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.

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.

<p>Theoretical studies of dusty plasmas have been performed by focusing attention principally on collective phenomena and on grain motion. This thesis consists of a collection of seven published papers that explore both the collective behavior of a complex plasma system as well as the dynamics of grains in plasmas. In paper 1, a mechanism that explains the energy gain which leads to the self excited grain oscillations is theoretically formulated. The newly developed mechanism explains the observed self excited oscillations through the coupling of plasma sheath fluctuations with the electrostatic force, which holds the dust grain. In paper 2, theoretical and simulation studies have been conducted to study the vertical oscillations of dust grains that are levitated in plasma sheaths, under low pressure conditions. The oscillations were driven either by an external force or by a plasma number density modulation. The proposed model gives a full picture of the dust grains dynamics and is capable of successfully explaining the experimental observations. Paper 3 explores both theoretically and numerically the origin of the nonlinearities that lead to the observed oscillation resonances. The feature of the confining potential well which traps the grain, the influence of an electrode voltage modulation on the trapping well, and hence on the grain dynamics, and the resulting nonlinear resonances are analyzed in detail. The numerical simulations presented successfully reproduce a broad range of dynamical phenomena, including the self excited oscillations, for a range of different parameters. Paper 4 is dedicated to the analysis of the propagation of Dust Acoustic Waves (DAW) in a medium with an equilibrium dust density distribution. It has been theoretically shown that only some harmonics of the dust density distribution will influence the propagation of the DAW, thus modifying its frequency. Paper 5 presents a theoretical and numerical analysis of the excitation of higher harmonics of electrostatic dust cyclotron waves. The instability is driven by the ion and electron currents flowing along the magnetic field. The dispersion relation and the wave instability conditions have been derived, and a detailed numerical analysis has been performed. In Paper 6, we explore theoretically some cross field instabilities of low frequency, long wavelength electrostatic modes in fully and weakly ionized plasmas. It is shown that in a magnetoplasma with a transverse equilibrium dc electric field, the energy associated with the cross field motion of the plasma particles can be coupled to low frequency electrostatic waves. Paper 7 explores the properties and instabilities of low frequency electrostatic waves propagating in a current carrying magnetoplasma with equilibrium density and field aligned ion flow with a transverse gradient. The paper contains previous results as limiting cases, together with additional instabilities related to the equilibrium plasma density distribution. </p>

Theoretical studies of dusty plasmas have been performed by focusing attention principally on collective phenomena and on grain motion. This thesis consists of a collection of seven published papers that explore both the collective behavior of a complex plasma system as well as the dynamics of grains in plasmas. In paper 1, a mechanism that explains the energy gain which leads to the self excited grain oscillations is theoretically formulated. The newly developed mechanism explains the observed self excited oscillations through the coupling of plasma sheath fluctuations with the electrostatic force, which holds the dust grain. In paper 2, theoretical and simulation studies have been conducted to study the vertical oscillations of dust grains that are levitated in plasma sheaths, under low pressure conditions. The oscillations were driven either by an external force or by a plasma number density modulation. The proposed model gives a full picture of the dust grains dynamics and is capable of successfully explaining the experimental observations. Paper 3 explores both theoretically and numerically the origin of the nonlinearities that lead to the observed oscillation resonances. The feature of the confining potential well which traps the grain, the influence of an electrode voltage modulation on the trapping well, and hence on the grain dynamics, and the resulting nonlinear resonances are analyzed in detail. The numerical simulations presented successfully reproduce a broad range of dynamical phenomena, including the self excited oscillations, for a range of different parameters. Paper 4 is dedicated to the analysis of the propagation of Dust Acoustic Waves (DAW) in a medium with an equilibrium dust density distribution. It has been theoretically shown that only some harmonics of the dust density distribution will influence the propagation of the DAW, thus modifying its frequency. Paper 5 presents a theoretical and numerical analysis of the excitation of higher harmonics of electrostatic dust cyclotron waves. The instability is driven by the ion and electron currents flowing along the magnetic field. The dispersion relation and the wave instability conditions have been derived, and a detailed numerical analysis has been performed. In Paper 6, we explore theoretically some cross field instabilities of low frequency, long wavelength electrostatic modes in fully and weakly ionized plasmas. It is shown that in a magnetoplasma with a transverse equilibrium dc electric field, the energy associated with the cross field motion of the plasma particles can be coupled to low frequency electrostatic waves. Paper 7 explores the properties and instabilities of low frequency electrostatic waves propagating in a current carrying magnetoplasma with equilibrium density and field aligned ion flow with a transverse gradient. The paper contains previous results as limiting cases, together with additional instabilities related to the equilibrium plasma density distribution.