Determination of deposited flux and energy of sputtered tungsten atoms on every stages of transport in HiPIMS discharge

International audienceA time-resolved tunable diode-laser (DL) induced fluorescence (TR-TDLIF) technique has been used to identify different populations of atoms (on different stages of transport) to determine their corresponding deposited energy and flux. The temporal dimension permits the splitting of the processes of sputtering during the discharge and particles transport in the post-discharge where atoms and flux velocity distribution functions (AVDF, FVDF) of each population were measured varying the discharge parameters (power, voltage, pressure, and distance from target). Tungsten (W) was chosen, being an interesting case in terms of sputtered atom transport, considering its weight which implies weak changes of directivity or energy transfer after collisions with the buffer gas. The high temporal and spectral resolutions of TR-TDLIF are the keys for the distinction of the atoms populations and the stage corresponding to the transition from the ballistic to diffusive regime of transport was observed for the first time and named quasi-diffusive regime. Thus, the ability to dissociate populations of atoms and to determine their deposited flux and energy may be of great interest to adjust film properties as desired for applications

Cluster Agglomeration Induced by Dust-Density Waves in Complex Plasmas

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Etude expérimentale d'une source plasma RF à configuration hélicon dans le mélange Ar/H2 (application à la gravure chimique de surfaces graphitiques dans le cadre des interactions plasma-paroi du divertor d'ITER)

Les interactions plasma-paroi représentent l'un des principaux problèmes à résoudre pour avancer dans la recherche sur la fusion contrôlée. Ce travail de thèse a pour objectif de développer une source d'hydrogène atomique à basse pression (< Torr) dans un réacteur à configuration hélicon en mélange H2/Ar pour étudier la gravure chimique du graphite et de composites à fibres de carbone utilisés dans le tokamak Tore Supra. Selon les conditions expérimentales, le réacteur peut générer les modes de couplages capacitif, inductif, Trivelpiece-Gould et hélicon à bas champ. Leur caractérisation a montré que le mode inductif est, dans ce cas, celui présentant le plus grand intérêt pour la source d'hydrogène atomique. Les études en mode inductif ont révélé un phénomène de décroissance de la densité relative de deux niveaux métastables de l'ion Ar+ et d'un niveau métastable de l'argon neutre lors de l'augmentation du champ magnétique de confinement. Un modèle simple a confirmé que ces niveaux métastables sont détruits par collisions électroniques vers des niveaux de plus grande énergie. La gravure du graphite par la source d'hydrogène atomique est relativement efficace (jusqu'à 3 m/h) à 10 mTorr et diminue avec la pression. Une analyse qualitative de la cinétique de l'hydrogène atomique a permis de conclure que cette baisse de la vitesse de gravure est due au flux d'hydrogène atomique arrivant sur l'échantillon qui décroît lorsque la pression augmente. Les premières observations de la surface après gravure ont mis en évidence la présence de structures carbonées (agglomérats de nanoparticules et dépôts). Ces structures ressemblent à celles observées dans différents tokamaksPlasma-wall interactions are one of the main issues in fusion research and must be thoroughly studied to progress in this topic. The objective of this work is to develop an atomic hydrogen source at low pressure (< Torr) in a helicon configuration reactor working in H2/Ar gas mixture. This source is then used to study the chemical etching of graphite and carbon fiber composites composing the limiter of the Tore Supra tokamak. Depending on the experimental conditions, the RF power coupling of the reactor can be in capacitive, inductive, Trivelpiece-Gould or low field helicon mode. The characterization of these modes determined that in this case the inductive one presents the greatest interest for the atomic hydrogen source. Further studies in inductive mode showed that increasing the confinement magnetic field leads to a decrease of measured relative densities of two metastable levels of argon ion and one metastable state of neutral argon. A simple model simulating neutral metastable state behavior confirmed that these levels are destroyed by electronic collisions towards upper levels. The chemical etching of graphite exposed to the atomic hydrogen source is relatively efficient (up to 3 m/h) at 10 mTorr and drops with the pressure. A qualitative analysis of atomic hydrogen kinetics concluded that this behavior is due to the decrease of atomic hydrogen flux on the sample with increasing pressure. Finally, first observations of the etched surface underlined different structures (nanoparticles clusters and deposits). These can be compared to the ones observed in different tokamaksMETZ-SCD (574632105) / SudocNANCY1-Bib. numérique (543959902) / SudocNANCY2-Bibliotheque electronique (543959901) / SudocNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Agglomeration processes sustained by dust density waves in plasma: From injection to the formation of an organized structure

International audienceIn this paper, an experimental investigation of dust particle agglomeration in a capacitively coupled RF discharge is reported. Carbonaceous particles are produced in an argon plasma using acetylene. As soon as the particle density becomes sufficient, dust density waves (DDWs) arespontaneously excited within the cathode sheath. Recently, it was proven that DDWs can significantly enhance the agglomeration rate between particles by transferring them a significant kinetic energy. Thus, it helps them to overcome Coulomb repulsion. The influence of this mechanism is studied from acetylene injection to the formation of very large agglomerates forming an organized structure after a few dozens of seconds. For this purpose, three diagnostic tools are used: extinction measurements to probe nanometer-sized particles, fast imaging for large agglomerates and a dust extraction technique developed for ex-situ analysis

Wide variations of SiCxNy:H thin films optical constants deposited by H2/N2/Ar/hexamethyldisilazane microwave plasma

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Argon Ion and Neutral Metastable Levels Destruction in a Magnetically Enhanced Inductively Coupled Plasma Reactor

International audienceThis paper deals with pure argon plasma studied in a magnetically enhanced inductively coupled reactor. Laser induced fluorescence technique was performed with an optical parametric oscillator laser on the 3d4F7/2 and 3d2G9/2 Ar + metastable levels and on the (2P0 3/2)4s² neutral metastable state to determine their relative densities. Langmuir probe was used as a complementary diagnostic to measure the electron energy distribution functions. When increasing the confinement magnetic field, the metastable state density collapses for all cited levels, by contrast with ne which is continuously growing. Calculations were carried out for neutral argon to explain this behaviour. The results show that the metastable population is depleted by electron-impact excitation and ionization, these loss processes becoming dominant compared to the metastable state creation term

Mechanisms and dynamics of unipolar arcs in magnetized plasmas

International audiencePost-mortem analyses suggest that arcs in contact with metallic walls are the main cause of impurities and dust formation in tokamaks [1,2]. Such dust and impurities represent important operational and safety issues for next-step fusion devices like ITER and DEMO. If dipolar arcs, which occur in between adjacent Plasma Facing Components (PFCs), are well known and can in most of the cases be avoided by adapting the design of the PFCs, it is not the case for unipolar arcs taking place in between PFCs and the plasma. On addition to dust production, unipolar arcs also damage mirrors required by many diagnostics used for studying the plasmas as well as for ensuring a safe operation of the fusion device. Several models for explaining the triggering of unipolar arcs have been developed, but these models are based on assumptions which are difficult to assess experimentally, mainly due to the spatial and temporal scales involved (a few µm to <1mm and ~1ns for the triggering and few µs for the lifetime, respectively). As a result, defining the right threshold required for triggering the arcs is a tricky operation due to the uncertainties in the relative importance of the numerous input parameters which also influence the arc dynamics: surface roughness, secondary electron emission, thermionic emission, desorption of gas trapped in thick deposits, sheath modifications induced by ELMs… With the aim to provide more reliable input parameters and to assess the validity of the models, an experimental setup making possible highly resolved measurements in the linear low-beta ALINE plasma device has been developed. Stereoscopic fast camera measurements are performed and analyzed with the TRACK software, which enables a statistical investigation of the influence of various parameters on the triggering and dynamics of unipolar arcs

Mechanisms and dynamics of unipolar arcs in magnetized plasmas

International audiencePost-mortem analyses suggest that arcs in contact with metallic walls are the main cause of impurities and dust formation in tokamaks [1,2]. Such dust and impurities represent important operational and safety issues for next-step fusion devices like ITER and DEMO. If dipolar arcs, which occur in between adjacent Plasma Facing Components (PFCs), are well known and can in most of the cases be avoided by adapting the design of the PFCs, it is not the case for unipolar arcs taking place in between PFCs and the plasma. On addition to dust production, unipolar arcs also damage mirrors required by many diagnostics used for studying the plasmas as well as for ensuring a safe operation of the fusion device. Several models for explaining the triggering of unipolar arcs have been developed, but these models are based on assumptions which are difficult to assess experimentally, mainly due to the spatial and temporal scales involved (a few µm to <1mm and ~1ns for the triggering and few µs for the lifetime, respectively). As a result, defining the right threshold required for triggering the arcs is a tricky operation due to the uncertainties in the relative importance of the numerous input parameters which also influence the arc dynamics: surface roughness, secondary electron emission, thermionic emission, desorption of gas trapped in thick deposits, sheath modifications induced by ELMs… With the aim to provide more reliable input parameters and to assess the validity of the models, an experimental setup making possible highly resolved measurements in the linear low-beta ALINE plasma device has been developed. Stereoscopic fast camera measurements are performed and analyzed with the TRACK software, which enables a statistical investigation of the influence of various parameters on the triggering and dynamics of unipolar arcs

Functionalization and characterization study of MOFs by plasma process

International audienceMetal Organic Framework (MOFs), also known as porous coordination polymers (PCPs), have been intensively studied during the last decade due to their porous structure and high surface area 1-3. Enhancing the porosity and adsorption properties of these materials, especially for hydrogen storage applications is a significant challenge. Low-temperature plasma such as dielectric barrier discharge (DBD) could be used in order to functionalize these materials by grafting some functional groups such as amino groups in the organic ligands of MOFs. Two organic ligands "terephtalic acid and trimesic acid" were treated by DBD plasma in argon gas in order to analyze the physical impact of the plasma on the materials. During the treatment, optical emission spectroscopy (OES) was carried out to identify the chemical species present in the plasma. The global morphology of the organic ligands was determined by SEM. X-Ray diffraction analysis was performed to determine the crystal structures of the organic ligands before and after exposure to the DBD plasma. Subsequently, we investigated the experimental conditions in order to functionalize the materials by executing DBD discharges in Ar/N2/H2 and Ar/NH3 gas mixtures to form the desired amino groups. References: [1] M