The Cassini spacecraft orbits Saturn since 2004, carrying a multitude of instruments for studies of the plasma environment around the planet as well as the constituents of the ring system. Of particular interest to the present thesis is the large E ring, which consists mainly of water ice grains, smaller than a few micrometres, referred to as dust. The first part of the work presented here is concerned with the interaction between, on the one hand, the plasma and, on the other hand, the dust, the spacecraft and the Langmuir probe carried by the spacecraft. In Paper I, dust densities along the trajectory of Cassini, as it passes through the ring, are inferred from measured electron and ion densities. In Paper II, the situation where a Langmuir probe is located in the potential well of a spacecraft is considered. The importance of knowing the potential structure around the spacecraft and probe is emphasised and its effect on the probe's current-voltage characteristic is illustrated with a simple analytical model. In Paper III, particle-in-cell simulations are employed to study the potential and density profiles around the Cassini as it travels through the plasma at the orbit of the moon Enceladus. The latter part of the work concerns large-scale currents and convection patterns. In Paper IV, the effects of charged E-ring dust moving across the magnetic field is studied, for example in terms of what field-aligned currents it sets up, which compared to corresponding plasma currents. In Paper V, a model for the convection of the magnetospheric plasma is proposed that recreates the co-rotating density asymmetry of the plasma. / QC 20120507

No description available.

Recently, experimental observations have shown that Polar Mesospheric Summer Echoes PMSE may be modulated by radio wave heating the irregularity source region with a ground-based ionospheric heating facilities. It is clear from these past investigations that the temporal behavior of PMSE during ionospheric heating shows promise as a diagnostic for the associated dust layer. To investigate the temporal behavior of plasma irregularities thought to produce PMSE, this work describes a new model that incorporates both finite diffusion time effects as well as dust charging. The hybrid model utilizes fluid ions described by continuity and momentum equations, electrons whose behavior is determined from quasi-neutrality, and charged dust described by the standard Particle-In-Cell PIC method. The model has been used to investigate the temporal behavior of charged dust associated electron irregularities during electron temperature enhancement associated with radio wave heating. The model predicts that the temporal behavior of the irregularities depends on the ratio of the electron-ion ambipolar diffusion time to the dust particle charging time Td/Tc. The results indicate that typically for Td/Tc << 1, an enhancement in electron irregularity amplitude occurs for a period after turn-off of the radio wave heating. The work also predicts that for Td/Tc >> 1, an enhancement in electron irregularity amplitude occurs for a time period after the turn-on of the radio wave heating. Due to the dependence of Td on irregularity scale-size, these results have important implications for observations of PMSE modification at different radar frequencies. Both continuous and discrete charging model were embedded into this computational model, the results were compared and analyzed. It is evident that significant diagnostic information may be available about the dust layer from the temporal behavior of the electron irregularities during the heating process which modifies the background electron temperature. Particularly interesting and important periods of the temporal behavior are during the turn-on and turn-off of the radio wave heating. Although a number of past theoretical and experimental investigations have considered both these on and off period, this dissertation considers further possibilities for diagnostic information available as well as the underlying physical processes. Approximate analytical models are developed and compared to a more accurate full computational model as a reference. Then from the temporal behavior of the electron irregularities during the turn-on and turn-off of the radio wave heating, the analytical models are used to obtain possible diagnostic information for various charged dust and background plasma quantities. Finally, two experiment campaigns have been performed at HAARP, Gakona, Alaska. Preliminary observation results look promising for the existence of PMSE turn-on overshoot. However, more careful experiments need to be done before firm conclusions can be drawn. The new designed Echotek digital receiver is ready for use now. It will be much superior to the experimental setup used for measurements in the previous campaign.Therefore, future experimental campaigns are planning next year to support the theoretical research. / Ph. D.

Dusty plasmas, defined as plasmas of ions, electrons, neutrals, and charged micron to sub-micron dust particles, support a rich diversity of physical states. These states (ranging from solids to liquids to gas) are determined by the ratio of the Coulomb potential energy between dust particles to the particles kinetic energy and allow for a broad range of phenomena, from crystallization to dust acoustic waves. Due to various dusty plasma interactions, dust acoustic waves can be nonlinear and exhibit various wave phenomena, from topological wave defects to shock waves to structure formations. In this thesis, I investigate a spectrum of plasma and wave interactions in liquid-like dusty plasmas and focus on a range of dust acoustic wave phenomena observed experimentally in a dc discharge dusty plasma. By developing various experimental techniques, dust acoustic wave diffraction and topological wave defects, dust acoustic shock waves, temporal dust acoustic wave growth, and structure forming dust acoustic waves were observed. I begin in Chapter 2 with the diffraction of dust acoustic waves, which I investigated by introducing a glass rod into the dusty plasma. The resulting diffraction pattern was compared to acoustic wave diffraction in a neutral gas. In addition to the diffraction pattern, topological wave defects were observed to form. I continue Chapter 2 with a preliminary investigation into topological wave defects in dust acoustic waves. Chapter 3 follows with three nonlinear dust acoustic wave experiments. I created a shock tube like profile for dust acoustic waves using a single slit. The shock-tube like potential resulted in two sets of nonlinear dust acoustic waves, coalescing high and low amplitude waves and dust acoustic waves that developed into dust acoustic shock waves. The self-excited dust acoustic shock waves were compared to theoretical models. The third nonlinear dust acoustic wave phenomenon that I investigated was a reverse drift mode that appears in high amplitude dust acoustic waves. I propose a wave process based on dust particle dynamics in high amplitude dust acoustic waves to explain the observations. In Chapter 4, I describe an experimental technique that I developed to create a quiescent drifting dusty plasma. The drifting dusty plasma was used to observe dust acoustic wave growth from thermal density fluctuations. The observed growth rate and frequency were compared kinetic and fluid models. In Chapter 5, I describe experimental observations of a structure forming instability in dusty plasmas. By increasing the discharge current, transient and aperiodic dust density striations formed. I characterized the transient and stationary modes and compared the stationary mode to an ionization/ion-drag instability and a polarization instability.

Brownian dynamics (BD) simulation method has been widely used for studying problems in dispersed systems, such as polymer solutions, colloidal suspensions and more recently, complex (dusty) plasmas. The main problem addressed with this simulation technique is that of time scale separation, which occurs when one form of motion in the system is much faster than the other. This can be a serious problem in Molecular dynamics (MD) simulation where very short time steps are needed to handle the fast motions and thus, requiring very long time runs for the proper evolution of slower modes making the simulation very expensive. More importantly, the fast motions may not be of much interest within themselves, as will be the case in a dusty plasma. The motion of neutral atoms or molecules comprising the plasma occurs at a very fast time scale with respect to the motion of dust particles, and is usually of very little interest, though a large number of such neutrals are present. In such cases, an approximate method is usually adopted, whereby the neutral particles are omitted from the simulation and their effect upon the dynamics of dust particles modeled by a combination of random forces and frictional terms. This leads to a recasting of the Newton's Equation of motion solved in MD, to a Langevin equation, solved in BD. Adopting this approach, we simulate a system of charged dust particles interacting via Yukawa potential in a 2-Dimensional layer, and extract relevant equilibrium statistical features such as the radial distribution function, static structure factor and the low frequency dust wave modes. We then propose the use of a Generalized Hydrodynamical (GH) approach to provide a semi-analytical model for the dust collective modes, which not only provides us with good predictions of the wave dispersion but also provides reasonable estimates for wave-number dependent wave damping, both of which will be compared against the results obtained from BD simulation. Finally, through our simulations, we also observe the equilibrium configuration of dust particles in the presence of cold ions streaming perpendicularly into the 2-Dimensional layer of dust particles. This provides us with novel results in the regime of sub-sonic ion flow speeds.

Brownian dynamics (BD) simulation method has been widely used for studying problems in dispersed systems, such as polymer solutions, colloidal suspensions and more recently, complex (dusty) plasmas. The main problem addressed with this simulation technique is that of time scale separation, which occurs when one form of motion in the system is much faster than the other. This can be a serious problem in Molecular dynamics (MD) simulation where very short time steps are needed to handle the fast motions and thus, requiring very long time runs for the proper evolution of slower modes making the simulation very expensive. More importantly, the fast motions may not be of much interest within themselves, as will be the case in a dusty plasma. The motion of neutral atoms or molecules comprising the plasma occurs at a very fast time scale with respect to the motion of dust particles, and is usually of very little interest, though a large number of such neutrals are present. In such cases, an approximate method is usually adopted, whereby the neutral particles are omitted from the simulation and their effect upon the dynamics of dust particles modeled by a combination of random forces and frictional terms. This leads to a recasting of the Newton's Equation of motion solved in MD, to a Langevin equation, solved in BD. Adopting this approach, we simulate a system of charged dust particles interacting via Yukawa potential in a 2-Dimensional layer, and extract relevant equilibrium statistical features such as the radial distribution function, static structure factor and the low frequency dust wave modes. We then propose the use of a Generalized Hydrodynamical (GH) approach to provide a semi-analytical model for the dust collective modes, which not only provides us with good predictions of the wave dispersion but also provides reasonable estimates for wave-number dependent wave damping, both of which will be compared against the results obtained from BD simulation. Finally, through our simulations, we also observe the equilibrium configuration of dust particles in the presence of cold ions streaming perpendicularly into the 2-Dimensional layer of dust particles. This provides us with novel results in the regime of sub-sonic ion flow speeds.

I report experimental and numerical studies of microparticle motion in a dusty plasma. These microparticles are negatively charged and are levitated in a plasma consisting of electrons, ions and neutral gas atoms. The microparticles repel each other, and are confined by the electric fields in the plasma. The neutral gas damps the microparticle motion, and also exerts random forces on them. I investigate and characterize microparticle motion. In order to do this, I study velocity distributions of microparticles and correlations of their motion. To perform such a study, I develop new experimental and analysis techniques. My thesis consists of four separate projects. In the first project, the battle between deterministic and random motion of microparticles is investigated. Two particle velocity distributions and correlations have previously studied only in theory. I performed an experiment with a very simple one dimensional (1D) system of two microparticles in a plasma. My study of velocity correlations involves just two microparticles which is the simplest system that allows interactions. A study of such a simple system provides insight into the motions of the microparticles. It allowed for the experimental measurement of two-particle distributions and correlations. For such a system, it is shown that the motion of the microparticles is dominated by deterministic or oscillatory effects. In the second project, two experiments with just two microparticles are performed to isolate the effects of ion wakes. The two experiments differ in the alignment of the two microparticles: they are aligned either perpendicular or parallel to the ion flow. To have different alignments, the sheath is shaped differently in the two experiments. I demonstrate that microparticle motion is more correlated when they are aligned along the ion flow, rather than perpendicular to the ion flow. In the third project, I develop a model with some key assumptions to compare with the experiments in the first two projects. My model includes all significant forces: gravity, electrical forces due to curved sheath and interparticle interaction, and gas forces. The model does not agree with both the experiments. In the last project, I study the non-Gaussian statistics by analyzing data for microparticle motion from an experiment performed under microgranity conditions. Microparticle motion is studied in a very thin region of microparticles in a three dimensional dust cloud. The microparticle velocity distributions exhibit non-Gaussian characteristics.

Statistical physics has been the foundation for much of our understanding about plasma physics. Often, plasma physics phenomena are explained using statistical physics principles and theories. Here, I reverse this paradigm to instead use plasma experiments to test statistical physics principles. In this thesis, I test statistical physics principles with an experimental dusty plasma, which is a four-component mixture of micron-sized ``dust'' particles, electrons, ions, and neutral gas molecules. When immersed in the plasma, the dust particles acquire large negative charges, since they accumulate more electrons than ions. Due to their large electric charges, the dust particles have interparticle potential energies that greatly exceed their kinetic energies, so that the collection of dust particles is considered to be a strongly coupled plasma. Like other strongly coupled plasma, the collection of dust particles can exhibit solid-like or liquid-like behavior. A key advantage offered by dusty plasma experiments is the ability to track the motion of individual dust particles. Dust particles are sufficiently large to allow for direct imaging using a video camera, so that time series data can be obtained for particle positions and velocities. These particle-level data provide a richer description of the dynamics and structure than can be obtained for most other strongly coupled plasmas, simple liquids, or solid materials. In particular, the particle-level data of positions and velocities are often required inputs for testing statistical physics theories or principles. The dusty plasma data I analyze are from the experiment of Haralson~\textit{et al.} [1,2], where dust particles were electrically levitated in a single horizontal layer within a vacuum chamber. The collection of dust particles initially settled into a crystalline lattice with solid-like behavior. To reach a liquid-like state, or to drive a shear flow, dust particles were manipulated using the radiation pressure force of lasers. In this thesis, I test three different statistical physics principles using an experimental dusty plasma. First, I test the fluctuation theorem, which was first was presented in 1993 by Evans, Cohen, and Morriss [3]. The fluctuation theorem, which is one of the most important recent developments in statistical physics, quantifies the probability that the entropy production rate will temporarily fluctuate to negative values in ``violations'' of the second law of thermodynamics. The original formulation of the fluctuation theorem described the entropy production due to viscous heating in a shear flow; this version of the fluctuation theorem had never been experimentally demonstrated in a liquid of any kind. In Chapter 2, I provide the first such demonstration by showing that the entropy production rate in a liquid-like dusty plasma shear flow satisfies the fluctuation theorem. This result also serves as the first demonstration that a strongly coupled plasma obeys the fluctuation theorem. Second, I measure the Einstein frequency $\Omega_E$, which describes the stochastic process of collisions in a strongly coupled plasma, and I compare my measurement to predictions made in the literature that used simulation data. Often, for weakly coupled plasma, a collision frequency is obtained to provide a measure of the strength of particle-particle interactions. However, for strongly coupled plasma (and likewise for liquids and solids), a collision frequency is not well defined since collisions are multibody and occur continuously. Another quantity is needed to describe the strength of particle-particle interactions. I propose that the Einstein frequency $\Omega_E$, a concept more commonly used in solid physics, is better suited for describing particle-particle interactions in a strongly coupled plasma. In Chapter 3, I present and use a new method to obtain the Einstein frequency of a 2D dusty plasma in both a liquid-like state and a crystalline state. My measurement of the Einstein frequency, which serves as a proxy for a collision frequency, is consistent with simulation predictions in the literature. Third, I present particle-coordination survival functions, which provide a richer description of microscopic dynamics in a liquid than the commonly used relaxation time. Relaxation times have been used, for example the Maxwell relaxation time, to describe the characteristic time scale for the crossover between elastic and viscous behavior in viscoelastic liquids. However, relaxation times are single-value measures that cannot fully describe the complexity of a liquid. In Chapter 4, using a survival function that retains temporal information about the local structural in a liquid, I discover that the microscopic arrangements in a liquid-like 2D dusty plasma have multiple time scales. Unexpectedly, non-defects have two time scales, while defects have one. My survival functions are time-series graphs of the probability that a particle's number of nearest neighbors, i.e., its coordination, remains the same. The two time scales for non-defects are revealed by an elbow in their survival-function curve. As a spinoff with a considerable amount of importance, I performed the simplest fluctuation theorem experiment to date, using an aerosol. An aerosol is simply a particle that is immersed in air. In Chapter 5, I show that the fluctuation theorem is applicable for an aerosol particle undergoing free-fall in air due to gravity. While the particle typically fell downwards, it is observed to occasionally fall upwards, against the force of gravity. For such upward displacements, the work done on the particle is negative, which is a temporary violation of the second law. I find that the probability of these temporarily violations obeys the work fluctuation theorem. This result also allowed an application: a novel diagnostic method to measure the mass of aerosol particles.

Thesis advisor: Gabor Kalman / Complex plasma systems have been studied for a long time. In this thesis we focus on a quasi-2D layer system. In fact, most experimental studies of complex plasmas are based on 2D systems, because it is easy to use camera to record the in-plane movement of particles. Unfortunately, due to the finite confining strength, the system is not a strictly 2D layer, it is a quasi-2D layer. We firstly studied the density profile of such a quasi-2D system by density functional theory(DFT). From the density profile research result, we found that the system can form a trilayer structure with proper parameters. Then we studied the dynamical properties of a trilayer system, and for simplicity, we only studied an ideal three layer model, both in liquid and lattice case. In lattice case, we firstly searched the stable lattice structure at different inter-layer distance. Then we used lattice sites summation to construct the dynamical matrix and solve the dispersion relation. For liquid case, we did the theoretical prediction for the collective dispersion by quasi localized charge approximation(QLCA), then we extracted the collective mode information from the molecular dynamics(MD) simulation. The QLCA and MD results were compared and discussed. The reason for the previous gap discrepancy problem is discovered. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

La métrologie des nanoparticules est devenue un enjeu scientifique et industriel capital pour pouvoir contrôler les caractéristiques des nano-objets (taille, densité, etc.) dans les procédés industriels tant pour la qualité des produits fabriqués que pour la protection des personnes et de l’environnement. Il est important, par conséquent, de trouver des méthodes de caractérisation innovantes et simples à mettre en oeuvre. L’objectif de ce travail de recherche fût de développer et d’optimiser des solutions pour caractériser des nanoparticules en voie sèche à l’aide d’un plasma (favorisant la désagglomération de l’échantillon de poudres à étudier). La présence de nanoparticules modifiant sensiblement les caractéristiques électriques du plasma, nous avons, dans un premier temps, développé un diagnostic basé sur la caractérisation électrique de la décharge et du plasma. Cette méthode nous renseignant sur la taille et la densité moyenne des nanoparticules. Puis, dans un second temps, nous avons développé un diagnostic basé sur la diffusion multi-angle de la lumière laser, nous permettant d’obtenir, là aussi, la taille et la densité moyenne des nanoparticules présent dans le plasma mais également leur indice de réfraction. Nous nous sommes, enfin, intéressés à la sédimentation assistée par plasma permettant d’obtenir la distribution en taille des nanoparticules, en l’optimisant. La corrélation de ces trois diagnostics nous donne, donc, un diagnostic efficace et fiable permettant la caractérisation en taille, en densité et en propriétés optiques des nanoparticules piégées dans le plasma. / Dust nanoparticles metrology has become a major scientific and industrial issue in order to control the characteristics of nano-objects (size, density, etc.). For industrial interests it concerns processes control and monitoring, manufactured products quality, human being and environment protection. It is therefore, crucial to find innovative methods of characterization and easy to implement and to handle. The objective of this research program was to develop and optimize solutions for characterizing nanoparticles in dry process and environment using a plasma (promoting disagglomeration of the studied powders). The presence of nanoparticles substantially altering the electrical characteristics of the plasma, we have, at first, developed a diagnostic based on the electrical characteristics of the discharge and the plasma. This method allows the determination the average nanoparticles size and concentration trapped in the plasma gas phase. Then, in a second step, we developed a diagnostic based on multi-angle laser light scattering, allowing us to get the size and the average density of the nanoparticles in the plasma, but also their refractive index. We finally focused our interest on the optimization of the plasma-assisted sedimentation of nanoparticles to obtain their size distribution especially for particles having sizes below 10 nm. The correlation of these three diagnostics gives us, so an efficient and reliable diagnostic for the global characterization in terms of size, density and optical properties of nanoparticles trapped in the plasma.