and the effect of dust particles on theplasma have been investigated from the plasma ignition to the afterglow. It was shown that plasma and discharge parameters are greatly affected by the dust particles. Furthermore, plasma instabilities can be triggered by the presence of the dust particles. These instabilities can be due to dust particle growth or they can be instabilities of a well established dust cloud filling the interelectrode space. When the discharge is switched off, the dust particles act like a sink for the charge carrier and consequently affect the plasma losses. It was shown that the dust particles do keep residual chargeswhich values are greatly affected by the diffusion of the charge carriers and especially the transition from ambipolar to free diffusion.Les plasmas complexes (poussiéreux) sont un sujet d'intérêt croissant. Ce sont des gaz ionisés contenant des poudres chargées. Dans les décharges RF capacitives, deux méthodes peuvent être utilisées pour produire les poussières: les plasmas chimiquement actifs ou la pulvérisation. La croissance de poudres dans une décharge d'argon par pulvérisation ainsi que ses effets sur le plasma ont été étudiés depuis l'allumage de la décharge jusqu'à la phase post-décharge. Il a été montré que les paramètres du plasma et de la décharge sont grandement affectés par la présences de poudres. De plus, des instabilités du plasma peuvent être induites par la présence des poussières. Ces instabilités peuvent être dues à la croissance des poudres ou elles peuvent être des instabilités auto-excitées d'un nuage dense de poudres remplissant l'espace inter-électrode. Quand on éteint la décharge, les poudres agissent comme un piège pour les porteurs de charge et par conséquent modifient les mécanismes d'extinction du plasma. Il a aussi été montré que les poudres gardent une charge électrique résiduelle qui dépend fortement du processus de diffusion du plasma et notamment de la transition de la diffusion ambipolaire vers la diffusion libre

Stability of two-dimensional complex plasma monolayers in asymmetric capacitively-coupled radio-frequency discharges

Submitted to Phys. Rev. EIn this article, the stability of a complex plasma monolayer levitating in the sheath of the poweredelectrode of an asymmetric capacitively coupled radio-frequency argon discharge is studied. Com-pared to earlier studies, a better integration of the experimental results and theory is achieved byoperating with actual experimental control parameters such as the gas pressure and the dischargepower. It is shown that for a given microparticle monolayer at a fixed discharge power there existtwo threshold pressures: (i) above a specific pressure p cryst , the monolayer always crystallises; (ii)below a specific pressure p MCI , the crystalline monolayer undergoes the mode-coupling instability andthe two-dimensional complex plasma crystal melts. In-between p MCI and p cryst , the microparticlemonolayer can be either in the fluid phase or the crystal phase: when increasing the pressure frombelow p MCI , the monolayer remains in the fluid phase until it reaches p cryst at which it recrystallises;when decreasing the pressure from above p cryst , the monolayer remains in the crystalline phase untilit reaches p MCI at which the mode-coupling instability is triggered and the crystal melts. A simpleself-consistent sheath model is used to calculate the rf sheath profile, the microparticle charges and themicroparticle resonance frequency as a function of power and background argon pressure. Combinedwith calculation of the lattice modes the main trends of p MCI as a function of power and backgroundargon pressure are recovered. The threshold of the mode-coupling instability in the crystalline phaseis dominated by the crossing of the longitudinal in-plane lattice mode and the out-of plane latticemode induced by the change of the sheath profile. Ion wakes are shown to have a significant effecttoo

Tracking and Linking of Microparticle Trajectories During Mode-Coupling Induced Melting in a Two-Dimensional Complex Plasma Crystal

In this article, a strategy to track microparticles and link their trajectories adapted to the study of the melting of a quasi two-dimensional complex plasma crystal induced by the mode-coupling instability is presented. Because of the three-dimensional nature of the microparticle motions and the inhomogeneities of the illuminating laser light sheet, the scattered light intensity can change significantly between two frames, making the detection of the microparticles and the linking of their trajectories quite challenging. Thanks to a two-pass noise removal process based on Gaussian blurring of the original frames using two different kernel widths, the signal-to-noise ratio was increased to a level that allowed a better intensity thresholding of different regions of the images and, therefore, the tracking of the poorly illuminated microparticles. Then, by predicting the positions of the microparticles based on their previous positions, long particle trajectories could be reconstructed, allowing accurate measurement of the evolution of the microparticle energies and the evolution of the monolayer properties

Caractérisation in-situ de la dynamique de croissance d'un nuage de nanoparticules de tungstène dans une décharge luminescente

National audienceCette communication présente une étude de la dynamique de croissance, au sein d'une décharge luminescente, d'un nuage de nano-poudres de tungstène formées par pulvérisation sous argon d'une cathode de tungstène. Pour cela, la distribution en taille ainsi que la concentration en nombre des nano-poudres de tungstène a été analysée in-situ par spectroscopie d'extinction pour différentes positions verticales entre les électrodes; donnant ainsi accès à leur répartition spatiale. En parallèle, la dynamique du nuage de nano-poudres a été étudiée via une technique de tomographie laser ; tandis que la forme, la composition et la taille de nano-poudres collectées en bas de l'enceinte plasma ont été analysées ex-situ par microscopie électronique et spectroscopie Raman. Il se dégage de ces différentes analyses qu'après allumage de la décharge, le nuage nano-particulaire est rapidement le siège d'un phénomène d'agglomération, très probablement de nanocristallites. Pendant cette phase de croissance, le nuage se rapproche de l'anode (en bas d'enceinte plasma) et les bords de la décharge plasma, permettant à une nouvelle génération de nano-poudres de croître dans l'espace laissé vacant, et ainsi de suite..

Hydro-carbon material design in a capacitively coupled radio-frequency discharge

International audienc

Dust density influence on complex plasma decay

Copyright 2008 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in AIP Conference Proceedings and may be found at http://link.aip.org/link/?APCPCS/1041/185/1International audienceIn this paper, the influence of dust particles on the plasma losses in a complex plasma afterglow is studied. It is shown that the dust particles can drastically shorten the plasma loss time by absorption-recombination onto their surfaces. The dust particle absorption frequency increases with the dust density but the dependence is not linear for high dust density. Finally, the possible use of dust absorption frequency measurements as a diagnostics for complex plasmas is mentioned and supported by comparison to existing experimental data