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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Caractérisation d'une décharge micro-onde pulsée dans le mélange CH4-H2 en vue de son optimisation pour la synthèse de films de diamant

Texte intégral accessible uniquement aux membres de l'Université de LorraineOwing to its very good physico-chemical properties, diamond is an excellent candidate for applications in optics and in electronics intended to work in hard conditions (high power, high frequençy and high temperature). These applications need high purity diamond to be synthetize. As a consequence, many research works showed that Microwave Plasma Assisted Chemical VapourDeposition (MWPACVD) is the most suitable process for diamond films deposition. The original aspect of this work concerns the study of the influence of pulsed microwave dis charge parameters - especially the power pulse duration and repetition rate- on the plasma reactivity and the diamond layer properties. Sample characterization was carried out by means of Scanning Electron Microscopy and Raman Spectroscopy. These analyses allowed us to show that an optimum indiamond film quality is reached, without lowering the film growth rate, when both the microwave pulse duration and the afterglow are fixed at 1 ms. The plasma diagnostic was perfomed by using Time Resolved Optical Emission Spectroscopy (TROES), Laser Induced Fluorescence (LIF) and the "double pulse technique" that allows the ground state of species to be re-excited during the afterglow. The influence of the time parameters of the plasma on the gas phase composition wascorrelated to the modification in the diamond film properties. The time and space resolved study of the main reactive species shows that a gooq adjustment of the power pulse duration and repetition rate allows the concentration of these species to be controlled. Furthermore, we point out that the main processes which govern such pulsed plasma are thermal effects (heating and cooling of theneutral gas) and diffusion to the walls. These points make necessary the time and space resolved plasma characterization.Le diamant possédant des propriétés physico-chimiques exceptionnelles, il est un excellent candidat pour des applications en optique et en électronique destinées à travailler dans des conditions extrêmes (haute puissance, haute fréquence et haute température). Ces applications nécessitant la synthèse de films de diamant d'une très grande pureté, de nombreuses investigations ont permis de montrer que le meilleur procédé de synthèse est le dépôt chimique en phase vapeurassisté par plasma micro-onde (MWPA CVD). L'originalité de ce travail est l'étude de l'effet des décharges micro-onde pulsées (temps de décharge et temps de post-décharge) sur la réactivité du plasma et les propriétés des films de diamant. Afin de mener,cette étude, l'analyse des échantillons a été réalisée par microscopie électronique à balayage (SEM) et spectroscopie Raman. L'analysedes films de diamant déposés a montré qu'un optimum de qualité est atteint sans perte de vitesse de croissance, pour un temps de décharge et un temps de post-décharge égaux et valant 1 ms. En ce qui concerne la caractérisation du plasma, nous avons utilisé la fluorescence induite par laser (LIF) à deux photons, la spectroscopie optique d'émission résolue dans le temps (TROES), l'actinométrie et une technique (appelée technique du "double pulse") qui permet de réexciter lesatomes (ou molécules) en post-décharge. La caractérisation des plasmas a permis de corréler l'influence des temps de décharge et de post-décharge sur la composition de la phase gazeuse et les dépôts. L'étude spatio-temporelle des principales espèces réactives montre que le choix des temps de décharge et de post-décharge nous permet de contrôler leurs concentrations. De plus, nousavons mis en évidence que les plasmas pulsés nécessitent une caractérisation spatio-temporelle car les principaux mécanismes gouvernant ce type de plasmas sont la diffusion des espèces (neutres et chargées) et le chauffage ou le refroidissement du gaz

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

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

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

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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