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The PlasmaKristall-4 (PK-4) experiment on the International Space Station (ISS) has been widely used to study collective behavior and self-organization of dust grains immersed in a plasma environment under microgravity conditions. Recently it has been shown that the PK-4 BU, a ground-based replica of the PK-4, supports the presence of inhomogeneous features related to ionization waves moving through the positive column on the microsecond timescale 1 . Plasma conditions within the ionization waves, such as plasma density and electric fields, have been shown to vary by up to an order of magnitude from background values. Previous investigations of dust dynamics within the PK-4 environment have focused on time-averaged plasma conditions, assuming a homogeneous environment. Given the importance of plasma density and electric fields for dust self-organization, it is important to understand the impact of their observed variability. Here results will be presented from an N-body simulation which models dust and ion dynamics, employing initial boundary conditions derived from a 2D particle-in-cell with Monte Carlo collisions (PIC/MCC) simulation of the observed ionization waves.

The role of charged, solid particulate matter (i.e., "dust"’ in plasmas has been considered for several decades. However, beginning with observations of structures in the dust tails of comets and the Voyager observations of radial structures ("spokes"’ in Saturn’s rings, the role of charged dust in the solar system led to the emergence of the field of "dusty" (alternatively, "complex"’ plasmas. Since the early 1990’s there has been a complementary effort in the area of laboratory dusty plasmas that has, over almost four decades, led to the observation of numerous types of plasma and dusty plasma behavior – ranging from appearance of strongly-coupled, self-organized dusty plasma crystals to new types of collective, dust-driven modes, such as the dust acoustic and dust density waves.

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Complex plasmas contain, in addition to the usual electrons, ions, and neutral atoms, macroscopic electrically charged (nanometer to micrometer) sized dust particles. Based on the ratio of the electrostatic potential to kinetic energy, these microparticles can exhibit gaseous, fluid, and crystal-like behavior. For this reason, complex plasmas are a unique testing ground to study multiparticle systems. The dynamics of these systems can be studied using the particle tracking velocimetry (PTV) analysis technique. The PTV technique provides a spatially resolved particle phase space distribution function, which can be used to calculate correlation functions and thermal properties of the system. There are experimental settings, such as microgravity experiments, where technical and data storage limitations make it desirable to have near real-time video analysis techniques that allow an experimenter to tune operating conditions until appropriate velocity profiles or velocity distributions are obtained. This article discusses PTV software that allows for real-time particle tracking that, we believe, can be applied to a broad range of physical systems.

Dust acoustic waves were investigated in an experiment under microgravity conditions, using the European Space Agency-Roscosmos facility PK-4 on the International Space Station. A three-dimensional dust cloud was confined in a glow-discharge plasma powered by a radio-frequency coil in low-pressure neon gas. Low-frequency dust acoustic waves (DAWs) were spontaneously excited due to the flowing ions in the plasma. Dust motion was imaged using video cameras, yielding the position for individual dust particles. Besides these particle-level measurements, the dust was also measured as a fluid continuum to obtain its velocity and density fluctuations. A space–time diagram of the dust-fluid velocity reveals that the waves changed properties as they propagated across the dust cloud, in the direction of the ion flow. Density fluctuations were characterized by a wave spectrum, indicating the emergence of collective vibration modes in a broad range of wave numbers. The decay of the modes was characterized by density correlation functions. Fitting the correlation functions to an analytical model yields an experimental measurement of the dispersion relation for DAWs, which is compared to that of linear theory.

Filamentary structures in low-pressure, low-temperature plasmas are produced when strong magnetic fields are applied parallel to the electric field defined by parallel electrodes. Filamentary structures are regions within the plasma that have distinct properties such as optical brightness and extend along the magnetic field lines. In our experiments, an argon, radio frequency discharge is exposed to a strong background magnetic field in the magnetized dusty plasma experiment at Auburn University. Different forms of filamentary structures emerge in the plasma such as columns, target, or spiral-like structures. To investigate the origin and the characteristics of these patterns, we have developed a three-dimensional fluid model that can reproduce the experimental observations which enables us to investigate the underlying mechanisms of the filamentation process.

First results of complex plasma experiments in microgravity with grid electrodes in the Ekoplasma facility are presented. These electrodes separate the region of plasma production from the working volume where the microparticles are injected. Grid electrodes allow a control of plasma parameters, and provide a nearly force-free environment for the microparticles. The experiments show that the grid electrode method can be applied to complex plasmas and that this technology yields a promising new experimental method for future research topics.

A description is given of the Magnetized Dusty Plasma Experiment (MDPX) at Auburn University that was designed to study the physics of highly magnetized low temperature plasmas and magnetized dusty plasmas. The central components of the MDPX are a superconducting solenoid capable of operation at magnetic fields up to 4 T, and a parallel plate RF discharge plasma. Results from some of the initial experiments will be presented.A description is given of the Magnetized Dusty Plasma Experiment (MDPX) at Auburn University that was designed to study the physics of highly magnetized low temperature plasmas and magnetized dusty plasmas. The central components of the MDPX are a superconducting solenoid capable of operation at magnetic fields up to 4 T, and a parallel plate RF discharge plasma. Results from some of the initial experiments will be presented.

The Magnetized Dusty Plasma Experiment (MDPX) device is a newly constructed research instrument for the study of dusty (complex) plasmas. The MDPX device is envisioned as an experimental platform in which the dynamical behavior of all three charged plasma components, the electrons, ions, and charged microparticles (i.e., the ‘dust’) will be significantly influenced by the magnetic force. This brief paper will provide a short overview of the design, magnetic performance, and initial plasma measurements in the MDPX device.

The last decade has seen the development of new experimental devices to explore the physics of magnetized dusty plasmas. Because of the small charge-to-mass ratio of the charged microparticles, it is necessary to operate these experiments at high magnetic fields of several Tesla in order to observe the direct effect of the magnetic forces on the transport properties of the charged microparticles. While the study of magnetized dusty plasmas is still the ultimate goal, these experiments have also provided new opportunities to studies regimes of strongly magnetized, low temperature, laboratory plasmas that have not been extensively explored. Experiments show the formation of new types of self- and imposed-ordered structures that form in both the plasma and among the microparticles. This paper summarizes recent experimental observations of plasma filamentation (in the plasma) and will discuss possible connections to 'dust gridding' phenomena that are observed in the magnetized dusty plasma experiment device.

We address an experimental observation of pattern formation in a magnetised rf plasma. The experiments are carried out in a electrically grounded aluminium chamber which is housed inside a rotatable superconducting magnetic coil. The plasma is formed by applying a rf voltage in parallel plate electrodes in push-pull mode under the background of argon gas. The time evolution of plasma intensity shows that a homogeneous plasma breaks into several concentric radial spatiotemoral bright and dark rings. These rings propagate radially at considerably low pressure and a constant magnetic field. These patterns are observed to trap small dust particles/grains in their potential. Exploiting this property of the patterns, a novel technique to measure the electric field associated with the patterns is described. The resulting estimates of the corresponding field intensity are presented. At other specific discharge parameters the plasma shows a range of special type of characteristic structures observed in certain other chemical, mechanical and biological systems., It contains four pages with four figures

Experiments with complex plasmas in a dc discharge using the PK-4 facility will be discussed, focusing on the formation of strings corresponding to an electrorheological fluid observed in recent parabolic flight experiments.

The magnetized dusty plasma experiment (MDPX) is a newly commissioned plasma device that started operations in late spring, 2014. The research activities of this device are focused on the study of the physics, highly magnetized plasmas, and magnetized dusty plasmas. The design of the MDPX device is centered on two main components: an open bore, superconducting magnet that is designed to produce, in a steady state, both uniform magnetic fields up to 4 Tesla and non-uniform magnetic fields with gradients of 1–2 T m−1and a flexible, removable, octagonal vacuum chamber that provides substantial probe and optical access to the plasma. This paper will provide a review of the design criteria for the MDPX device, a description of the research objectives, and brief discussion of the research opportunities offered by this multi-institution, multi-user project.

The current status of experimental and theoretical research into plasma crystals is reviewed. We start with the derivation of a generalized phase diagram for complex plasmas, followed by crystal structures observed in laboratory experiments and in space under microgravity conditions, and finish with an outlook into developing areas - homogeneous nucleation and nonequilibrium crystallization.

Experiments under microgravity conditions were carried out to study 'condensed' (liquid and crystalline) states of a colloidal plasma (ions, electrons, and charged microspheres). Systems with approximately 10(exp 6) microspheres were produced. The observed systems represent new forms of matter--quasineutral, self-organized plasmas--the properties of which are largely unexplored. In contrast to laboratory measurements, the systems under microgravity are clearly three dimensional (as expected); they exhibit stable vortex flows, sometimes adjacent to crystalline regions, and a central 'void,' free of microspheres.

Colloidal plasmas may “condense” under certain conditions into liquid and crystalline states, while retaining their essential plasma properties. This “plasma condensation” therefore leads to new states of matter: “liquid plasmas” and “plasma crystals.” The experimental discovery was first reported in 1994, and since then many researchers have begun to investigate the properties of condensed plasma states. In this paper we describe some of the basic physics required to understand colloidal plasmas and discuss experiments conducted to investigate the details of the interaction between the plasma particles (in particular, the interaction potential), the melting phase transition, and the thermodynamics of this new state of matter.

A dusty (or complex) plasma is a four-component plasma system consisting of electrons, ions, neutral atoms, and charged, nanometer to micrometer-sized particles (i.e., the “dust”). Because these dust grains are charged, they fully participate in the plasma dynamics and can be used to reveal details about transport, instabilities, and charging properties of plasmas. However, one important area that has not been studied extensively is the area of magnetized dusty plasmas. Even though the charged dust grains in a typical laboratory experiment can acquire several thousand elementary charges, the large mass of the grains ensures that the charge-to-mass ratio is quite low. As a result, it is technically challenging to design an experiment that can achieve full magnetization of ions, electrons, and the charged dust grains.

Experimental studies are presented where dust particles are suspended in the lower sheath region of an argon rf discharge at a strong vertical magnetic field from B=1.5 T up to 2.27 T. There the particles arranged in an ordered pattern imposed by the upper mesh electrode. It is observed that the particles jump to a new equilibrium position, where they exhibit self-excited vertical oscillations when illuminated by a horizontal laser beam. The dust motion is weakly damped during an upward jump and strongly damped during the return to the equilibrium after the laser is switched off. A model based on delayed charging is presented that can describe the observed behavior.

The Magnetized Dusty Plasma Experiment at Auburn University has been operational for over one year. In that time, a number of experiments have been performed at magnetic fields up to B = 2.5 T to explore the interaction between magnetized plasmas and charged, micron-sized dust particles. This paper reports on the initial results from studies of: (a) the formation of imposed, ordered structures, (b) the properties of dust wave waves in a rotating frame, and (c) the generation of plasma filaments.

This special issue contains papers solicited at the 14th Workshop on the Physics of Dusty Plasmas, which was hosted by Auburn and Wittenberg University and held in Auburn, AL, USA, from 26 May, 2015 to 29 May, 2015. The publication of this issue follows the tradition of earlier workshops, which resulted in similar special issues in 1994, 2001, 2004, 2007, 2010, and 2013, and is intended to provide a snapshot of recent progress in the field of dusty plasma research.

It is shown that the quantitative interpretation of recent experiments to determine the ion drag force in complex (dusty) plasmas [C. Zafiu, A. Melzer, and A. Piel, Phys. Plasmas 9, 4794 (2002); 10, 1278 (2003)] is not correct. A comparison of different models of the ion drag force is carried out to illustrate the complexity of this issue and to highlight the current level of the research.

Summary form only given. The next generation of plasma chambers for the research on complex plasmas in microgravity conditions onboard of the International Space Station (ISS), developed in the scope of the PlasmaLab project, is presented, including the first scientific results from microgravity and ground experiments.

A defocusing imaging technique has been used as a diagnostic to identify binary agglomerates (dimers) in complex plasmas. Quasi-two-dimensional plasma crystal consisting of monodisperse spheres and binary agglomerates has been created where the agglomerated particles levitate just below the spherical particles without forming vertical pairs. Unlike spherical particles, the defocused images of binary agglomerates show distinct, stationary/periodically rotating interference fringe patterns. The results can be of fundamental importance for future experiments on complex plasmas.

Low-pressure room-temperature neon, argon, krypton, and air plasmas were studied in magnetic fields up to flux densities of 2.3 T. Filaments appeared parallel to the magnetic field lines, and patterns such as spirals and concentric circles formed in the perpendicular direction. We link these effects to the magnetization of the ions. We also used a layer of embedded microparticles as probes in the plasma. Their motion changed dramatically from a collective rotation of the whole ensemble in moderate magnetic fields to a rotation in several small vortices centered at the filaments.

Agglomeration of highly charged microparticles was observed and studied in complex plasma experiments carried out in a capacitively coupled rf discharge. The agglomeration was caused by strong dust density waves triggered in a particle cloud by decreasing neutral gas pressure. Using a 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 some particles were sufficiently high to overcome the mutual Coulomb repulsion and hence to result in agglomeration. After stabilising the cloud again through the increase of the pressure, we were able to observe the aggregates directly with a long-distance microscope. We show that the agglomeration rate deduced from our experiments is in good agreement with theoretical estimates. In addition, we briefly discuss the mechanisms that can provide binding of highly charged microparticles in a plasma., submitted to Phys. Plasma

We report on the first three-dimensional (3D) complex plasma structure analysis for an experiment that was performed in an elongated discharge tube in the absence of striations. The low frequency discharge was established with 1 kHz alternating dc current through a cylindrical glass tube filled with neon at 30 Pa. The injected particle cloud consisted of monodisperse microparticles. A scanning laser sheet and a camera were used to determine the particle position in 3D. The observed cylindrical-shaped particle cloud showed an ordered structure with a distinct outer particle shell. The observations are in agreement with performed molecular dynamics simulations.

Electrostatic potential around a small dust charged particle in the near-wall region of a low pressure rf discharge is one of the fundamental problems in the physics of complex (dusty) plasmas because the screening of charged dust particles determines interactions between them and thus governs their dynamics (e.g., formation of dust crystals). An important feature of the plasma-wall transition layer is the (high degree of) anisotropy of the ion velocity distribution. The form of the screened potential in such an anisotropic plasma is still under debate: some authors attribute the screening to electrons, some attribute the screening to ions, and others suggest that both species provide comparable contributions to the screening. The objective of the present work is to test the hypothesis that the screening under typical conditions is primarily due to ions and not electrons. Here, two different theoretical models for the ion screening in the presence of anisotropy of the ion velocity distribution are derived (for collisional and collisionless cases, respectively) and compared with existing experimental data. The first model is a kinetic model that takes the ion-neutral collisions into account and defines the zeroth-order state as a homogeneous plasma with the balance of acceleration of ions in a homogeneous external electric field and collisions of ions with neutrals. The second one is a collisionless kinetic model that defines the zeroth order state as a homogeneous plasma with the ion velocity distribution being a shifted delta-function. In both models, the first-order perturbation due to a charged dust particle (considered as a non-absorbing point charge) is self-consistently calculated. The models are found to be in agreement with experimental data of Konopka et al. (PRL 84, 891, 2000).

Dusty plasmas have been studied in argon, radio frequency (rf) glow discharge plasmas at magnetic fields up to 2.5 T where the electrons and ions are strongly magnetized. Plasmas are generated between two parallel plate electrodes where the lower, powered electrode is solid and the upper electrode supports a dual mesh consisting of #24 brass and #30 aluminum wire cloth. In this experiment, we study the formation of imposed ordered structures and particle dynamics as a function of magnetic field. Through observations of trapped particles and the quasi-discrete (i.e., “hopping”) motion of particles between the trapping locations, it is possible to make a preliminary estimate of the potential structure that confines the particles to a grid structure in the plasma. This information is used to gain insight into the formation of the imposed grid pattern of the dust particles in the plasma.

Dusty plasmas have been studied in argon, rf glow discharge plasmas at magnetic fields up to 2 T, where the electrons and ions are strongly magnetized. In this experiment, plasmas are generated between two parallel plate electrodes where the lower, powered electrode is solid and the upper, electrically floating electrode supports a semi-transparent, titanium mesh. We report on the formation of an ordered dusty plasma, where the dust particles form a spatial structure that is aligned to the mesh. We discuss possible mechanisms that may lead to the formation of the “dust grid” and point out potential implications and applications of these observations.

A monolayer plasma crystal consisting of micron-sized particles levitated in the sheath of a rf discharge was melted by applying a short electric pulse to two parallel wires located at the height of the particles. Structural properties and the particle temperature were examined during the stage of recrystallization. A liquidlike phase was followed by a transient state characterized by energy release and the restoring of long range translational order while the defect fraction was low. No long range orientational order was found, though highly ordered domains formed locally. Numerical simulations revealed the same regimes of recrystallization as those observed in the experiment.

Melting of a monolayer plasma crystal was induced by an electric pulse. We investigated, how structural parameters like defect fraction and correlation lengths as well as dynamical properties like the particle kinetic energy changed during the recrystallisation. As an indication of the phase transition, the change of the Lindemann parameter and the Coulomb coupling parameter were considered.

Complex plasmas are an ideal model system to investigate laminar fluids as they allow to study fluids at the kinetic level. At this level we are able to identify streamlines particle by particle. This gives us the ability to research the behaviour of these streamlines as well as the behaviour of each individual particle of the streamline.We carried out our experiments in a modified GEC‐RF‐Reference cell. We trapped the particles within two glass rings and forced them to form a circular flow by using several stripe electrodes. In this flow the particles behave like an ideal fluid and form streamlines. By putting an obstacle into the flow we reduce the cross‐section. To pass through this constricted cross‐section some streamlines have to reconnect. After the obstacle the streamlines split up again. An analysis how streamlines split up and reconnect as result of external pressure on the fluid in our system is presented here.Streamlines also occur if two clouds of particles penetrate each other. We call this ...

To complete our picture of general complex plasmas, experiments under the influence of high magnetic fields have been carried out in a radio frequency (rf) discharge with and without embedded micro‐particles. The influence of the strong magnetic field on the plasma with respect to its homogeneity as well as on the isotropy of the particle interaction was studied. We observed a filamentation of the plasma at low pressures and low powers even in the absence of particles. The plasma filaments moved around — traced by embedded particles — and suddenly changed to a crystalline like arrangement.

Summary form only given. Micro particles that are introduced into a plasma environment charge up to a high negative value due to ion and electron bombardment. As a result of the strong Coulomb interaction? between the particles often so-called "plasma crystals" are built up. These systems show a variety of wave phenomena like dust acoustic waves, dust shear waves, etc. that have been investigated partially only. In this paper, the wave dispersion relation is analyzed in a two-dimensional plasma crystal under magnetic field influence by theoretical analysis and molecular dynamics simulation. An expression for the wave dispersion relation clearly shows that the longitudinal and the transverse modes in a plasma crystal are coupled due to the Lorentz force acting on dust particles. The dispersion relation has a high- and a low- frequency branch. Our analysis shows that the high-frequency branch has a cut-off at the cyclotron frequency of the dust particles, and the wave polarization drastically changes with magnetic field strength. In this presentation, we also report on the experimental results of the wave propagation in a magnetized dusty plasma.

Dust-free regions around a Langmuir probe are studied in a complex plasma under microgravity. The dust particles settle in the presheath of the probe, where an equilibrium of the electric field force and the ion-drag force is established. The size and shape of the dust cloud are discussed with simple models. A more sophisticated presheath model is solved numerically to analyze the acting forces and the equilibrium position of the dust. The formation of distinct particle layers in the dust shell can be explained by the force gradients of the effective potential well.

Fluid flow around an obstacle was observed at the kinetic (individual particle) level using ``complex (dusty) plasmas'' in their liquid state. These ``liquid plasmas'' have bulk properties similar to water (e.g., viscosity), and a comparison in terms of similarity parameters suggests that they can provide a unique tool to model classical fluids. This allows us to study ``nanofluidics'' at the most elementary---the particle---level, including the transition from fluid behavior to purely kinetic transport. In this (first) experimental investigation we describe the kinetic flow topology, discuss our observations in terms of fluid theories, and follow this up with numerical simulations.

The wave dispersion relation in a two-dimensional strongly coupled plasma crystal is studied by theoretical analysis and molecular dynamics simulation taking into account a constant magnetic field parallel to the crystal normal. The expression for the wave dispersion relation clearly shows that high-frequency and low-frequency branches exist as a result of the coupling of longitudinal and transverse modes due to the Lorenz force acting on the dust particles. The high-frequency and the low-frequency branches are found to belong to right-hand and left-hand polarized waves, respectively.

Melting of a monolayer plasma crystal in a radio-frequency discharge with no particles suspended above or below is studied. The experimental data are compared with results of molecular dynamics simulations and theory. It is shown that the melting is caused by the resonance coupling between the longitudinal and the transverse dust-lattice wave modes, due to the interaction of particles with the plasma wakes.

Coagulation of charged particles was studied using the mean-field Smoluchowski equation. The coagulation equation was generalized for the case of a conserved system of charged particles. It was shown that runaway cluster growth (gelation) solutions exist if the charge-dipole (induced) interaction of clusters is included. When clusters are in thermal equilibrium with the ambient gas, the charge-dipole interaction dramatically enhances the aggregation process and considerably increases the likelihood of a gelation transition.

The plasma parameters of a complex plasma have been measured under microgravity conditions using a cylindrical Langmuir probe. One‐ and two‐dimensional profiles of dusty and void regions are obtained, which show a flattened distribution of plasma potential and electron density. Characteristic changes of the plasma parameters correlate with the geometry of the particle cloud.

Experiments with complex (“dusty”) plasmas in microgravity often show a particle-free region in the center of the discharge, which is the so-called “void.” The cause of this unexpected feature is assumed to be the ions streaming outward from the discharge center toward the radio-frequency electrodes and walls, dragging the microparticles with them out of the center. In several experiments onboard the International Space Station, single particles moving out of the void could be traced. The obtained trajectories reveal the distribution of the forces inside the void.

Experiments were carried out to investigate a three-dimensional (3D) plasma crystal. A method of determining the positions of each individual microparticle has been developed. A crystal volume of about $2\ifmmode\times\else\texttimes\fi{}{10}^{4}$ particles in 19 horizontal planes was analyzed. Direct imaging and the 3D pair correlation function show that ``domains'' of fcc and hcp lattices coexist in the crystal. Other structures, in particular, the theoretically predicted bcc lattice, were not observed.

Microrods were levitated in the collisional sheath of a rf plasma. Rods below a critical length settle vertically, parallel to the electric field, while longer rods float horizontally. Usually rods with other inclinations spin about a vertical axis. These experimental features fit well with a model that includes a theoretical profile for the sheath, a plasma model for the screening length, which increases going deeper in the sheath, and a plasma theory for the charging of the rod's elements. Despite the agreement this paper highlights the need for a better understanding of the charging mechanism of bodies in sheaths and of the transition region in collisional sheaths.

The theory of dust particle oscillations in the plasma sheath is presented, taking into account particle charging kinetics and neutral gas friction. Effects of ``regular'' and stochastic charge variations are considered. It is shown that whilst regular variations generally enhance the damping of horizontally propagating dust lattice waves, they can also cause an instability in the vertical oscillations of single particles. The stochastic charge variations, if sufficiently strong, result in exponential growth of the mean energy of both types of oscillations.