Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session BO3: Plasma Wakes and Dusty Plasmas |
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Chair: Eric Gillman, Naval Research Laboratory Room: Salon D |
Monday, October 27, 2014 9:30AM - 9:42AM |
BO3.00001: Simulation of Unsteady Wakes in Magnetized Plasmas Christian Bernt Haakonsen, Ian H. Hutchinson, Chuteng Zhou Wakes occur in plasmas moving past obstacles such as probes or the moon, and can take a variety of forms for different plasma parameters. The present work addresses relative motion perpendicular to a magnetic field, where the plasma Debye length and typical ion gyroradius are smaller than the obstacle. We have discovered such wakes to be unsteady, using Particle-In-Cell simulations, both for subsonic and supersonic relative motion. The underlying drive for the unsteadiness is provided by counterstreaming ions in the wake, and for supersonic motion kinetic electron effects lead to unsteadiness closer to the obstacle than what is seen in simulations with a Boltzmann electron response. It is thus sometimes necessary to resolve the electron time-scale in simulations, requiring parallel simulations using significant computational resources. The subsonic simulations are for a regime applicable to (among other things) Mach probes in the scrape-off layers of tokamaks, where unsteady wake effects could potentially be detected. The supersonic simulations are for instance relevant to the solar wind flowing past the moon, where the present work dramatically improves the resolution and understanding of plasma phenomena in the wake. [Preview Abstract] |
Monday, October 27, 2014 9:42AM - 9:54AM |
BO3.00002: Is the compressibility positive or negative in a strongly-coupled dusty plasma? John Goree, W. D. Suranga Ruhunusiri In dusty plasmas, dust particles are often strongly coupled with a large Coulomb coupling parameter $\Gamma$, while the electrons and ions that share the same volume are weakly coupled. In most substances, compressibility $\beta$ must be positive; otherwise there would be an explosive instability. In a multicomponent plasma, however, one could entertain the idea that $\beta$ for a single strongly coupled component could be negative, provided that the restoring force from charge separation overwhelms the destabilizing effect. Indeed, the compressibility for a strongly-coupled dust component is assumed to be negative in three theories we identified in the literature for dust acoustic waves. These theories use a multi-fluid model, with an OCP (one component plasma) or Yukawa-OCP approach for the dust fluid. We performed dusty plasma experiments designed to determine the value of the inverse compressibility $\beta^{-1}$, and in particular its sign. We fit an experimentally measured dispersion relation to theory, with $\beta^{-1}$ as a free parameter, taking into account the systematic errors in the experiment and model. We find that $\beta^{-1}$ is either positive, or it has a negligibly small negative value, which is not in agreement with the assumptions of the OCP-based theories. [Preview Abstract] |
Monday, October 27, 2014 9:54AM - 10:06AM |
BO3.00003: Dust density measurements in 3D dust clouds by tomography Andre Melzer Dusty plasmas usually consist of (micron-sized) dust particles trapped in a gaseous discharge plasma. Volume-filling dust clouds can be generated in the laboratory by thermophoretic levitation of the particles against gravity or under the microgravity conditions of parabolic flights. In these discharges, the dust density is typically so high that together with the high charge on the particles, the dust charge density can compete with the ion and electron (charge) density indicating a regime of charge depletion. Here, we present a technique that allows to measure the spatially resolved 3D dust density in such dusty discharges. For that purpose, the dust cloud is transilluminated by a homogeneous light source and the transilluminated cloud is measured under different angles in a tomographic-like manner. This allows to reconstruct the full 3D dust density within the discharge volume and further to deduce the force balance for the dust component. [Preview Abstract] |
Monday, October 27, 2014 10:06AM - 10:18AM |
BO3.00004: Measurement of Interbundle/Interstring Forces in Vertically Aligned Dust Particle Systems Truell Hyde, Jie Kong, Oleg Petrov, Bo Zhang, Lorin Matthews Particle-particle, cluster-cluster and string-string interactions have long been of interest across a variety of scientific research disciplines. Complex plasmas have proven to be a versatile analog for studying such systems, often providing a mechanism for determining the fundamental physics behind the strong correlation effects observed. In this talk, the interaction between two, two-particle vertically aligned strings, two, three--particle vertically aligned strings and two vertically aligned dust particle clusters is examined experimentally. The energy storage held within the Coulomb field between particles will be discussed using data collected employing a technique using externally assigned potentials on the walls of an Indium Tin Oxide (ITO) box. [Preview Abstract] |
Monday, October 27, 2014 10:18AM - 10:30AM |
BO3.00005: Dust as In-Situ Probes for Plasma Magnetic Field Interactions in a Dusty Plasma Michael Dropmann, Rene Laufer, Georg Herdrich, Lorin Matthews, Truell Hyde A series of experiments were conducted inside a GEC rf reference cell to map the forces in three dimensions above a magnet placed in a dusty plasma and employing both horizontal and vertical orientations. Micron sized dust particles were used as in-situ probes to investigate the interaction between the low-temperature plasma produced and a magnetic field close to a non-conductive surface. Dust particles were dropped into the plasma where they obtained a negative charge leading to trajectories, which were strongly influenced by both electric and ion drag forces. By recording the trajectories of the particles, which were illuminated by a vertical laser plane, the forces onto the particles were determined. A strong influence of the magnetic field onto the plasma sheath was observed. Given the electrons are strongly magnetized by the magnet while ions remain comparatively unaffected by the magnet a charge separation takes place, which leads to strong electric fields. As a result the sheath thickness varies significantly within the magnetic field, showing strong horizontal force components. Based on these observations, analogies to the interaction of the lunar plasma with known lunar magnetic anomalies will be drawn to contribute to the explanation of the formation of lunar swirls. [Preview Abstract] |
Monday, October 27, 2014 10:30AM - 10:42AM |
BO3.00006: Dust particles as probe in a complex plasma Raziyeh Yousefi, Allen Davis, Jorge Carmona-Reyes, Lorin Matthews, Truell Hyde Understanding the behavior of dust particles in a complex plasma requires a knowledge of the basic properties such as the net electrostatic charge and dipole moment of the dust as well as the local electrostatic fields. In this study, dust aggregates are formed from gold coated monodisperse spherical melamine-formaldehyde monomers in a radio-frequency (rf) argon discharge plasma. The behavior of observed dust aggregates is analyzed both by studying the particle trajectories and by employing computer models examining 3D structures of aggregates and their interactions and rotations as induced by torques arising from their dipole moments. These allow the basic characteristics of the dust aggregates, such as the electrostatic charge and dipole moment, and local electrostatic fields to be determined from the behavior of particles. It is shown that the experimental results agree with predicted values from computer models for aggregates in these environments. [Preview Abstract] |
Monday, October 27, 2014 10:42AM - 10:54AM |
BO3.00007: The Potential Field within a Biased Indium Tin Oxide Glass Box Located in a Dusty Plasma Environment Jorge Carmona-Reyes, Lorin Matthew, Truell Hyde The number of studies in complex plasmas have increased rapidly due to the field's ability to act as an analog for research in other areas such as metallic glasses in engineering, coulomb interactions in pure physics, and double helical formation in biophysics. Much of the data collected in such studies occurs inside a glass box placed on the lower powered electrode of a GEC rf reference cell. Recently, several research groups have expanded this technique through use of an Indium Tin Oxide (ITO) glass box for better control of the confining potential. Unfortunately, a proper understanding of the underlying physics producing the confinement inside either glass box is lacking. This work will provide results from a mapping of the potential well inside an ITO box, employing a user positioned reference probe installed in a Zyvex S-100 nanomanipulator. The shape of the potential well, the plasma sheath and the particle interaction inside the ITO box will be discussed. [Preview Abstract] |
Monday, October 27, 2014 10:54AM - 11:06AM |
BO3.00008: Small Dust Cluster Probes within a Dusty Plasma Jie Kong, Ke Qiao, Lorin Matthews, Truell Hyde Small-number dust particle clusters are often seen in dusty plasmas. Interestingly, such clusters can often be used as in-situ probes providing plasma diagnostics. The number of dust particles, as well as the cluster size and shape, can be easily controlled employing a glass box placed on the powered lower electrode within a GEC rf reference chamber to provide confinement of the dust. Adjusting the rf power alters the plasma conditions creating structural changes within the cluster. This effect can be used to probe the relationship between the rf power and other plasma parameters of interest. This experiment employs the sloshing and breathing modes of small cluster oscillations to examine the relationship between the system's rf power and the plasma screening length inside the glass box as well as determine the particle charge. Experimental results indicate that both the screening length and the dust charge decrease as the rf power inside the box increases. [Preview Abstract] |
Monday, October 27, 2014 11:06AM - 11:18AM |
BO3.00009: Mode Coupling and Resonance Instabilities in a Dust Chain Ke Qiao, Jie Kong, Lorin Matthews, Truell Hyde Mode couplings and resonance instabilities have recently received tremendous attention in both large plasma crystals [1] and small dust clusters [2, 3]. In this research, normal modes are investigated using both simulation and experiment to examine a horizontal finite chain consisting of 3-50 dust particles in a complex plasma. The resultant mode coupling and resonance instabilities are analyzed and compared with previous research on large crystals and circular dust clusters. [Preview Abstract] |
Monday, October 27, 2014 11:18AM - 11:30AM |
BO3.00010: Sheared and unsheared rotation of driven dust clusters Dietmar Block, Jan Schablinski, Jan Carstensen, Franko Greiner, Alexander Piel In dusty (complex) plasmas rotating dust clusters with either rigid body rotation, horizontally sheared rotation, or vertical sheared rotation are observed in axial magnetic fields, rotating electric fields, rotating neutral gas columns, and in laser manipulation experiments. Our experiments and simulations now add a small anisotropy of only a few percent to the confinement potential. Such anisotropies are hardly avoidable in experiments and therefore their role should be carefully studied to avoid misinterpretation of the experimental observations. This contribution reports on systematical investigations of the motion of driven dust clusters in a slightly anisotropic confinement. Special attention is paid to the questions whether an unsheared drive always results in an unsheared cluster rotation and how symmetry and particle arrangement affect the dynamical response of the system. [Preview Abstract] |
Monday, October 27, 2014 11:30AM - 11:42AM |
BO3.00011: Selective mode excitation in 2D dust clusters Jan Schablinski, Dietmar Block, Andre Melzer, Alexander Piel The dynamical properties of dust clusters are still a hot topic in recent research activities. In many applications intense lasers are used as a tool for the manipulation of the dynamics of particle systems. For example, specific particle motion patterns like the intershell rotation of small clusters can be driven or the systems can be effectively heated by randomized momentum transfer using multiple lasers. In this contribution we present a method to drive a selection of eigenmodes of the particle system, which are mainly associated with radial particle oscillations, and give a characterization of the driving mechanism. Further, the impact of a localized driving force on the ability to excite either breathing or wave-like modes is discussed. [Preview Abstract] |
Monday, October 27, 2014 11:42AM - 11:54AM |
BO3.00012: Shock Propagation in Dusty Plasmas by MD Simulations Mathieu Marciante, Michael Murillo The study of shock propagation has become a common way to obtain statistical information on a medium, as one can relate properties of the undisturbed medium to the shock dynamics through the Rankine-Hugoniot (R-H) relations. However, theoretical investigations of shock dynamics are often done through idealized fluid models, which mainly neglect kinetic properties of the medium constituents. Motivated by recent experimental results,\footnote{N. P. Oxtoby, E. J. Griffith, C. Durniak, J. F. Ralph and D. Samsonov. Ideal Gas Behavior of a Strongly Coupled Complex (Dusty) Plasma. \textit{Phys. Rev. Lett.} \textbf{111}, 015002 (2013).} we use molecular dynamics simulations to study the propagation of shocks in 2D-dusty plasmas, focusing our attention on the influence of kinetic aspects of the plasma, such as viscosity effects. This study is undertaken on two sides. On a first side, the shock wave is generated by an external electric field acting on the dust particles, giving rise to a shock wave as obtained in a laboratory experiment. On another side, we generate a shock wave by the displacement of a two-dimensional piston at constant velocity, allowing to obtain a steady-state shock wave. Experiment-like shock waves propagate in a highly non-steady state, what should ask for a careful application of the R-H relations in the context of non-steady shocks. Steady-state shock waves show an oscillatory pattern attributed to the dominating dispersive effect of the dusty plasma. [Preview Abstract] |
Monday, October 27, 2014 11:54AM - 12:06PM |
BO3.00013: Intrinsic Charge Fluctuations of Dust in Plasmas Containing Multiply Charged Ions Babak Shotorban A master equation, formulated for states of the charge of a grain in a plasma containing various kinds of singly or multiply charged ions (Shotorban, Phys Plasmas, 2014) is presented. From the master equation, a Fokker-Planck equation is derived through van Kampen's system-size expansion method. The derived Fokker-Planck equation has a Gaussian solution with a mean and variance governed by two initial-value differential equations involving the ions and electron attachment rates. Also, a Langevin equation, statistically equivalent to the Fokker-Planck equation, and a discrete stochastic method, statistically representing the master equation, are developed to model the grain charge fluctuations in time. Grain charging in a plasma containing electrons, protons, and alpha particles is considered as a test problem. It is shown that the Gaussian solution is in very good agreement with the master equation's solution obtained numerically for this problem. [Preview Abstract] |
Monday, October 27, 2014 12:06PM - 12:18PM |
BO3.00014: ABSTRACT WITHDRAWN |
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