Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session NO6: Complex Plasmas |
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Chair: Jeremiah Williams, Wittenberg University Room: Hanover FG |
Wednesday, November 4, 2009 9:30AM - 9:42AM |
NO6.00001: Observation of frequency cutoff for self-excited dust acoustic waves V. Nosenko, S.K. Zhdanov, G.E. Morfill, S.-H. Kim, J. Heinrich, R.L. Merlino Complex (dusty) plasmas consist of fine solid particles suspended in a weakly ionized gas. Complex plasmas are excellent model systems to study wave phenomena down to the level of individual ``atoms''. Spontaneously excited dust acoustic waves were observed with high temporal resolution in a suspension of micron-size kaolin particles in a dc discharge in argon. Wave activity was found at frequencies as high as 400 Hz. At high wave numbers, the wave dispersion relation was acoustic-like (frequency proportional to wave number). At low wave numbers, the wave frequency did not tend to zero, but reached a cutoff frequency $f_c$ instead. The value of $f_c$ declined with distance from the anode. We propose a simple model that explains the observed cutoff by particle confinement in plasma. The existence of a cutoff frequency is very important for the propagation of waves: the waves excited above $f_c$ are propagating, and those below $f_c$ are evanescent. [Preview Abstract] |
Wednesday, November 4, 2009 9:42AM - 9:54AM |
NO6.00002: Laboratory observations of self-excited dust acoustic shock waves Robert L. Merlino, Jonathon R. Heinrich, Su-Hyun Kim Dust acoustic waves have been discussed in connection with dust density structures in Saturn's rings and the Earth's mesosphere, and as a possible mechanism for triggering condensation of small grains in dust molecular clouds. Dust acoustic waves are a ubiquitous occurrence in laboratory dusty plasmas formed in glow discharges. We report observations of repeated, self-excited dust acoustic shock waves in a dc glow discharge dusty plasma using high-speed video imaging. Two major observations will be presented: (1) The self-steepening of a nonlinear dust acoustic wave into a saw-tooth wave with sharp gradient in dust density, very similar to those found in numerical solutions [1] of the fully nonlinear fluid equations for nondispersive dust acoustic waves, and (2) the collision and confluence of two dust acoustic shock waves. \\[4pt] [1] B. Eliasson and P. K. Shukla, Phys. Rev. E 69, 067401 (2004). [Preview Abstract] |
Wednesday, November 4, 2009 9:54AM - 10:06AM |
NO6.00003: Externally driven dust density waves at low and high modulation frequencies Iris Pilch, Torben Reichstein, Alexander Piel An individual dust cloud, confined in an anodic glow discharge, immersed in a radio-frequency-produced plasma, is shown to support self-excited dust density waves (20-30 Hz), if the number of dust grains is sufficiently large. When the anode voltage is modulated at low frequencies (1-15 Hz) the self-excited wave is disturbed by a sloshing and stretching motion of the entire dust cloud. For higher frequencies (16-60 Hz), the self-excited wave is entrained at the external modulation frequency but a spatial wave decay can be found for frequencies above the eigenfrequency of the self-excited wave. If the frequency is further increased, the wave is not anymore entrained at the modulation frequency but the wave propagation is more coherent compared with the self-excited wave. We report on these dynamical features of externally modulated dust density waves. [Preview Abstract] |
Wednesday, November 4, 2009 10:06AM - 10:18AM |
NO6.00004: Time-dependent correlation buildup in spherical Yukawa balls Hanno Kaehlert, Michael Bonitz In recent years it has become possible to create 3D dust crystals in experiments~[1], where the particles arrange on concentric spherical shells. Compared to confined ions the interaction between the dust particles is screened, which has been shown to affect the shell occupation of the ground state~[2], and the probability of metastable states~[3,4]. Here we study dynamical processes in a trapped Yukawa plasma by means of Langevin dynamics simulations, which fully include the Coulomb correlations, the confinement and friction with the neutral gas. By cooling a weakly correlated initial state towards the strong coupling regime, the formation of concentric shells is observed. While in systems with Coulomb interaction the shells clearly emerge at the cluster boundary, they appear almost simultaneously for sufficiently large screening. Monte Carlo simulations are used to show that the sequence, in which radial order is established, is determined by the confinement potential.\\[4pt] [1] O. Arp, D. Block, A. Piel, and A. Melzer, PRL \textbf{93}, 165004 (2004)\\[0pt] [2] H. Baumgartner et al., New J. Phys. \textbf{10}, 093019 (2008)\\[0pt] [3] D. Block et al., Phys. Plasmas \textbf{15}, 040701 (2008)\\[0pt] [4] H. K\"{a}hlert et al., Phys. Rev. E \textbf{78}, 036408 (2008) [Preview Abstract] |
Wednesday, November 4, 2009 10:18AM - 10:30AM |
NO6.00005: On the influence of wakefields on 3D particle arrangement Mattias Kroll, Jan Schablinski, Dietma Block, Alexander Piel So far basically two different particle arrangements have been observed in 3D dust clouds. On the one hand,plasma crystals, trapped in the lower plasma sheath, feature among the hexagonal structure within the horizontal plane a distinctive chain structure in vertical direction. On the other hand, Yukawa balls show an isotropic structure of concentric shells [1]. The difference in structure is caused by the directed ion flow in the sheath. Because of this ion flow, the shielded interaction between particles is not the isotropic Yukawa potential, but rather takes the form of a wakefield [2]. However, the picture of either particle chains or hcp- odered dust clouds is too simple. The structure of plasma crystals is strongly affected by the confinement. In this contribution experiments with two particles trapped in the lower plasma sheath are presented. The particles were manipulated by a laser and their response is analyzed. Stereoscopic digital in-line holography as a novel diagnostic for dusty plasmas allows to observe the particle trajectories with high spatial and temporal resolution. The excitation of metastable particle positions will be demonstrated and their role for the 3D particle arrangement in dust clouds is discussed.\\[4pt] [1] O. Arp et al., Phys. Rev. Lett. 93, 195002 (2004)\\[0pt] [2] A. Melzer et al., Phys. Rev. E, Vol. 54 (1996) [Preview Abstract] |
Wednesday, November 4, 2009 10:30AM - 10:42AM |
NO6.00006: Neutral Wind Effects on 3D-Clusters in RF Plasmas at Low Pressure Franko Greiner, Jan Carstensen, Lu-Jing Hou, Alexander Piel For the investigation of 2D dust clusters in RF plate reactors the neutral gas pressure is typically around 10Pa. In this regime, the flow is Knudsen like, i.e. the flow regime is between the hydrodynamical and the molecular dynamical flow regime. In this regime, a rotating electrode disk can be used to introduce a shear flow rotation of the neutral gas column. Using 2D dust clusters as high precision velocity detectors we found, that the flow can be described by hydrodynamic equations with no-slip boundary conditions.The fact that viscous neutral gas flows are possible even at low pressure can explain the observation,that the rotation of 2D dust clusters in a magnetic field parallel to the electric field of the plasma boundary, show rotation frequencies which are much higher then predicted. The ``standard model'' for rotation of dust in a magnetic field by pure ion wind force is based on the assumption, that neutral gas flows are not possible [POP 16, 013702, 2009]. In addition the rotating electrode can be used as a new technique to introduce torque to the dust grains without any drawback of the plasma. This torque can be used to modify horizontal confinement of the dust clusters. In this way the effective screening length in the plasma boundary can be measured without any prior knowledge of the plasma parameters. [Preview Abstract] |
Wednesday, November 4, 2009 10:42AM - 10:54AM |
NO6.00007: Oscillatory Particle Motion in Dusty Plasma under Microgravity Conditions John Goree, Bin Liu, V.E. Fortov, A.M. Lipaev, V.I. Molotkov, O. Petrov, G.E. Morfill, H.M. Thomas, R. Rothermel, A. Ivlev In an experiment aboard the International Space Station, performed using the PK-3 Plus instrument, 6.8-micron diameter polymer microspheres were introduced into a neon RF discharge at 0.2 Torr. The microspheres became highly charged in this dusty plasma. Microspheres were tracked by video micrography. The suspension had a void, and on one side there was only a single layer of microparticles. The random motion in this single layer was analyzed by spectral analysis methods, revealing the presence of oscillations. Microspheres oscillate about equilibrium positions determined by confining forces. The transverse, or out-of-plane, motion had a resonance frequency that allows us to compute a force constant $k=2\times10^{-10}$ Nm$^{-1}$ for the radial confining forces. These forces, $QE$ and ion drag, are equal and opposite. Their magnitudes correspond to an acceleration in the range $0.2 - 0.4g$ [Preview Abstract] |
Wednesday, November 4, 2009 10:54AM - 11:06AM |
NO6.00008: Void formation and closure under microgravity for different gases Victor Land, Diana Bolser, Lorin Matthews, Truell Hyde A self-consistent 2D dusty plasma fluid model used to model micro-gravity experiments in argon has been extended to allow the modeling of dusty plasmas in additional noble gases, including helium, krypton, neon, and xenon. The electron transport coefficients were obtained using BOLSIG+[1], whereas the ion transport coefficients were obtained from many literature sources, together with the thermal conductivity of the gases and the thermal accommodation coefficients. In this presentation we focus on the formation and closure of voids in microgravity experiments at different pressures and driving potentials, with the different noble gases. The goal is to provide a two-dimensional plot of the pressures and potentials at which 3D dust clouds under microgravity contain a dust-free void, or are void-free, for different carrier gases. This plot could serve as a road map for future experiments. \\[4pt] [1] G. J. M. Hagelaar and L. C. Pitchford, Plasma Sources Sci. Technol. 14 (2005) 722-733 [Preview Abstract] |
Wednesday, November 4, 2009 11:06AM - 11:18AM |
NO6.00009: Interparticle Forces Between Dust Particles Confined within a Glass Box in a GEC Chamber Jie Kong, Truell Hyde, Lorin Matthews, Ke Qiao, Zhuanhao Zhang, Brandon Harris, Gary Shetler, Steve Rapp, Jimmy Schmoke, Mike Cook The additional confinement provided by a glass box placed on the lower electrode of a GEC rf reference cell easily allows the generation of individual long vertical dust chains. This extended 1D~vertical dust structure is ideal for the investigation of dust crystal vertical dispersion relations. It can also be used as a probe to allow the investigation of plasma parameters within the sheath. In this work, the oscillation spectrum of an extended 1D dust particle chain, driven through the addition of an external DC bias applied to the lower electrode, was examined. The vertical oscillation spectrum obtained using this experimental setup will be discussed and shown to exhibit significant differences as compared to the spectrum obtained for a dust particle pair without the glass box confinement. [Preview Abstract] |
Wednesday, November 4, 2009 11:18AM - 11:30AM |
NO6.00010: Investigation of the Wakefield Interaction within 3D Dust Particle Clusters in a Complex Plasma Ke Qiao, Truell Hyde, Lorin Matthews, Jie Kong, Jorge Carmona Reyes, Zhuanhao Zhang, Jimmy Schmoke, Mike Cook Two-dimensional (2D) dust particle systems confined to a horizontal plane within a capacitively coupled rf discharge have been studied extensively over the past decade. However three-dimensional (3D) clusters, where the wake field interaction between particles often plays a major role, have not yet received the same amount of attention. In this research, stable 3D dust particle clusters, consisting of 1-5 particles, were formed in a complex plasma confined within a glass box. These clusters were examined over a range of powers in order to determine the various structural phases allowed by the system. As rf power was increased, the system was observed to exhibit a series of structural phase transitions. (For example, a 3-particle cluster can exist as a linear chain, a triangle in the vertical plane or a triangle in the horizontal plane.) Analysis employing a force-balance method was used to quantitatively determine the wakefield interaction between the particles. [Preview Abstract] |
Wednesday, November 4, 2009 11:30AM - 11:42AM |
NO6.00011: A Study of the Resonance Frequency of Conductive Grains in a Dusty Plasma Zhuanhao Zhang, Truell Hyde, Lorin Matthews, Jie Kong, Jorge Carmona Reyes, Ke Qiao, Jimmy Schmoke, Mike Cook Over the past decade, the dynamic behavior of insulating (melamine formaldehyde) dust particles immersed in a complex plasma have been studied extensively. Phenomena such as disordered or ordered structures, the interaction of dust particles with the wake field produced by the ion flow, and the interaction between particles have all been the subject of both experimental and numerical investigation. However, the investigation of conducting materials in dusty plasmas has not yet received the same attention. In this work, the resonance frequency of conductive grains at various pressures and powers within a standard GEC reference cell is examined and the results compared to those for insulating grains (MF). Since the resonance frequency of a dust grain is proportional to its charge-to-mass ratio, such studies are beneficial in helping determine the particle charge. Preliminary results will be presented showing that in similar plasma environments, conducting grains do not exhibit the same behavior as non-conducting grains possibly due to differences in the charging process. [Preview Abstract] |
Wednesday, November 4, 2009 11:42AM - 11:54AM |
NO6.00012: Layer Splitting in a Complex Plasma Bernard Smith, Truell Hyde, Lorin Matthews, Megan Johnson, Mike Cook, Jimmy Schmoke 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. An interesting facet of complex plasma behavior is that particle layers appear to split as the DC bias is increased. This splitting of layers points to a phase transition differing from the normal phase transitions found in two-dimensional solids. In 1993, Dubin noted that as the charged particle density of an initially two-dimensional Coulomb crystal increases the system's layers split at specific charge densities. This work modeled ions in a Paul or Penning trap, but may be applicable to dusty plasma systems as well. This work will discuss this possibility along with splitting observed in the CASPER GEC rf Reference Cell at specific pressures and powers. [Preview Abstract] |
Wednesday, November 4, 2009 11:54AM - 12:06PM |
NO6.00013: Potential Structure Formation in the Gas-Liquid Interfacial Plasmas Toshiro Kaneko, Takashi Harada, Qiang Chen, Rikizo Hatakeyama Specific kinds of plasmas composed of ionic liquids and gas phase discharge plasmas are created. These novel gas-liquid interfacial plasmas could contribute to the effective creation and modification of nano-bio materials, where the control of ion behavior in both the gas and liquid phases is desired. In this study, the plasma source containing the ionic liquids is developed for clarifying the effects of the ionic liquids on the plasma generation and the potential structure formed in the gas-liquid interfacial region is investigated for controlling the ion motion A direct current (DC) or a pulsed DC discharge plasma is generated just above the ionic liquid by applying the DC or the pulsed DC voltage to an electrode immersed in the ionic liquid against a grounded electrode set in the gas phase region. The precise potential structure between these electrodes through the gas-liquid interfacial region is clarified, and the dynamics of the plasma ions is found to be controlled using the sheath electric field in the interfacial region. The control of the plasma ion irradiation flux and energy to the ionic liquid leads to the creation of various kinds of nanoparticles. [Preview Abstract] |
Wednesday, November 4, 2009 12:06PM - 12:18PM |
NO6.00014: Scaling Studies of Pair Production by Ultraintense Lasers Edison Liang, Alexander Henderson, Tal Einav, Pablo Yepes, Hui Chen, Scott Wilks Using a combination of particle-in-cell plasma kinetic simulations and the CERN GEANT code for pair production, we systematically study the pair production by ultraintense lasers irradiating gold targets. We will present results for the pair production yield as a function of laser and target parameters. Using these we will develop road maps for future experiments to optimize the pair yield. Applications of these results to both laboratory astrophysics and high density positronium physics will be discussed. [Preview Abstract] |
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