Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session NO5: Dusty, Complex, and Strongly Coupled Plasmas |
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Chair: James Danielson, University of California, San Diego Room: Ballroom F |
Wednesday, November 16, 2011 9:30AM - 9:42AM |
NO5.00001: Electron screening and disorder-induced heating in ultracold neutral plasmas Mary Lyon, Scott Bergeson We report measurements on the effect of electron screening on disorder-induced heating (DIH) in strongly coupled ultracold neutral plasmas. DIH is a nonequilibrium, ultrafast relaxation process that occurs when laser-cooled atoms are photo-ionized to make an ultracold plasma. Its effects dominate the ion motion at early times and limit the strong coupling of the system to order unity. Using tools of atomic physics we study DIH with ns time resolution for different plasma densities and temperatures. By changing the frequency of the laser beam we use to probe the ions, we map out the time evolution of the velocity distribution. This allows us to observe and characterize effects due to electron screening on ion equilibration over a wide range of plasma conditions [Preview Abstract] |
Wednesday, November 16, 2011 9:42AM - 9:54AM |
NO5.00002: Wave spectra of strongly coupled magnetized plasmas Hanno Kaehlert, Alexi Reynolds, Torben Ott, Michael Bonitz Results are presented for the wave propagation in a strongly coupled, magnetized one-component plasma. For different angles of the wave vector with respect to the external magnetic field we discuss the dispersion and polarization based on the quasi-localized charge approximation (QLCA) [1]. Further, the results of the QLCA are compared with molecular dynamics simulations, extending previous results for two-dimensional systems, e.g. [2,3]. The dependence of the wave spectra on the coupling parameter and the magnetic field strength is examined.\\[4pt] [1] G. Kalman and K. I. Golden, Phys. Rev. A \textbf{41}, 5516 (1990).\\[0pt] [2] L.-J. Hou, P. K. Shukla, A. Piel, and Z. L. Miskovic, Phys. Plasmas \textbf{16}, 073704 (2009).\\[0pt] [3] M. Bonitz \textit{et al.}, Phys. Rev. Lett. \textbf{105}, 055002 (2010); T. Ott \textit{et al.}, Phys. Rev. E \textbf{83}, 046403 (2011). [Preview Abstract] |
Wednesday, November 16, 2011 9:54AM - 10:06AM |
NO5.00003: Diffusion of strongly correlated plasmas in a magnetic field Torben Ott, Michael Bonitz The self-diffusion properties of dusty plasmas and non-neutral plasmas are of basic importance for a detailed understanding of the dynamics of these systems. In the presence of a magnetic field, the diffusion coefficients for field-parallel and cross- field diffusion need to be investigated separately. We present a molecular dynamics study of a one-component plasma which is both strongly coupled ($ \Gamma>1$) and subjected to a strong magnetic field ($\omega_c/ \omega_p>1$). As in a weakly coupled plasma, the cross-field diffusion scales as $1/B$ in the strong-field limit. The field-parallel diffusion is also strongly affected by the magnetic field and approaches a $1/B$ scaling, in contrast to earlier predictions. \\[1ex] [1] T. Ott, M. Bonitz, arXiv:1106.3717 [physics.plasm-ph] [Preview Abstract] |
Wednesday, November 16, 2011 10:06AM - 10:18AM |
NO5.00004: Growth and nonlinearity in a self-excited dust-density wave J. Goree, T.M. Flanagan Exponential growth and nonlinear saturation of waves in a dusty plasma were observed. Dust particles (4.8 micron polymer spheres) were introduced into an argon radio-frequency plasma. They collected a charge of order $-10^3 e$. Due to a vertical dc $E$ field, these charged dust particles were levitated against gravity and suspended as a 3D cloud within the plasma. A downward ion flow caused an unstable growth of the dust density wave. This wave is like an ion-acoustic wave, but with the inertia provided by heavy dust particles instead of ions, so that it had a low frequency of about 25 Hz. Frictional drag of the dust particles on the neutral gas damped the wave, in competition with the ion-driven instability mechanism. Below a critical gas pressure, a self-excited wave grew in amplitude as it propagated. We measured the exponential growth rate $k_i$ during the wave's linear stage. Nonlinear waves were observed by further reducing the gas pressure. In this nonlinear stage we found that the harmonics had wave numbers and growth rates that were near integer multiples of those for the fundamental. [Preview Abstract] |
Wednesday, November 16, 2011 10:18AM - 10:30AM |
NO5.00005: Viscosity Quantified in a 2D Dusty Plasma Experiment Yan Feng, J. Goree, Bin Liu Experiments were performed in a low-temperature dusty plasma that was a four-component mixture of negatively-charged polymer microspheres, positive argon ions, electrons, and neutral argon gas. The microspheres were electrically suspended in a single layer in the sheath above a lower electrode. Due to their large electric charge, the microspheres were strongly coupled, and self-organized in a crystalline lattice. Laser heating was used to melt the lattice, so that the collection of charged microspheres was a liquid-phase strongly coupled plasma. The viscosity $\eta$ of this liquid was determined using a variant of the fluctuation-dissipation theorem called the Green-Kubo (GK) relation, which relies on the fluctuating random motion of microspheres. Video microscopy allowed the tracking of individual microspheres, yielding time series for their positions and velocities, which are inputs for the GK calculation. The resulting viscosity, for our experiment without macroscopic flow of the microspheres, is in agreement with a previous experiment with a steady macroscopic shear flow. [Preview Abstract] |
Wednesday, November 16, 2011 10:30AM - 10:42AM |
NO5.00006: Dust Dynamics Simulations with the Consideration of Wake Effects Patrick Ludwig, Hanno K\"ahlert, Michael Bonitz, Wojciech Miloch The theoretical description of complex plasmas requires multiscale concepts that adequately incorporate the correlated interplay of streaming electrons and ions, neutrals, and dust grains. By means of an effective dust potential, the multiscale problem can be effectively reduced to a one-component plasma model of the dust subsystem. The statically screened Coulomb (Yukawa) potential has been proven to yield good agreement with the experiments for various specific setups [1,2]. Streaming of the plasma leads, however, to strong deviations from the Yukawa potential by giving rise to a distinct oscillating wake structure behind each grain. Here we present a systematic evaluation of the electrostatic dust potential in the presence of a streaming plasma environment by (i) a high precision computation of the dynamically screened Coulomb potential from the dynamic dielectric function, and (ii) a detailed assessment of these linear response results, particularly, in view of non-linear effects and dynamical grain charging processes by means of self-consistent full 3D PIC simulations [3].\\[0pt] [1] Introduction on Complex Plasmas, M. Bonitz, N. Horing, and P. Ludwig (eds.), Springer (2010) [2] P. Ludwig et al., Plas. Phys. Control. Fusion \textbf{52}, 124013 (2010) [3] P. Ludwig, W.J. Miloch, H. K\"ahlert, and M. Bonitz, submitted for publication in New J. Phys. (2011) [Preview Abstract] |
Wednesday, November 16, 2011 10:42AM - 10:54AM |
NO5.00007: The potential of an emissive dust-grain Amnon Fruchtman, Gennady Makrinich The potential of a sherical dust-grain that emits electrons is calculated. A photoelectric emission is assumed so that there are three types of particles: plasma ions, plasma electrons, and emitted electrons. The ions are assumed collisionless in their motion towards the dust-grain. The energy and angular momentum of the ions in the bulk plasma are assumed zero, so that orbital motion effects are neglected. For a small Debye number (the ratio of the Debye length to the dust-grain radius), we perform a two-scale analysis to determine the potential of the grain relative to the plasma and the fluxes of the various particles. At space charge saturation, we derive approximate analytical expressions for the potential and the fluxes. For a finite Debye number, numerical solutions show that the grain potential comes closer to the plasma potential. We then allow charge - exchange collisions for the ions during their motion towards the grain. It is shown that these collisions increase the potential drop in the presheath, so that the grain potential becomes more negative relative to the plasma. [Preview Abstract] |
Wednesday, November 16, 2011 10:54AM - 11:06AM |
NO5.00008: Nonstationary Stochastic Process of Dust Particle Charging in Plasmas Babak Shotorban The one-step process master equation of dust particle charging [T. Matsoukas and M. Russell, J. Appl. Phys. 77, 4285 (1995)] was expanded through the system size expansion method of Van Kampen and then having made the linear noise approximation, a linear Fokker-Planck equation with a Gaussian solution was derived. The mean and variance of the Gaussian solution are governed by two time-dependent ODEs that can describe nonstationary stochastic charge of the dust particle. A test problem in which the electron and ion currents were calculated through the orbital motion limited theory was solved using these equations. The results were in very good agreement with the results obtained through directly solving the master equation. [Preview Abstract] |
Wednesday, November 16, 2011 11:06AM - 11:18AM |
NO5.00009: Influence of rf oscillations on the dust particle dynamics in an rf plasma discharge C. Horn, M. Davoudabadi, B. Shotorban The dynamics of a dust particle in an argon rf plasma discharge in a low pressure reactor is investigated. The Lagrangian equations of motion along with the electrical charge equation are solved to track the dust particle from its release at the top of the reactor to its steady-state settling location for particles of different size and different density. The electrical, ion drag, neutral drag, and gravity forces act on the particles. The dependency of the dust particle on rf oscillations is explored through a comparison made between two cases. In the first case, the rf period-averaged plasma variables are utilized to solve the dust particle equations whereas in the second case, the instantaneous rf oscillations of the plasma variables are utilized to solve these equations. The dust particle positions and charges, and the forces acting on it have been compared for two cases and significant differences have been found. [Preview Abstract] |
Wednesday, November 16, 2011 11:18AM - 11:30AM |
NO5.00010: Dust Particle Charge in Complex Plasma Experiments Angela Douglass, Victor Land, Ke Qiao, Lorin Matthews, Truell Hyde A self-consistent fluid model which includes effects due to increasing ion flow speed and electron depletion near the powered electrode was used to obtain representative plasma parameters throughout the plasma in a GEC radio-frequency discharge cell. Employing these parameters in the dust charging equation, the dust charge as a function of height above the powered electrode was calculated. The results obtained are used to explain previous, seemingly contradictory data recently presented in the literature as well as results from recent experiments conducted in the CASPER GEC cell. The shape of the electric force is also investigated experimentally and compared to the model. Finally, the maximum and minimum levitation height for charged dust particles within the GEC cell will be discussed. [Preview Abstract] |
Wednesday, November 16, 2011 11:30AM - 11:42AM |
NO5.00011: The Dust Temperature Distribution Within a Dust Density Wave Formed in a Complex Plasma Zhuanhao Zhang, Ke Qiao, Jie Kong, Lorin Matthews, Truell Hyde In recent years, dust particles in complex plasma confined to a glass box placed on the lower powered electrode in a GEC rf reference cell have attracted attention. Phenomena such as dust chains, waves and vortices have all been observed under such conditions and in both terrestrial labs and micro-gravity experiments. This talk will discuss the spatial distribution of the dust temperature within a self-excited dust density wave produced inside a glass box such as the one described. It will be shown that by increasing the discharge power or reducing the neutral gas pressure, the dust cloud in which dust particles form stable chains can become unstable, producing a density wave. Due to instabilities within the wave, particles at different locations can oscillate at different equilibrium positions and different amplitudes, affecting the dust temperature. [Preview Abstract] |
Wednesday, November 16, 2011 11:42AM - 11:54AM |
NO5.00012: Normal Modes and Mode Coupling in a Dust Cluster Ke Qiao, Jie Kong, Zhuanhao Zhang, Lorin Matthews, Truell Hyde Mode couplings in complex plasma crystals have recently received attention both theoretically [1] and experimentally [2, 3]. In this research, the normal modes for a cluster consisting of 3-10 dust particles within a complex plasma are investigated employing a molecular dynamic (MD) simulation. The ion wakefield downstream from each particle and the particle's charge variation as it relates to the particle's location in the plasma are examined independently. The resultant coupling and corresponding resonances between modes will also be discussed and compared to experiment. \\[4pt] [1] V. V. Yaroshenko, A. V. Ivlev, and G. E. Morfill, Phys. Rev. E 71, 046405 (2005). \\[0pt] [2] L. Couedel et al., Phys. Rev. Lett. 104, 195001 (2010). \\[0pt] [3] Bin Liu, J. Goree, and Yan Feng, Phys. Rev. Lett. 105, 085004 (2010). [Preview Abstract] |
Wednesday, November 16, 2011 11:54AM - 12:06PM |
NO5.00013: Using a One-Dimensional Dust String as an In-Situ Probe in a Complex Plasma Jay Kong, Ke Qiao, Brandon Harris, Angela Douglass, Zhuanhao Zhang, Lorin Matthews, Truell Hyde It has recently been shown that dust particles within a complex plasma can be employed as an in-situ diagnostic. In this work a single, a one-dimensional dust string, formed inside a glass box placed on the lower powered electrode within a GEC rf reference cell, is used to investigate the ion drag effect within the sheath inside the box. The dynamic behavior for dust particles located above and below the ion rich region will be shown to be different and an oscillation phase change for dust particles located on opposite sides of this region is shown to exist. The manner in which the ion rich region, which is a function of the ion drifting speed, varies with applied DC bias and/or background pressure and its affect on the dynamic behavior of the dust particles will also be examined. [Preview Abstract] |
Wednesday, November 16, 2011 12:06PM - 12:18PM |
NO5.00014: Employing a Zyvex S100 Nanomanipulator Within a Complex Plasma Brandon Harris, Lorin Matthews, Truell Hyde Particles within a Coulomb crystal formed in the plasma sheath above the powered lower electrode in a GEC reference cell are examined using a Zyvex S100 nanomanipulator. Using the S100, a powered probe is positioned vertically within the cell transverse to the center of the crystal.~~It will be shown that as the probe is lowered toward the horizontal dust lattice plane, a circular dust void is formed. When the probe potential is driven employing a sinusoidal oscillation, an out of phase oscillation develops with dust near the probe moving in a primarily vertical counter clockwise loop and dust far from the probe moving in a primarily vertical clockwise loop. At the critical point between these two extremes, the dust particles oscillate along a linear vertical path due to the force balance between probe attraction, dust repulsion, and the confinement provided by the electrode geometry. A model describing the probe's interaction with the complex plasma system will be discussed and basic plasma parameters within the sheath will be examined. [Preview Abstract] |
Wednesday, November 16, 2011 12:18PM - 12:30PM |
NO5.00015: Effects of dust on the optical emission and electrical properties in a dusty plasma experiment Victor Land, Autumn Paro, Erin Middlemas, Jorge Carmona Reyes, Angela Douglass, Lorin Matthews, Truell Hyde Dust particles in plasma obtain a charge due to the continuous absorption of free electrons and ions from the surroundings. Depending on the dust size and number density, this can significantly alter the local plasma, as well as the global discharge characteristics. Here, we present measurements of the changes in optical emission originating from argon plasma, as well as changes in the electrical properties of the discharge, as a different number of dust particles is introduced into the plasma, with different sizes. Measurement of the electronic signals of the discharge, including the electrode potential, current and derivative signals, allows determination of the complex impedance and with that, determination of changes in the equivalent circuit of the discharge. Optical emission, on the other hand, is a tracer of energetic electrons, since in argon emission is mostly due to relaxation of energetic atomic states resulting from electron-impact excitation. The experimental results are compared with numerical results from a two-dimensional dusty plasma fluid model. We aim to reproduce the size and density dependent equivalent circuit of the suspended dust clouds. [Preview Abstract] |
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