Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session PO5: Dusty and Low Temperature Plasmas |
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Chair: William Amatucci, Naval Research Laboratory Room: Reunion C |
Wednesday, November 19, 2008 2:00PM - 2:12PM |
PO5.00001: Interaction Forces Between Two Vertical Particles in a Complex Plasma Jay Kong, Truell Hyde In the complex plasma generated in a GEC reference cell, charged dust grains levitate above the powered electrode. Fast-moving ions in the plasma sheath generate wake fields, creating vertical particle chains where the interaction forces between each particle in the chain are generally different due to the wake-field effect. An attenuated oscillation method designed to examine these anisotropic interaction forces has been developed based on an experimental technique whereby dust particles are raised to a height $\Delta $h above their natural equilibrium employing an external DC bias. Removal of this DC bias causes the dust particles to oscillate with attenuated amplitude, eventually returning to their original equilibrium positions. The resulting oscillation spectrum displays features unique to the interaction between the particles. Recent experimental results will be presented. [Preview Abstract] |
Wednesday, November 19, 2008 2:12PM - 2:24PM |
PO5.00002: Effects of particle dipole moments in a Coulomb Crystal Ke Qiao, Lorin Matthews, Truell Hyde The study of interparticle forces and potentials within Coulomb crystals has long been an important and fundamental topic in the field of dusty (complex) plasma research. In a typical complex plasma experiment, the dust particles form a two-dimensional (2D) system having a single layer or a quasi-2D system with several layers due to the vertical gravitational confinement. It is generally agreed that particles within a layer interact through a Yukawa (screened Coulomb) potential. Interactions between particles within different layers are much more complex and not as well understood. One factor influencing such interactions is the possible formation of an effective dipole moment on the dust particles. Such a dipole moment can be induced either by anisotropic charging of the dust grain or the (re)distribution of plasma particles surrounding the grains. In this research a molecular dynamics (Box{\_}Tree) simulation is employed to study the effect such a dipole --dipole interaction would have on a multilayer particle system. Simulation results will also be compared with recent experimental data. [Preview Abstract] |
Wednesday, November 19, 2008 2:24PM - 2:36PM |
PO5.00003: Complex Plasmas with Two Distinct Particle Sizes Bernard Smith, Lorin Matthews, Truell Hyde Dust particle clouds are found in most plasma processing environments and many astrophysical environments. Dust particles suspended within such plasmas often acquire an electric charge from collisions with free electrons in the plasma. Depending upon the ratio of interparticle potential energy to average kinetic energy, charged dust particles can form a gaseous, liquid or crystalline structure with short to longer range ordering. The majority of past and current experiments employed monodisperse spheres to form their complex plasma system. As a result, the manner in which the basic thermodynamic properties of the system are dependent upon the overall dust grain size distribution is still not well understood. In this experiment, two-dimensional plasma crystals were formed from mixtures of 11.93 $\mu $m, 8.89 $\mu $m and 6.50 $\mu $m monodisperse particles in Argon plasma. The pair correlation function and vibrational data were determined for varying pressures and powers and then compared with measurements obtained for experiments employing a single size distribution of monodisperse spheres. [Preview Abstract] |
Wednesday, November 19, 2008 2:36PM - 2:48PM |
PO5.00004: Coagulation of fractal aggregates in Lorentzian space plasma with ultraviolet radiation Lorin Matthews, Victor Land, Truell Hyde, Allison Youngblood Coagulation of dust particles occurs in many plasma environments and is especially important for the early stages of planet formation. Particles present in tenuous plasma around young stellar objects are likely to obtain a negative charge, which would hinder coagulation through the repulsive Coulomb interaction. However, due to the fractal nature of the particles, dipole moments arise, which promote coagulation. Both the charge and dipole moments depend on the energy distribution of the ambient plasma. In space, Lorentzian distributions are observed, with a large contribution of high energy particles. Furthermore, ultraviolet (UV) radiation from the young star causes photo-detachment, affecting the charge and dipole moments of the dust, and possibly the coagulation. We use a charging model based on a modified orbital-motion-limited theory, including the effect of Lorentzian plasma distributions and UV radiation, to calculate the charge and dipole moments on fractal aggregates. With a self-consistent N-body code, which includes the dipole-dipole interactions, we then model the coagulation of these aggregates. [Preview Abstract] |
Wednesday, November 19, 2008 2:48PM - 3:00PM |
PO5.00005: Manipulating particle traps in a GEC reference cell through thermophoresis Victor Land, Erica Shen, Lorin Matthews, Truell Hyde Laboratory dusty plasma contains electrons, ions, atoms, and solid particles, which usually are micrometer sized. These particles absorb ions and electrons, obtaining an overall negative charge. A proper description of dusty plasma thus requires knowledge of both the plasma and the dust parameters; however their experimental determination is difficult without perturbing the plasma. We apply a self-consistent 2D dusty plasma fluid model to a capacitively coupled radio-frequency discharge in a Gaseous Electronics Conference reference cell. The model will be shown to produce reliable results for determining the plasma parameters at varying discharge settings. The forces on the dust particles and the formation of particle traps within the discharge will also be discussed. Such particle traps are important since they allow for the formation of differing types of dust structures and symmetry. Finally, active heating or cooling of surfaces within the discharge chamber and the manner in which they alter these particle traps will be examined. [Preview Abstract] |
Wednesday, November 19, 2008 3:00PM - 3:12PM |
PO5.00006: Manipulation of dust particles in complex plasmas by electric field V. Nosenko, A. Ivlev, S. Zhdanov, G. Morfill Complex plasmas consist of fine solid particles suspended in a weakly ionized gas. It is often necessary to manipulate particles in a complex plasma of a gas discharge. Popular manipulation methods include modulating the discharge parameters (e.g., discharge voltage or current), using the radiation pressure of a focused laser beam, and using an electrically biased wire or external magnetic field. In the present study, additional electrodes were placed in a plasma of capacitively coupled rf discharge. Polymer microspheres were suspended in the plasma sheath above the lower rf electrode and in the vicinity of the additional electrodes. An ac voltage was applied to the additional electrodes with a frequency higher than the dust plasma frequency but lower than the ion plasma frequency. As a result, ions responded to the applied (screened) ac field and the particles responded to the time-averaged electric field and ion drag force. We discuss various regimes of particle manipulation using this scheme and their applications. [Preview Abstract] |
Wednesday, November 19, 2008 3:12PM - 3:24PM |
PO5.00007: Particle Dynamics in a Gravitationally Variable Argon DC Glow Discharge Dusty Plasma Michael Hvasta, Brandon Bentzley, Justin Nieusma, Rachel Sherman, Andrew Zwicker Dusty plasmas were studied in an experiment designed to investigate the impact of gravity on particle (80 micron) dynamics. The experiment used a CCD camera and thin laser-sheet to image silica dust clouds that were exposed to UV light (100W, 365nm) within an argon DC glow discharge plasma (300mT, 0.65ma) aboard NASA's ``Weightless Wonder.'' The Weightless Wonder is a plane capable of creating a 0-1.8g environment for 10-25 seconds by flying through a series of parabolic trajectories. Interparticle spacing was studied for a variety of equilibrium positions (where gravitational and electrostatic forces offset) during the flight. Particle position and velocity were analyzed as a function of g. Theory and results are presented. [Preview Abstract] |
Wednesday, November 19, 2008 3:24PM - 3:36PM |
PO5.00008: Critical Point of Fine Particle Plasmas and Density Fluctuations Hiroo Totsuji We analyze thermodynamic functions of fine particle plasmas in the domain of strong coupling between fine particles. It is shown [1] that, when the coupling is sufficiently strong, we have a separation into phases with different densities and an associated critical point. When we approach the critical point, there appear enhanced density fluctuations with diverging amplitude at the critical point. We derive related phase diagrams and enhancement factors of density fluctuations in the plane of dimensionless characteristic parameters. We also discuss the combination of experimental parameters of fine particle plasmas which enables one to observe these phenomena [2]. In order to make such an observation, it is necessary to have a bulk isotropic three-dimensional system of fine particle plasmas with a very strong coupling. Though it is not easy to realize such a system on the ground, we expect microgravity experiments may provide a chance of observation. [1] H. Totsuji, J. Phys. A: Math. Gen. 39, 4565 (2006). H. Totsuji, Non-Neutral Plasma Physics VI, ed. M. Drewsen et al., AIP, 2006, p.248. H. Totsuji, Phys. Plasmas 15, 072111(2008). [2] H. Totsuji, to appear in Plasma and Fusion Research, Vol.3. [Preview Abstract] |
Wednesday, November 19, 2008 3:36PM - 3:48PM |
PO5.00009: Molecular dynamics simulation of complex plasmas: interaction of nonlinear waves Celine Durniak, Dmitry Samsonov Complex plasmas consist of micron sized microspheres immersed into ordinary ion-electron plasmas. They exist in solid, liquid, gaseous states and exhibit a range of dynamic phenomena such as waves, solitons, phase transitions, heat transfer. These phenomena can be modelled in complex plasmas at the microscopic or ``molecular'' scale, which is almost impossible in ordinary solids and liquids. We simulate a monolayer complex plasma consisting of 3000 negatively-charged particles (or grains) with the help of molecular dynamics computer simulations. The equations of grain motion are solved using a 5$^{th}$ order Runge Kutta method taking into account interaction of every grain with each other via a Yukawa potential. The grains are confined more strongly in the vertical direction than in the horizontal. After seeding the grains randomly the code is run until the equilibrium is reached as the grain kinetics energy reduces due to damping force equal to the neutral friction in the experiments and a monolayer crystal lattice is formed. Then we investigate interactions between nonlinear waves in a monolayer strongly coupled complex plasma moving in three dimensions. Different excitations are applied during a short time symmetrically on both sides of the lattice. Structural properties and nonlinear waves characteristics are examined as the pulses propagate across the complex plasma in opposite directions. [Preview Abstract] |
Wednesday, November 19, 2008 3:48PM - 4:00PM |
PO5.00010: Experimental study of interacting solitons in a complex (dusty) plasma Paul Harvey, Dmitry Samsonov, Gregor Morfill A plasma is an ionised gas which consists of a mixture of electrons, positive ions and neutral molecules. In complex plasmas, small micron-sized microspheres are introduced. The grains become negatively charged and form a monolayer lattice. A soliton is a stable, solitary wave that retains its shape as it propagates through a medium. The apparatus for this experiment consists of a discharge chamber containing two electrodes. The lower electrode delivers RF-power into the chamber, maintaining the argon gas in the plasma state. The particles are confined radially within a bowl-shaped potential. Two parallel wires run along opposite sides of the monolayer lattice. A negative pulse on both wires excites two solitons to propagate inwards. A thin sheet of laser light illuminates the lattice which is then captured on video at a high frame rate. The kinetic movement of the microspheres can then be analysed. The propagation of the two solitons through this crystal lattice has been traced. Interaction has been observed to occur between two soliton waves within the complex plasma. [Preview Abstract] |
Wednesday, November 19, 2008 4:00PM - 4:12PM |
PO5.00011: Drift wave turbulence and associated transport in a collisional dusty magnetoplasma Nitin Shukla, P.K. Shukla, J.T. Mendon, Dastgeer Shaikh A set of nonlinear equations for the low-frequency, long and short wavelength (in comparison with the ion gyroradius) electrostatic drift waves in a nonuniform collisional dusty magnetoplasma is derived. In the linear limit, the presence of stationary charged dust grains is found to enhance the frequency and the growth rate of the long wavelength drift wave instability. Subsequently, nonthermal drift wave fluctuations induced cross-field plasma particle transport is also enhanced. Furthermore, nonlinear mode coupling equations for long and short wavelength drift waves are useful for studying the formation of structures and zonal flows in nonuniform collisional dusty magnetoplasmas, such as those in the Earth's mesopshere and cometary tails where collisional interactions between the electrons and ions with neutrals play an essential role. [Preview Abstract] |
Wednesday, November 19, 2008 4:12PM - 4:24PM |
PO5.00012: Modification to the Nyquest Thermal Noise Equation for Plasma Circuit Elements - a high frequency cut off Igor Alexeff, Ted Anderson The Nyquest Thermal Noise Equation for circuit elements is given as $\frac{2}{\pi }RKT$ where R is the resistance in Ohms, K is Boltzmann's constant (Joules per degree Kelvin) and T is the temperature in degrees Kelvin. When applied to plasma circuit elements such as plasma antennas\footnote{ Recent Results for Plasma Antennas, Igor Alexeff, Ted Anderson, Esmaeil Farshi, Naresh Karnam, and Nanditha Reddy Pulsani, Physics of Plasmas 15, 057104-1 (2008).}, this equation predicts a large amount of thermal noise, since plasmas are in general much hotter than metal circuit elements. However, this equation is an approximation in that it assumes that the electron collision frequency in the conductor is much higher than the frequency being studied. We have extended this equation as follows $\frac{2RKT}{\pi }(\frac{1}{1+(\frac{2\pi \nu }{\nu _c })^2})$ where $\nu$ is the frequency of interest and $\nu _c$ is the electron collision rate. Actually in our research in the 2 GHz frequency region, we find that the electron collision rate is much lower than the applied frequency. This results theoretically in a plasma antenna having much less thermal noise than a metal antenna. [Preview Abstract] |
Wednesday, November 19, 2008 4:24PM - 4:36PM |
PO5.00013: Neutral Depletion and Ion Acceleration in a Flowing High-Power Argon Helicon Plasma C. Mark Denning, Matt Wiebold, John Scharer Steady state measurements are performed on an argon helicon plasma with a static axial magnetic nozzle field (1 kG source, 1.5 kG nozzle peak)[1]. Flow rates are between 22 and 150 sccm with incident 13.56 MHz rf power levels of between 300 and 3000 W. Collisional-radiative (CR) models for Ar II and Ar I are used to spectroscopically determine the electron temperature (T$_{e})$ and the neutral density, respectively. The electron density (n$_{e})$ is measured with 105 GHz microwave interferometry and is an input to the CR models. In regions of low neutral depletion, where the ions are collisional with neutrals, T$_{e}$ remains constant while n$_{e}$ rises linearly with increasing power. In collisionless, high depletion regions, T$_{e}$ rises linearly with power while n$_{e}$ remains constrained. Regions of pressure balance and pressure gradients are present, and evidence of axially accelerated ion flows is observed. The ion energy distribution function is measured using tunable diode laser-induced fluorescence to determine the effect of neutral depletion on the axial ion velocity. Research supported by AFOSR Grant No. FA9550-06-1-0172. [1] C. M. Denning, M. Wiebold, J. Scharer, accepted for publication, Phys. Plasmas, 2008. [Preview Abstract] |
Wednesday, November 19, 2008 4:36PM - 4:48PM |
PO5.00014: Volt-Ampere characteristics and the anatomy of gas discharges Zoran Petrovi\'c, Dragana Mari\'c, Gordana Malovi\'c, Nikola \v{S}koro, Marija Radmilovi\'c-radjenovi\'c We are presenting time resolved recordings of the spatial profile (radial and axial) of cylindrically symmetric discharges recorded by ICCD camera. These pictures are closely associated with temporal development of voltage and current. Volta-Ampere characteristics are recorded in order to test the pd, jd$^2$ and E/N scaling in such discharges and we have performed measurements both and standard, cm size discharges and for micro discharges. Most importantly we have recorded carefully the area of the discharge in order to make proper normalization of the total measured current into current density j. The proper breakdown voltage versus current density normalized by the square of the gap length characteristics is obtained where entire glow discharge is a single point as expected by the basic phenomenology. This has never been directly proven by measurements. We have also found that the Paschen curves and Volt- Ampere characteristics hold well, if properly normalized down to 200 micro meters. We have also found several modes, presumably associated with spatial profiles in hollow cathode and even in parallel plate discharges when radial dimension exceeds the mean free path by a large amount. We have also found evidence that flat Paschen curves recorded to the left of the minimum are in some cases due to the long path breakdown. This work was funded by project 155 of the Serbian Academy of Sciences and arts. [Preview Abstract] |
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