Cécile Arnas

Reorganization of graphite surfaces into carbon micro- and nanoparticles under high flux hydrogen plasma bombardment

Fine-grain graphite samples were exposed to high density low temperature (ne∼1020u2009m−3,u2009Te∼1u2009eV) hydrogen plasmas in the Pilot-PSI linear plasma generator. Redeposition of eroded carbon is so strong that no external precursor gas injection is necessary for deposits to form on the exposed surface during the bombardment. In fact, up to 90% of carbon is redeposited, most noticeably in the region of the highest particle flux. The redeposits appear in the form of carbon microparticles of various sizes and structures. Discharge parameters influence the efficiency of the redeposition processes and the particle growth rate. Under favorable conditions, the growth rate reaches 0.15u2009μm/s. The authors used high resolution scanning electron microscopy and transmission electron microscopy to study the particle growth mode. The columnar structure of some of the large particles points toward surface growth, while observation of the spherical carbon nanoparticles indicates growth in the plasma phase. Multiple nanoparticles...

Growth of tungsten nanoparticles in direct-current argon glow discharges

The growth of nanoparticles from the sputtering of a tungsten cathode in DC argon glow discharges is reported. The study was performed at fixed argon pressure and constant discharge current. The growth by successive agglomerations is evidenced. First, tungsten nanocrystallites agglomerate into primary particles, the most probable size of which being ∼30u2009nm. Primary particles of this size are observed for all plasma durations and always remain the most numerous in the discharge. Primary particles quickly agglomerate to form particles with size up to ∼150u2009nm. For short plasma duration, log-normal functions describe accurately the dust particle size distributions. On the contrary, for long discharge durations, a second hump appears in the distributions toward large particle sizes. In the meantime, the discharge voltage, electron density, and emission line intensities strongly evolve. Their evolutions can be divided in four separate phases and exhibit unusual distinctive features compared to earlier observati...

Nanoparticle formation and dusty plasma effects in DC sputtering discharge with graphite cathode

We developed a model for the nucleation, growth and transport of carbon dust particles in a DC discharge. The carbon source comes from the sputtering of a graphite cathode resulting in the production of primary clusters and then of nanoparticles. We consider the ionic cluster growth as well as the particle growth and charging and the influence of both on the discharge equilibrium. We found that the discharge becomes electronegative for long duration when particle density reaches 10 9 cm-3 and particle size 45 nm. The corresponding transition modifies the electric field profile in the vicinity of the field reversal region in the negative glow. We then analyze the space and time evolution of the different discharge characteristics and the mechanisms involved in the discharge. We showed that particle density is governed by nucleation, coagulation and transport, while particle size is mainly governed by the deposition of the small neural clusters emitted at the cathode on the particle surface

In-situ characterisation of the dynamics of a growing dust particle cloud in a direct-current argon glow discharge

The growth and the dynamics of a tungsten nanoparticle cloud were investigated in a direct-current low pressure argon glow discharge. Real-time analyses of the dust particle size and number concentration were performed in-situ by light extinction spectrometry, while spatial dynamics of the cloud was investigated with the laser light-sheet scattering method. Additional off-line electron microscopy and Raman spectroscopy measurements were also performed for comparison purpose. This experimental work reveals the existence of an agglomeration phase followed by the appearance of a new dust particle generation. While growing, the dust cloud is pushed towards the anode and the discharge edge. Afterwards, a new dust particle generation can grow in the space freed by the first generation of nanoparticles. The continuous growth, below the light extinction spectrometry scanning positions, explains the apparent dissimilarities observed between the in-line optical and the off-line electron microscopy analyses.

A review on Light Extinction Spectrometry as a diagnostic for dust particle characterization in dusty plasmas

In this article, a detailed description of the light extinction spectrometry diagnostics is given. It allows the direct in-situ measurement of the particle size distribution and absolute concentration of a dust cloud levitating in plasmas. Using a relatively simple and compact experimental setup , the dust cloud parameters can be recovered with a good accuracy making minimum assumptions on their physical properties. Special emphasises are given to the inversion of light extinction spectra and all the required particle shape, refractive index and the light extinction models. The parameter range and the limitations of the diagnostic are discussed. In addition, two examples of measurements in low-pressure gas discharges are presented: in a DC glow discharge in which nanopar-ticles are growing from the sputtering of a tungsten cathode, and in an Argon-Silane radio-frequency discharge.

Detrapping of tungsten nanoparticles in a direct-current argon glow discharge

Nanoparticles are grown from the sputtering of a tungsten cathode in a direct current argon glow discharge. Laser light scattering of a vertical laser sheet going through the plasma reveals that the dust particle cloud is compressed and pushed towards the anode during the discharge. Scanning electron microscopy images of substrates exposed to the plasma for given durations show that dust particles are continuously falling down on the anode during the discharge. These observations are explained by the fact that the electrostatic force at the negative glow-anode sheath boundary cannot balance the ion drag, gravity, and thermophoresis forces for particles of more than a few tens of nanometres in diameter.

Nanoparticles in direct-current discharges: Growth and electrostatic coupling

The formation of nanoparticles from the sputtering of graphite and tungsten cathodes in direct-current discharges is investigated. The successive phases of growth present specificities according to the cathode material. The evolution of the discharge and plasma parameters during the growth phases accounts for the nanoparticle-plasma electrostatic coupling. This evolution also presents strong differences as a function of the cathode material. Features characterising each case are discussed.

Magnetized electron emission from a small spherical dust grain in fusion related plasmas

The effect of magnetic field on the electron emission yield from a small spherical dust grain immersed in a plasma is investigated. It is demonstrated that, due to their gyromotion, some electrons can be promptly recollected on the grains surface, which can reduce the emitted electron flux. The consequences are discussed mainly in the context of fusion-related applications, where the dust floating potential can be significantly reduced when positive while the dust lifetime remains weakly affected. In particular, we suggest that this effect should be included in the codes used to model the dust transport in tokamaks.

Electron collection and thermionic emission from a spherical dust grain in the space-charge limited regime

The collection and emission of electrons from a spherical body in the Space-Charge Limited (SCL) regime are investigated. When a Virtual Cathode (VC) in the potential profile around the body is present, the barrier in the effective potential energy of electrons is assumed to be located near the position of the minimum of the VC potential, for both collected and emitted electrons. This assumption is confirmed to be reasonable in the case of a double Yukawa potential profile and allows the SCL cross-section for electron collection and the emitted electron’s trapped-passing boundary to be written in a simple way. An expression for the collection current for Maxwellian electrons is derived and is shown to recover the classical Orbital Motion Limited (OML) theory when the VC vanishes. Using the same assumptions, an expression for the thermionic emission current in the SCL regime is also obtained and comparisons with the OML+ theory are made. Finally, an expression for the dust electric charge in the SCL regime...

Computational Prediction of Rate Constants for Reactions Involved in Al Clustering

Aluminum (Al) clustering processes via three types of association reactions are herein studied using classical molecular dynamics trajectory calculations. The simulations were carried out under realistic experimental conditions. The dependence of rate constants on temperature and cluster size was obtained. The association reactions have a very small activation barrier, and the activation energy increases with increasing temperature. Our prediction of reaction rate constants can be of interest for the study of Al nanoparticle growth using kinetic models.