Thesis (Ph. D.)--University of Missouri-Columbia, 1998. / Typescript. Vita. Includes bibliographical references (leaves 216-224). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1998. / Typescript. Vita. Includes bibliographical references (leaves 216-224). Also available on the Internet.

A côté des solides, des liquides et des gaz, le plasma est le quatrième état de la matière. Il est généré en ionisant un gaz. Dans l’univers, 99% de la matière est à l’état de plasma. L’émergence de plusieurs types de plasmas artificiels est due aux multiples et différentes applications, très intéressantes les unes que les autres, des plasmas dans des secteurs aussi variés que l’industrie, l’énergie, le biomédical et la science. Très souvent, des particules solides peuvent se former dans les plasmas. Ceci a tout particulièrement été observé dans ceux utilisés dans l’industrie. La compréhension des mécanismes de leur nucléation et croissance est d’une importance capitale en vue de trouver des solutions pour inhiber leur formation ou d’éviter qu’elles ne se déposent sur les surfaces en cours de traitement. L’objectif du travail de recherche entrant dans le cadre de cette thèse est l’étude de la formation de particules dans un plasma généré par décharge électrique continue. Il a été observé que ce phénomène dépend du flux de gaz neutre injecté dans l’enceinte du réacteur. Nous avons mis en évidence que la fréquence de formation est liée à ce paramètre. Les observations enregistrées à l’aide d’un spectroscope sont complétées et corrélées aux photographies obtenues par microscopie électronique à balayage. L’injection de particules dans le plasma plutôt que de procéder à les faire croitre présente certains avantages. On peut contrôler leur taille y compris du cas de mélanges de particules. On tout particulièrement injecter des particules ayant des tailles beaucoup plus importantes que celles que l’on forme sur des gammes de durées raisonnables des plasmas utilisés. Plusieurs expériences réalisées avec des particules injectées ont mis en exergue le caractère pluridisciplinaire du milieu plasma. Afin de comprendre les interactions mutuelles entre particules il est crucial de déterminer la charge portée par les particules. Plusieurs expériences réalisées au cours de cette thèse et présentées ici ont porté sur la détermination de ce paramètre fondamental. Dans ce cadre une série d’expérience a été réalisée à bord de la Station Spatiale Internationale (ISS) dans le but de déterminer la charge résiduelle des particules dans la phase de post-décharge. Dans la dernière partie, seront présentées et discutées des expériences portant sur l’observation de la rotation de clusters de particules soumis à un confinement supplémentaire. On montre que les particules tendent à former des alignements verticaux dus au faible champ qui se forme en aval de chaque particule. Enfin, les connaissances acquises sur les possibilités de moduler le potentiel d’interaction par l’intermédiaire d’un champ électrique seront discutées. Les résultats sont comparés aux prédictions des simulations. / Plasmas are next to the solid, liquid and gaseous phase the fourth state of matter. It is established by ionizing a gas. About 99% of the visible matter in the universe is in the state of plasma. The industrial, medical and scientific benefits of plasmas led to a variety of artificially produced plasmas. In plasmas dust particles can grow. Especially in industrial plasmas particle formation in the plasma gas phase is very common. The fundamental understanding of the growth is of vital importance in order to suppress undesired particle formation or to deposit particles and films in a certain region. In terms of this thesis the particle growth in a direct current (DC) discharge by using acetylene will be discussed. It has been observed that the particle growth depends on the neutral gas flow fed into the plasma chamber. Depending on the applied flow different growth frequencies and transport phenomena are shown. The observations recorded by a spectrometer will then be complimented by pictures from the particles taken by a scanning electron microscope. Introducing artificial particles into a plasma rather than growing them there yields several advantages. The particle sizes can be controlled, including the possibility of particle mixtures. Furthermore, particles with bigger diameter can be introduced than what can be grown on reasonable time scales in a plasma. Several possible experiments with injected particles underline the interdisciplinary character of the plasma environment. To understand the inter particle interactions the particle charge is a crucial parameter. In this thesis several experiments determining the particle charge will be discussed. In this frame the experiments on board of the International Space Station have been performed to measure the residual charge in the particle afterglow. In the last section experiments on particle cluster rotation as observed in an additional confinement will be discussed. It will be shown that the particles tend to form vertical strings due to the wake field that forms downstream of each particle. Finally the insight gained on the possibilities of tuning of the interaction potential by electric fields will be discussed. The results are then compared to the predictions of earlier simulations.

Nesta dissertação calculamos o parâmetro de fluxo elíptico v2, em função da pseudorapidez η, do momento transversal pt e da centralidade da colisão. Encontramos que v2, em função de η, reproduz a forma característica dos dados experimentais, apresentando um máximo em η = 0 e diminuindo conforme |η| aumenta. Obtivemos, também, que v2, em função de pt, cresce de forma, aproximadamente, linear, não apresentando a saturação observada nos dados experimentais, para pt > 1.5GeV (quando usamos a fórmula de Cooper-Frye). Usando o mecanismo de emissão contínua, há uma considerável redução do fluxo elíptico, na região de pt grande. Entretanto, tal redução não caracteriza uma saturação. No caso de v2, em função da centralidade, os resultados mostram-se consistentes com os dados experimentais. Verificamos, também, que os efeitos de uma equação de estado com ponto crítico é pouco significativo para o fluxo elíptico. Por outro lado, encontramos uma forte dependência de v2 em relação ao tipo de condições iniciais adotado (condições iniciais médias ou condições iniciais flutuantes). Finalmente, propomos uma fórmula para cálculo de v2, a partir de quantidades que podem ser medidas experimentalmente. O resultado obtido com esta fórmula mostra-se consistente com v2 calculado de maneira teórica (em relação à direção do parâmetro de impacto). / In this work, we calculate the elliptic flow parameter v2, as a function of the pseudorapidity η, the transversal momentum pt and the centrality of the collision. We found that v2,as a function of η, reproduces the characteristic shape of the experimental data, with a maximum at η = 0 and decreasing as |η| increases. We aIso observed that v2, as a function of pt, linearly increases, therefore it doesn't show the saturation observed in experimental data, for pt > 1.5 GeV (when we use the Cooper-Frye prescription). Using the continuous emission mechanism, we observed a considerable reduction of the elliptical flow in the range of high pt. However, such a reduction doesn't characterize a saturation. In the case of v2, as a function of centrality, the results are consistent with the experimental data. We also verified that the effect of a equation of state with critical point is of little importance to the elliptical flow. On the other side, we found a strong dependence of v2 on the type of initial conditions used (average or fluctuating initials conditions). Finally, we propose a prescrition to calculate v2 from quantities which can be experimentally measured. The result obtained with this prescrition is consistent with the v2 theoretically computed (using the impact parameter direction).

Plasmas are next to the solid, liquid and gaseous phase the fourth state of matter. It is established by ionizing a gas. About 99% of the visible matter in the universe is in the state of plasma. The industrial, medical and scientific benefits of plasmas led to a variety of artificially produced plasmas. In plasmas dust particles can grow. Especially in industrial plasmas particle formation in the plasma gas phase is very common. The fundamental understanding of the growth is of vital importance in order to suppress undesired particle formation or to deposit particles and films in a certain region. In terms of this thesis the particle growth in a direct current (DC) discharge by using acetylene will be discussed. It has been observed that the particle growth depends on the neutral gas flow fed into the plasma chamber. Depending on the applied flow different growth frequencies and transport phenomena are shown. The observations recorded by a spectrometer will then be complimented by pictures from the particles taken by a scanning electron microscope. Introducing artificial particles into a plasma rather than growing them there yields several advantages. The particle sizes can be controlled, including the possibility of particle mixtures. Furthermore, particles with bigger diameter can be introduced than what can be grown on reasonable time scales in a plasma. Several possible experiments with injected particles underline the interdisciplinary character of the plasma environment. To understand the inter particle interactions the particle charge is a crucial parameter. In this thesis several experiments determining the particle charge will be discussed. In this frame the experiments on board of the International Space Station have been performed to measure the residual charge in the particle afterglow. In the last section experiments on particle cluster rotation as observed in an additional confinement will be discussed. It will be shown that the particles tend to form vertical strings due to the wake field that forms downstream of each particle. Finally the insight gained on the possibilities of tuning of the interaction potential by electric fields will be discussed. The results are then compared to the predictions of earlier simulations.

Nesta dissertação calculamos o parâmetro de fluxo elíptico v2, em função da pseudorapidez η, do momento transversal pt e da centralidade da colisão. Encontramos que v2, em função de η, reproduz a forma característica dos dados experimentais, apresentando um máximo em η = 0 e diminuindo conforme |η| aumenta. Obtivemos, também, que v2, em função de pt, cresce de forma, aproximadamente, linear, não apresentando a saturação observada nos dados experimentais, para pt > 1.5GeV (quando usamos a fórmula de Cooper-Frye). Usando o mecanismo de emissão contínua, há uma considerável redução do fluxo elíptico, na região de pt grande. Entretanto, tal redução não caracteriza uma saturação. No caso de v2, em função da centralidade, os resultados mostram-se consistentes com os dados experimentais. Verificamos, também, que os efeitos de uma equação de estado com ponto crítico é pouco significativo para o fluxo elíptico. Por outro lado, encontramos uma forte dependência de v2 em relação ao tipo de condições iniciais adotado (condições iniciais médias ou condições iniciais flutuantes). Finalmente, propomos uma fórmula para cálculo de v2, a partir de quantidades que podem ser medidas experimentalmente. O resultado obtido com esta fórmula mostra-se consistente com v2 calculado de maneira teórica (em relação à direção do parâmetro de impacto). / In this work, we calculate the elliptic flow parameter v2, as a function of the pseudorapidity η, the transversal momentum pt and the centrality of the collision. We found that v2,as a function of η, reproduces the characteristic shape of the experimental data, with a maximum at η = 0 and decreasing as |η| increases. We aIso observed that v2, as a function of pt, linearly increases, therefore it doesn't show the saturation observed in experimental data, for pt > 1.5 GeV (when we use the Cooper-Frye prescription). Using the continuous emission mechanism, we observed a considerable reduction of the elliptical flow in the range of high pt. However, such a reduction doesn't characterize a saturation. In the case of v2, as a function of centrality, the results are consistent with the experimental data. We also verified that the effect of a equation of state with critical point is of little importance to the elliptical flow. On the other side, we found a strong dependence of v2 on the type of initial conditions used (average or fluctuating initials conditions). Finally, we propose a prescrition to calculate v2 from quantities which can be experimentally measured. The result obtained with this prescrition is consistent with the v2 theoretically computed (using the impact parameter direction).