Session BO5: Dusty Plasma and Multiphase Media

Measurement of the Interaction Force between Dust Particles within a Glass Box in a GEC RF Reference Cell

A wide variety of structural states (for example, Coulomb balls, one-dimensional vertically aligned dust particle chains, helical dust structures, etc.) have been observed for dust particles confined within a glass box placed on the lower electrode of a GEC rf reference cell. Both the interparticle interaction force and the confinement force play important roles in the formation of these structures. Unfortunately, since the exact nature of the confinement force produced by the walls of the glass box is yet unclear, it is difficult to differentiate between the effects produced by the interparticle interaction force and the effects created by the confinement force. In this experiment, a free-falling dust particle in the box acts as an in-situ probe, providing information on the structure of the confinement force. It will be shown that the data provided by this mapping procedure allows the interaction force between particles within various dust particle structures to be measured through perturbation of individual particles employing a diode pumped solid state laser (Coherent VERDI).   

The Relationship Between the Sloshing and Breathing Frequencies in a 1D Vertically Aligned Dust Particle Chain

When confined in a glass box placed on the lower powered electrode of a GEC rf reference cell, dust particles immersed in plasma can form vertically aligned 1D chains. Both the formation and subsequent structural changes within this vertically aligned dust chain are controlled by the rf power, since the rf power effects the ionization rate in the cell, the screening parameter and the charge on the dust particles. In this study, oscillations of a 1D vertically aligned dust particle chain are employed to investigate the dust charge and screening length through measurement of the resonance frequency. It will be shown that the relationship between the sloshing and breathing frequencies indicates that the ion streaming effect plays an important role in vertical oscillations and must be included in any structural analysis of the system.   

Vertical Modes and Discrete Instabilities in Two-Dimensional Dust Particle Clusters

The coupling observed between horizontal and vertical wave modes in large plasma crystals and their resulting instability formation are current topics of interest, both theoretically [1] and experimentally [2]. Similarly, horizontal [3] and vertical [4] normal modes in finite dust clusters have been examined theoretically but to date, only horizontal modes have been experimentally observed. In this research, dust clusters are formed within a modified GEC rf reference cell, using a glass box placed on the lower powered electrode to provide horizontal confinement. The resulting thermal motion of the dust particles is tracked and analyzed. Using the power spectra obtained, both horizontal and vertical normal modes are identified and discrete instabilities as predicted theoretically [5] are shown to be induced due to the coupling between the horizontal and vertical modes. \\[4pt] [1] V. V. Yaroshenko, A. V. Ivlev, and G. E. Morfill, Phys. Rev. E 71, 046405 (2005).\\[0pt] [2] L. Cou\"{e}del et al., Phys. Rev. Lett. 104, 195001 (2010).\\[0pt] [3] A. Melzer, Phys. Rev. E 67 016411 (2003).\\[0pt] [4] Ke Qiao and Truell W. Hyde, IEEE transactions on plasma science 36, 2753 (2008).\\[0pt] [5] Ke Qiao, Jie Kong, Eric Van Oeveren, Lorin S. Matthews and Truell W. Hyde, in preparation.   

Wave Modes of Vertical Dust Chains in Complex Plasma

Plasma sheaths are notoriously complicated; however, the recent use of micron size dust particles as in-situ probes of this region is beginning to provide data that can be employed to better understand these phenomena.~In this study, longitudinal and transverse waves are explored for vertically aligned dust particle chains consisting of 3 to 5~particles.~These spherical particles are levitated in the sheath above the powered lower electrode in a GEC reference cell and are confined in~the horizontal direction using a glass box. Under appropriate power and pressure conditions, the horizontal confinement provided by the box is great enough~to create the chains, which can then be perturbed by applying time-varying potentials to a vertical probe attached to a Zyvex S100~nanomanipulator. The probe can be positioned over a range of locations,~allowing both longitudinal and transverse waves to be~driven through the chains. Particles exhibit coupled oscillator motion, individually producing all of the~pure three-particle~longitudinal normal~modes.~Dispersion relations previously derived for particles aligned in the horizontal plane exhibit similar relevant forces; these are adapted~to the vertical direction and compared to experimental results.   

Interaction of single-layer plasma crystals with upstream charged particles

In experiments with single-layer plasma crystals, one often observes extra particles outside of the main layer after injecting particles into plasma. Those particles, which can be agglomerates or contamination, sometimes move at a high speed and disturb the lattice. When the extra particle speed is higher than the sound speed of the lattice, the disturbance forms a Mach cone. The Mach cones and wakes associated with extra particles moving {\it beneath} the lattice layer are well studied. In the present work, we study for the first time the interaction of a single-layer plasma crystal with charged extra particles located {\it above} it (upstream of the flow of ions) [1]. Upstream extra particles tend to move between the rows of particles in the crystal, accelerate to supersonic speeds, and excite attraction-dominated Mach cones and wakes in the crystal. We attribute the particle - lattice layer attraction to the ion wake formed underneath the upstream extra particle.\\[4pt] [1] C.-R. Du, V. Nosenko, S. K. Zhdanov, H. M. Thomas, and G. E. Morfill, EPL 99, 55001 (2012).   

Modeling dust crystal in a cylindrical rf plasma reactor

In this study, the effect of the radiofrequency on the modeling of three-dimensional dust crystal was examined. A low pressure cylindrical argon reactor was modeled numerically using the local field approximation model, then multiple small dust grains were released and tracked in a three-dimensional Lagrangian framework. The dust grain configuration was obtained for two different cases; one using the rf-period-averaged plasma variables and one using the instantaneous plasma variables. The results of two cases were compared and significant differences were observed.   

Pair and three-particle correlation functions of spherical dust clusters

Dust particles in a complex plasma usually accumulate a high negative charge inside a plasma which is responsible for their strong repulsive interaction and high coupling. When confined in a parabolic trap, these particles form spherical clusters with a characteristic shell structure. In recent years the phase transition-like crossover from a crystal to a liquid-like state has attracted high interest, e.g.~[1]. While the radial melting is now well understood, here we concentrate on the loss of intra-shell order. The radial pair correlation function $\rho(r_{ij})$ is well suited for homogeneous system but has to be adapted to the spherical symmetry for finite clusters. Here, we present the Center-Two-Particle correlation function that is equally sensitive to intra-shell as well as inter-shell correlation~[2,3]. As a second quantity, we present the Triple Correlation function that is calculated from the ``bonding angles`` of three particles. This quantity is particularly well suited to investigate the orientational order within spherical cluster shells.\\[4pt] [1] J. B\"oning et al., Phys. Rev. Lett. \textbf{100}, 113401 (2008)\\[0pt] [2] P. Ludwig et al., Plasma Phys. Control. Fusion \textbf{52}, 124013 (2010)\\[0pt] [3] A. Schella et al., Phys. Rev. E \textbf{84}, 056402 (2011)   

Phase Transitions, Structures and Transport of Charged Dust in Plasma under Laboratory and Microgravity Conditions

The charged dust system represent a non-neutral or quasi-neutral systems (dusty plasmas) containing micron-sized particles of a substance with electrical charges up to 1000-10000 e. As a result of strong interaction, the dust particles may form the ordered structures of liquid and crystal types. The laboratory dusty plasma is the unique object for studying the structures, phase transitions and transport phenomena on the ``kinetic level.'' The phase transitions in quasi-two-dimensional dust structures suspended in rf discharge were studied. The experimental results have revealed the existence of hexatic phase as well as solid-to-hexatic phase and hexatic-to-liquid transitions. The spatial distribution of pair interparticle interaction forces was recovered by the original method based on solving the inverse problem using Langevin equations. The measured phase-state points with the theoretical phase diagram of two-dimensional Yukawa system have been obtained. The formation of ordered structures from large number (about 10000) of charged diamagnetic dust particles in a cusp magnetic trap was studied under microgravity conditions onboard ISS. The magnetic susceptibility and charge of the particles have been estimated. An experimental study of the kinematic viscosity was carried out for dust particles of different sizes in weakly ionized plasma. Nonmonotonic dependence (a minimum) of the viscosity constants on the coupling parameter has been observed.   

Quenching a magnetized plasma close to the phase transition

The presence of an external magnetic field is shown to prevent the crystallization of a two-dimensional one-component plasma after a sudden quench, provided its field strength surpasses a critical threshold. This unexpected behavior---which seemingly is in violation of the Bohr-van Leeuwen theorem--- is shown to arise from the strong bending of the particle trajectories due to the magnetic field and is explained within a simple one-particle model which elucidates the geometric origin of the crystallization blocking. \\[1ex] T. Ott, H. L\"owen, M. Bonitz, Phys. Rev. Lett., \emph{accepted for publication} (2013)   

Cosmic Dust Aggregation with Stochastic Charging

The coagulation of cosmic dust grains is a fundamental process which takes place in astrophysical environments, such as presolar nebulae and circumstellar and protoplanetary disks. Cosmic dust grains can become charged through interaction with their plasma environment or other processes, and the resultant electrostatic force between dust grains can strongly affect their coagulation rate. Since ions and electrons are collected on the surface of the dust grain at random time intervals, the electrical charge of a dust grain experiences stochastic fluctuations. In this study, a set of stochastic differential equations is developed to model these fluctuations over the surface of an irregularly-shaped aggregate. Then, employing the data produced, the influence of the charge fluctuations on the coagulation process and the physical characteristics of the aggregates formed is examined. It is shown that dust with small charges (due to the small size of the dust grains or a tenuous plasma environment) are affected most strongly.   

Lunar Swirls: Plasma Magnetic Field Interaction and Dust Transport

In close collaboration between the Center for Astrophysics, Space Physics and Engineering Research (CASPER) at Baylor University, Texas, and the Institute of Space Systems (IRS) at the University of Stuttgart, Germany, two plasma facilities have been established using the Inductively heated Plasma Generator 6 (IPG6), based on proven IRS designs. A wide range of applications is currently under consideration for both test and research facilities. Basic investigations in the area of plasma radiation and catalysis, simulation of certain parameters of fusion divertors and space applications are planned. In this paper, the facility at Baylor University (IPG6-B) will be used for simulation of mini-magnetospheres on the Moon. The interaction of the solar wind with magnetic fields leads to the formation of electric fields, which can influence the incoming solar wind ion flux and affect dust transport processes on the lunar surface. Both effects may be partially responsible for the occurrence of lunar swirls. Interactions of the solar wind with such mini-magnetospheres will be simulated in the IPG6-B by observing the interaction between a plasma jet and a permanent magnet. The resulting data should lead to better models of dust transport processes and solar wind deflection on the moon.   

Observation of temperature peaks due to strong viscous heating in a dusty plasma flow

Profound temperature peaks are observed in regions of high velocity shear in a 2D dusty plasma experiment with laser-driven flow [1]. These temperature peaks are attributed to viscous heating, which occurs due to collisional scattering in a shear flow. In most plasmas and other substances, viscous heating has long been known to occur, but the temperature peaks in the high-shear regions are usually difficult or impossible to observe. This is so because thermal conduction in most substances is so strong as to flatten the temperature peaks. Dusty plasmas, on the other hand, have a sufficiently high viscosity and a sufficiently small thermal conductivity that the temperature peaks are easily observed. We also measure the shear viscosity and thermal conductivity in the same experiment; this is done by fitting the measured profiles of flow velocity and temperature to the dust hydrodynamic equations for momentum and energy.\\[4pt] [1] Feng, Goree and Bin Liu, PRL 109, 185002 (2012) and PRE 86, 056403 (2012)   

Dynamics of the self organized toroidal dust flow structures in plasma

The self-organized dynamical flow pattern of the dust cloud forming a toroidal structure in an unmagnetized glow discharge plasma is shown to result from a shear driven instability of the dust cloud. A condensed phase of the levitated dust cloud shows a steady rotation below the instability threshold. Crossing the threshold sets in the melting of the condensed state which saturates by forming a hollow toroidal dust flow structure. An analytical formulation is presented and characterized in order to model the dynamics of such self-organized dusty structures.   

Microwave measurements on a well-collimated dusty plasma sheet for communications blackout applications

A linear hollow cathode produces an electron beam that is accelerated into a low pressure (50 to 150 mTorr) background of Argon, producing an electron beam discharge. A relatively constant 170 Gauss axial magnetic field is produced by two electromagnet coils arranged in a Helmholtz configuration. This results in a well-collimated electron beam, producing a 2-dimensional discharge sheet (40 cm high by 30 cm wide by 1 cm thick) with densities as high as 10$^{\mathrm{12}}$ cm$^{\mathrm{-3}}$. The plasma sheet is intended to replicate the parameters of the plasma layer produced around hypersonic and reentry vehicles. The electron beam is accelerated vertically towards a grounded beam dump electrode. This electrode is modified to include an array of six piezo buzzers modified and filled with alumina powder. When powered with a modest voltage, the piezoelectric shakers drop dust particles into the plasma sheet discharge directly below. A transmitting microwave horn is oriented normal to the dense plasma sheet while the receiving horn is mounted on a stage that can be rotated up to 180 degrees azimuthally. Microwave transmission and scattering measurements of the plasma sheet are made in the S-band and X-band for applications related to communications blackout.