Session NO6: Basic Plasma Physics: Dusty and Low Temperature Plasmas

Agglomeration of Microparticles in Complex Plasmas

Dust agglomeration plays an important role in astrophysics, atmospheric science, fusion physics as well as plasma physics. Usually, microparticles in plasmas acquire rather high negative charges on the surface so that the Coulomb repulsion prohibits their agglomeration. In our experiment, we excited strong dust-acoustic waves by decreasing gas pressure in a capacitive discharge. Using high-speed camera during this unstable regime, it was possible to resolve the motion of individual microparticles and to show that the relative velocities of a fraction of particles were sufficiently high to overcome the mutual Coulomb repulsion and hence to result in agglomeration. After increasing pressure to a level where the dust cloud was stabilized, we observed the agglomerates directly with a long distance microscope. The agglomeration rate deduced from our experiments is in good agreement with theoretical estimates. The aggregates are stable since the attractive force due to van de Waals interaction and ``charge discreteness'' can provide a sufficient binding of highly charged microparticles in plasmas. Our experiments unambiguously show, for the first time, that even highly charged microparticles can form stable agglomerates.   

Laboratory observation of naturally occurring dust-density waves

Dust-density waves are electrostatic compressional waves that propagate in dusty plasma. Compared to the more familiar ion-acoustic waves, in dust-density waves, inertia is provided by dust particles (instead of ions) while pressure is provided by the ions as well as electrons (not just electrons). In a laboratory experiment, we observed dust density waves in a 3D void-free dusty plasma. The waves occur naturally due to an ion-flow instability. Dust particles (4.8 microns) are levitated within the volume of a glass box that rests atop an electrode in a radio-frequency glow discharge plasma. Horizontal confinement of dust particles is provided by the plasma's natural electric field that is enhanced by the walls of the glass box, while vertical confinement is due to the electrode's sheath. We observed dust-density waves with planar wave fronts propagating in alignment with flowing ions. By directly imaging the dust particles with a 500 frame-per-second camera, we monitor the dust density modulations in both space and time. A typical wave propagates at 40 mm/s with a frequency of 24 Hz. In this work, we characterize these waves and their growth as they propagate.   

Mode coupling for waves in a single-layer dusty plasma crystal

Mode coupling of waves in a dusty plasma was experimentally observed. Polymer microspheres (8 $\mu $m) were introduced into an rf plasma, where they became negatively charged and confined in a single layer. They self organized in a crystalline-like lattice. There are dispersion relations for longitudinal and transverse modes for in-plane motion, and a transverse mode for out-of-plane motion. Due to a negative bias on the lower electrode, ions flow by each particle, and they are focused so that the wake downstream of each particle has a net positive space charge. This space charge, which moves along with the upstream particle, alters the interparticle interaction so that a vertical displacement of one particle results in an enhanced horizontal force on another particle. As a result, the wave dispersion relations are modified by the ion wake, especially where the two dispersion relations cross, in a phenomenon termed mode coupling. In an experiment, we observed mode coupling as well as some previously unpredicted modes, including one that we term the non-dispersive mode and a hybrid mode. This work was supported by NSF and NASA.   

Equilibrium structure and perturbative dynamics of three-dimensional dust clouds suspended in a glass box

In a modified GEC cell, dust particles are suspended above the powered lower electrode. The electrode can be cooled or heated in order to produce a vertical temperature gradient in the background gas, resulting in additional thermophoretic forces. Three dimensional dust clouds are formed by using a glass box (placed on the lower electrode) to vertically extend the radial confinement of the dust particles. Information about the horizontal confinement is obtained from single particle trajectories inside the glass box. The three-dimensional equilibrium structure is then reconstructed by combining vertical and horizontal slices through the dust cloud, captured by CCD cameras. By applying perturbations to the discharge power and DC bias, transitions between different equilibrium structures are captured using high speed cameras. The crystal structure in a horizontal and vertical slice is obtained both before and after the perturbation, as well as the particle trajectories during the transition. Finally, the experimental results are compared with numerical results from an N-Body model.   

Spectrum Studies of a One-Dimensional Vertical Dust String

A vertical dust string is formed at the center of a glass box placed on the lower electrode inside a GEC cell. Through modulation of an external DC bias applied to the lower electrode, the oscillation spectrum of this one-dimensional vertical dust string was investigated. Individual particle oscillation amplitudes, resonance frequencies, damping and relative oscillation phases were compared in order to analyze the interparticle interactions within the string. Both experimental and analytical results will be presented.   

Interactions between multiple sized particle bilayers

Numerous experiments have been conducted on dusty plasmas using monodisperse micron-sized dust particles. In a GEC reference cell, these dust particles levitate above the lower electrode at a height where the gravitational and the electrostatic forces on the particle are equal. In this case, particles with the same charge to mass ratio float at nearly the same height, creating a thin layer. By adding particles with a different charge to mass ratio, distinct layers can be formed which often exhibit interesting interactions with one another. For instance, particles in the lower layer will align with particles within the upper layer forming vertical chains due to the ``ion wakefield'' which creates an asymmetric force between them. In this paper, vertical interactions between layers of dust for various particle sizes, numbers of particles, and layer separations will be discussed.   

Mode spectra and mode conversion of dust particle clusters in complex plasmas

Vibrational mode spectra are obtained for small (2 $\le $ N $<$ 10) dust particle clusters in complex plasmas using both an analytical method and numerical simulation. The clusters are modeled as dust particles levitated in the sheath above the lower electrode in a GEC rf reference cell. Both the wake field effect and height-dependent charge variation are taken into account. Dependence of mode frequencies on the wake field and charge variation is analyzed. Mode conversions occur as the frequencies of different modes approach each other. These mode conversions are identified and experimental measurements of the modes and conversions are compared with the analysis of theoretical results.   

Interaction between two dust grains immersed in plasma

We have used a 2-D PIC simulation code in cylindrical (r-z) geometry to calculate self-consistently the charge on each of two dust grains immersed in plasma, and the complete plasma-mediated interaction force between the grains. The results differ from the charge and shielded electrostatic potential of a single grain, due to nonlinear response of the plasma to the presence of two nearby grains. The calculation includes charge-exchange collisions, trapped ions, and the attractive shadowing force due to anisotropic momentum-depositing collisions of ions with the grains. The results will be compared with theoretical models.   

Effects of Charged-Magnetic Grains in Protoplanetary Disks

The interaction and growth of dust grains is an important process in early planetesimal formation. The structure of aggregates formed from dust depends largely on the initial properties within the dust population, whether the grains are charged or uncharged, magnetic or non-magnetic. Theoretical simulations examining pair-wise interactions between aggregates indicate that charged magnetic grains exhibit different growth behavior than populations consisting of exclusively charged or exclusively magnetic grains. This study extends that work to predict how charged-magnetic grains influence grain growth within an astrophysical environment. An N-body simulation containing various mixtures of dust materials is used to examine the differences in dust coagulation in the presence of charged magnetic aggregates. The growth of the dust aggregates is analyzed to determine the effects that charged magnetic grains contribute to the evolution of the dust cloud. Aggregate structure is also analyzed to determine how the morphology differs from aggregates built completely from ballistic collisions to determine how gas-coupling changes as aggregates grow to larger sizes.   

Charging of aggregates in the heliosphere

Dust in space environments is often charged by photoelectron emission, collection and recombination of plasma particles, secondary electron emission, thermionic emission and field emission. The relative importance of each charging process depends on the radiation and plasma environment as well as on size and material composition of the grains. Data from spacecraft in situ observations show that Local Interstellar Cloud (LIC) dust particles with masses $>$ 10$^{-17}$ kg most likely have an aggregate structure and consist of smaller particles. We have developed a 3D model for the charging of interstellar aggregate dust particles exposed to solar radiation within the heliosphere. Using a self-consistent iterative approach, the equilibrium charge and dipole moment are calculated for the dust aggregate at different locations of the heliosphere. The deflection of interstellar dust in the magnetic field near the heliopause is investigated. We also examine the porosity effect on the photoelectron emission and secondary electron emission from fluffy aggregates.   

Naturally Occurring Dusty Plasmas in the Solar Wind

Interplanetary Field Enhancements (IFEs) appear as smoothly varying cusp-shaped enhancements in the interplanetary magnetic field which last minutes to many hours. They are seen throughout the inner solar system from 0.3 AU to 5 AU, at a rate of close to one per month. Multispacecraft observations show that these enhancements travel at the solar wind velocity. They produce a pressure ridge in the plasma pressure that acts against the solar wind flow on one side and pushes outward on the other side, possibly carrying charged dust outward. Such mass must consist of individual dust grains of only a few nm in diameter. To have the observed coherent interaction over distances much larger than the Debye length, the dust cloud must be sufficiently dense over those distances to form a dusty plasma. Such dusty plasma clouds could be created by releases from comets or in collisions between meteoroids. We present the observational evidence for such dusty plasma and discuss the charging processes and dynamic processes of these dust particles.   

The impulse exerted on the outward particle flux from a plasma ball

A plasma ball has been produced near the anode in a configuration that, when magnetized, operates as a radial plasma source (RPS) [1]. The plasma particle flux outward of the plasma ball seems to be larger than that expected by the Langmuir relation in double layers [2]. The forced oscillations of a pendulum induced by the flow in the vicinity of the plasma ball are also of an unexpectedly large amplitude. We examine the possibility that the ions gain most of the momentum in the quasi-neutral plasma rather than in the double layer. The impulse enhancement is suggested to result from ion-neutral collisions in the plasma. The electric force is being felt by ions for a longer time; their residence time in the acceleration region is increased due to their slowing-down collisions with neutrals. We previously suggested the ion-neutral collisions as a source of impulse enhancement in the RPS of a radially- outward flow with magnetized electrons. [1] G. Makrinich and A. Fruchtman, Phys. Plasmas 16, 043507, 2009; Appl. Phys. Lett. 95, 181504 (2009). [2] I. Langmuir, Phys. Rev. 33, 954 (1929); B. Song, N. D'Angelo, R.L. Merlino, J. Phys. D: Appl. Phys. 24, 1789 (1991).   

The potential of a cylindrical emissive probe

When a probe is emissive, the electron flux from the plasma is partially balanced by the opposite emitted electron flux, so that the probe floating potential gets closer to the plasma potential. Emissive probes can then be used for estimating the plasma potential. However, it is desirable to know how small the voltage beteen the emissive probe and the plasma is. It can be shown analytically that for an emissive planar probe, the plasma-probe voltage is reduced from 5.2 Te for argon to about 0.95 Te. For a cylindrical probe, the voltage is smaller than for the planar probe. Chen and Arnush calculated the dependence on the ratio of the Debye length and the probe radius of the voltage between a non-emissive cylindrical probe and a plasma [1]. We extend their analysis to an emissive cylindrical probe. As in [1], we take the ion angular momentum as zero, so that orbital motion effects are absent [2]. We find how the voltage between an emissive cylindrical probe and a plasma is reduced as the emitted current increases and reach saturation, and calculate how this voltage decreases when the ratio of Debye length and probe radius increases. [1] F. F. Chen and D. Arnush, Phys. Plasmas 8, 5051 (2001). [2] J. E. Allen, Physica Scripta 45, 497 (1992).   

Temporal and spatial characteristics of Dielectric Barrier Plasma Actuators

The temporal and spatial images of surface dielectric barrier plasma actuators with a wire/planar electrode combination and 1 kHz triangular applied voltages were taken using an intensified charge-coupled device (ICCD) camera and in addition a monochromator was used to measure the optical emission lines of discharges in air. Parallel and perpendicular images recorded with respect to discharge surface. Time resolved (nanosec order) images were used to calculate the discharge speed. Arc shaped discharges jumping from above the powered wire across the dielectric above the grounded planar electrode were observed for the first time. Nitrogen molecular second and first positive band lines were measured and electron and molecular temperatures were calculated using the SPECAIR code (Laux et al. Plasma Sources Sci. Technol. 2003,\textbf{12},1255-138) and the Boltzmann plot method. The electron temperatures were found to be 6020 $\pm $ 1200 and 6800$\pm $400 K from the Boltzmann method and SPECAIR code respectively.