Victor Land

Charging and Coagulation of Dust in Protoplanetary Plasma Environments

Combining a particle-particle, particle-cluster, and cluster-cluster agglomeration model with an aggregate charging model, the coagulation and charging of dust particles in plasma environments relevant for protoplanetary disks have been investigated, including the effect of electron depletion in high dust density environments. The results show that charged aggregates tend to grow by adding small particles and clusters to larger particles and clusters, and that cluster-cluster aggregation is significantly more effective than particle-cluster aggregation. Comparisons of the grain structure show that with increasing aggregate charge the compactness factor, σ, decreases and has a narrower distribution, indicating a fluffier structure. Neutral aggregates are more compact, with larger σ, and exhibit a larger variation in fluffiness. Overall, increased aggregate charge leads to larger, fluffier, and more massive aggregates.

CHARGING OF AGGREGATE GRAINS IN ASTROPHYSICAL ENVIRONMENTS

The charging of dust grains in astrophysical environments has been investigated with the assumption that these grains are homogeneous spheres. However, there is evidence which suggests that many grains in astrophysical environments are irregularly shaped aggregates. Recent studies have shown that aggregates acquire higher chargeto-mass ratios due to their complex structures, which in turn may alter their subsequent dynamics and evolution. In this paper, the charging of aggregates is examined including secondary electron emission and photoemission in addition to primary plasma currents. The results show that the equilibrium charge on aggregates can differ markedly from spherical grains with the same mass, but that the charge can be estimated for a given environment based on structural characteristics of the grain. The “small particle effect” due to secondary electron emission is also important for de terming the charge of micron-sized aggregates consisting of nano-sized particles.

Determination of the levitation limits of dust particles within the sheath in complex plasma experiments

Experiments are performed in which dust particles are levitated at varying heights above the powered electrode in a radio frequency plasma discharge by changing the discharge power. The trajectories of particles dropped from the top of the discharge chamber are used to reconstruct the vertical electric force acting on the particles. The resulting data, together with the results from a self-consistent fluid model, are used to determine the lower levitation limit for dust particles in the discharge and the approximate height above the lower electrode where quasineutrality is attained, locating the sheath edge. These results are then compared with current sheath models. It is also shown that particles levitated within a few electron Debye lengths of the sheath edge are located outside the linearly increasing portion of the electric field.

Dust particle charge in plasma with ion flow and electron depletion near plasma boundaries

The charge on micrometer-sized dust particles suspended in plasma above the powered electrode of radio-frequency discharges is studied. Using a self-consistent fluid model, the plasma profiles above the electrode are calculated and the electron depletion towards the electrode, as well as the increasing flow speed of ions toward the electrode are considered in the calculation of the dust particle floating potential. The results are compared with those reported in literature and the importance of the spatial dust charge variation is investigated.

Manipulating Dust Charge Using Ultraviolet Light in a Complex Plasma

Making use of a 1-D particle-in-cell plus Monte Carlo code, which includes the scattering and collection of plasma particles on dust immersed in the plasma, we study the effect of photodetachment by ultraviolet (UV) photons in a radio frequency dusty argon plasma. This is done by adding a UV source in the model, which results in a UV flux entering the plasma from the left side. The dust charge is reduced for dust clouds consisting of 5- mum particles at a dust density relevant for both experiments and plasma applications. Using UV photons with energies above the work function, but below the ionization threshold, at fluxes of a few tens of watts per square meter, provides an interesting tool to control the dust charge in such plasmas. This could result in a way to control the dust particle coagulation or dust transport. For very high dust densities, the dust charge is not reduced by the incident UV photons

Probing the Sheath Electric Field With a Crystal Lattice by Using Thermophoresis in Dusty Plasma

A 2-D dust crystal levitated in the sheath of a modified Gaseous Electronics Conference reference cell is manipulated by heating or cooling the lower electrode. The dust charge is obtained by measuring the global characteristics of the levitated crystal obtained from top-view pictures. From the force balance, the electric field in the sheath is reconstructed. From the Bohm criterion, we conclude that the dust crystal is levitated mainly above and just below the classical Bohm point.

Experimental and computational characterization of a modified GEC cell for dusty plasma experiments

A self-consistent fluid model developed for simulations of micro-gravity dusty plasma experiments has for the first time been used to model asymmetric dusty plasma experiments in a modified Gaseous Electronics Conference (GEC) reference cell with gravity. The numerical results are directly compared with experimental data and the experimentally determined dependence of global discharge parameters on the applied driving potential and neutral gas pressure is found to be well matched by the model. The local profiles important for dust particle transport are studied and compared with experimentally determined profiles. The radial forces in the midplane are presented for the different discharge settings. The differences between the results obtained in the modified GEC cell and the results first reported for the original GEC reference cell are pointed out.

Hydrodynamic and kinetic modelling of complex radio-frequency plasmas

In this paper hydrodynamic and kinetic approaches to model low-pressure capacitively coupled complex radio-frequency discharges are discussed and applied to discharges under micro-gravity. Complex plasmas contain dust grains with a large negative charge and are characterized by a strong coupling between the properties of the plasma and those of the dust grains. After a discussion of the physics and methods involved, examples are presented from modelling of experiments under micro-gravity in the PKE-Nefedov reactor on board the International Space Station. These discharges are simulated with a 2D cylindrically symmetric hydrodynamic model. Kinetic effects are studied with a 1D particle-in-cell plus Monte Carlo model in which capture and scattering by dust grains is included. Since experiments are often performed at low pressures, the electron energy distribution function is no longer determined by the local plasma properties. This has consequences for the charging of the dust. Results of simulations with this model are compared with the hydrodynamic results. In addition, we address the behaviour of the dust charge in decaying plasmas.

Guest Editorial Special Issue on Dusty Plasmas

This special issue contains papers solicited at the 13th Workshop on the Physics of Dusty Plasmas held May 20-23 in Waco, TX, USA and follows the tradition of earlier workshops, which resulted in such issues in 1994, 2001, 2004, 2007, and 2010. Dusty plasmas constitute a fully developed interdisciplinary field with direct connections to astrophysics, nanoscience, fluid mechanics, and material science as defined through experimental, theoretical, and numerical studies. The field achieved an interdisciplinary research focus when the 1986 fly by of Halleys Comet and Voyagers images of Saturns rings focused the attention of plasma physicists on a field that had previously been considered as purely planetary science. Joint efforts between these disciplines quickly led to rapid advancement in both plasma physics (e.g., discovery of dustacoustic waves, dust-ion-acoustic waves) and the planetary sciences (e.g., spokes in Saturns rings, Jovian stream particles). The primary objectives of the 13th Workshop on the Physics of Dusty Plasmas were to provide a review of recent advancements in the field of complex plasma, define new/existing/outstanding issues and research challenges, and strengthen engagement between the field of complex plasma and other research related disciplines. Researchers from universities all over the world, including twenty-six graduate students evenly split between universities in the United States and international universities, participated in the workshop. Eighty-six paper and poster presentations were accepted covering topics including laboratory, theoretical, and computational studies of dust plasma interactions. Sixteen papers from the Waco meeting appear in this special issue covering topics across a variety of theoretical and experimental areas. Instabilities in dusty plasma systems, the manner in which such instabilities are believed to be linked to the onset of dust density or dust-acoustic waves and the mechanism by which these waves evolve and are related to the ambient plasma are reported. Dust dynamics, mass loss, and a method for employing the plasma glow to probe the plasma sheath in the vertical direction are discussed. A numerical method allowing calculation of the grain charge while including secondary electron emission and the application of this method to the lunar environment is provided. Finally techniques for stereoscopic observations, three-dimensional models of dust plasma interactions and dusty plasma experiments allowing inspection of magnetized dusty plasmas and dusty plasma environments containing chemically reactive gases are reported.

Glow and Dust in Plasma Boundaries

The sheath region is probed in different complex plasma experiments using dust particles in addition to the measurement of the optical emission originating from the plasma. The local maximum in the optical emission coincides with the breaking of quasi-neutrality at the sheath boundary, as indicated by the vertical-force profile reconstructed from dust-particle trajectories as well as by the local onset of dust-density waves in high-density dust clouds suspended in a dielectric box.