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International audience; Arcs between carbon electrodes present some specific differences compared with metallic arcs. The arc voltage is higher, but does not attain a stable value displaying large fluctuations. Indeed, the arcs are produced by the direct sublimation of the electrodes, without passing through a molten phase. The arc production is also facilitated by both circuit breaking and electric field breakdown. In this paper, arcing has been examined under various conditions (voltage, current, and opening or fixed interelectrode gap) and particular attention has been given to the ability of the system to sustain the arc due to reignition. Smallangle X-ray scattering has been used to examine the formation of particles within carbon arcs and again the results are different from what is found with metallic electrodes, the particles being larger with rougher surfaces. The ultimate aim of this paper was to gain new knowledge concerning these arcs and their consequences for electrical safety (arc faults in wires).

In dusty plasmas, determining the three-dimensional particle positions and trajectories of individual particles is often required. This paper benchmarks two approaches capable of reconstructing the trajectories of particles in three dimensions. The influences of the particle number, the particle number density, and the orientation of the individual cameras are studied. Additionally, the demands on the desired image quality, required for these algorithms, are discussed. The reader is given practical information for the appropriate reconstruction approach and camera positioning that should/could be used in a specific application.

International audience; This paper deals with the formation of nanoparticles in a pulsed discharge. Experiments are performed in a capacitively coupled discharge operated in a mixture of argon and acetylene. The paper focuses especially on the influence of the pulse frequency on the dust formation. The experiments reveal the existence of a rather narrow frequency band that separates a frequency region with no dust formation from a frequency region where dust formation occurs. The decisive point in the observations is that a small change in the pulse frequency (from 700 to 725 Hz) is enough to induce or, respectively, suppress the formation of dust particles. The experimental results are discussed by means of a simple model that allows one to calculate the density of negative ions (C2H−, C4H−, etc.) as a function of the pulse frequency.

In this letter, we present scanning electron microscopy (SEM) results that confirm in a direct way our earlier explanation of an abrupt coagulation event as the cause for the void hiccup. In a recent paper, we reported on the fast and interrupted expansion of voids in a reactive dusty argon–acetylene plasma. The voids appeared one after the other, each showing a peculiar, though reproducible, behavior of successive periods of fast expansion, abrupt contraction, and continued expansion. The abrupt contraction was termed “hiccup” and was related to collective coagulation of a new generation of nanoparticles growing in the void using relatively indirect methods: electron density measurements and optical emission spectroscopy. In this letter, we present conclusive evidence using SEM of particles collected at different moments in time spanning several growth cycles, which enables us to follow the nanoparticle formation process in great detail.

International audience; In a capacitively-coupled radio-frequency (RF) discharge (13.56 MHz) in CH4, dust-particles are generated at low pressure (120 Pa). The gas is injected in the chamber with a flow rate of 5.6 sccm (9.45.10-3 Pa.m3.s-1). The plasma is generated between two parallel plane aluminum electrodes: the upper one is powered (80 W) by the RF generator and the lower one is grounded. An impedancemeter follows the evolution of the plasma electrical parameters. In our conditions, the dissociation of CH4 leads to the formation of dust-particles in the plasma bulk, and to the growth of a coating on the powered electrode. The dust-particles are trapped at the sheath boundary because of forces exerted on them, leading to the formation of dust-particle clouds parallel to the electrodes. When the dust-particles become too big and too heavy, they fall onto the grounded electrode, so they can be collected.The purpose of this experimental study is to understand how the nature of the powered electrode can influence the plasma parameters and the dust-particles generation. First of all, the electrodes are cleaned with ethanol and sand paper. Then, thin plates of different materials are cleaned with ethanol and taped on the powered electrode with conductive tape. 60% of the powered electrode is covered with the plate. After this, the pressure is lowered and the chamber is filled with CH4, and the plasma is turned on. After 15 minutes, the plasma is turned off. The dust-particles generated and the plates are collected in order to observe them by Scanning Electron Microscopy. The morphology of the coating formed on the powered electrode differs in function of the material of the plate. The diameter of the dust-particles and the electrical parameters of the discharge are also affected by the nature of the plate.Moreover, less dust-particles are generated when an insulator plate is taped on the powered electrode, compared to when a conductive plate is taped.

International audience; The methane decomposition in a planar RF discharge (13.56 MHz) leads both to a dust-particle generation in the plasma bulk and to a coating growth on the electrodes. Growing dust-particles fall onto the grounded electrode when they are too heavy. Thus, at the end of the experiment, the grounded electrode is covered by a coating and by fallen dust-particles. During the dust-particle growth, the negative DC self-bias voltage (VDC) increases because fewer electrons reach the RF electrode, leading to a more resistive plasma and to changes in the plasma chemical composition. In this paper, the cleanliness influence of the RF electrode on the dust-particle growth, on the plasma characteristics and composition is investigated. A cleanliness electrode is an electrode without coating and dust-particles on its surface at the beginning of the experiment.

In this paper, the nonlinear dust acoustic waves (DAW) in a magnetized dusty plasmas with different dust grains are analytically investigated. New analytical solutions of the governing equation for this system have been obtained for the first time. The exact mathematical expressions of the nonlinear dust waves have been canvassed for the general case in magnetized dusty plasma containing different dust particles.

Oral; International audience; In this paper, we report on a particularly impressive feature of dusty plasma instabilities where moving bright plasma spots (plasmoids) appear in between the electrodes of a low-pressure capacitively-coupled rf discharge (ccrf). These plasmoids show complex behaviors like mutual interactions consisting in their merging or splitting. The interest of this study is beyond the field of dusty plasmas as well-delimited regions of enhanced emission are observed in many different kinds of plasmas (for example at atmospheric pressure in dielectric barrier discharges (DBDs).

Oral; When dust particles are present in a plasma, they acquire a negative charge by capturing electrons. When the dust particle density is high, this huge loss of electrons can trigger unstable behaviors in the plasma like dust particle growth instabilities (DPGI). These experiments are performed in a capacitively coupled rf discharge in the PKE-Nefedov reactor where dust particles are grown by sputtering a polymer layer previously deposited on the electrode. A high-speed camera (till 10000 frames per second) is used to study more precisely these instabilities. During DPGI, a new phenomenon is detected in front of each electrode (top and bottom) and localized on their periphery. It consists of the appearance of small plasma spheroids, of about a few mm and with a slightly enhanced luminosity. In this paper, two cases are presented which reveal the existence of several regimes for the spheroid behavior: spheroid rotation around the electrodes, a chaotic regime consisting in the fast appearance and disappearance of the spheroids, and a regime without any spheroid. A correlation is performed between these regions and the evolution of the plasma glow in between the electrodes.

4 pages Best student paper award; International audience; Numerical simulations using a 1D self-consistent plasma-aerosol model were compared to experimental measurements of a dusty argon-silane RF plasma with corresponding operating conditions. Experimental measurements were made of the spatiotemporal profiles of the intensities of laser light scattering and plasma emission. Qualitative agreement was found for several major aspects of the evolution of the emission and scattering intensity profiles.

This paper reports the modelling of CCP‐RF discharges (13.56 MHz) in pure nitrogen, produced within a cylindrical parallel‐plate reactor, similar to a GEC reference cell surrounded by a lateral grounded grid, at 0.1–2 mbar pressures and 10–50 W coupled powers. This study is a first step in simulating Titan’s chemistry at laboratory scale, using the PAMPRE experiment. Modelling results are compared with experimental measurements of the average electron density, and the self‐bias potential at the polarized electrode.