Complex Plasma Crystals Research Articles

Impurity modes in two-dimensional strongly coupled complex plasma crystals

Complex plasma fluctuation processes have been extensively studied in many aspects, especially lattice waves in strongly coupled plasma crystals, which are of great significance for understanding fundamental physical phenomena. A challenge of experimental investigations in two-dimensional strongly coupled complex plasma crystals is to keep the main body and foreign particles of different masses on the same horizontal plane. To solve the problem, we have proposed a potential well formed by two negatively biased grids to bind the negatively charged particles in a two-dimensional (2D) plane, thus achieving a 2D plasma crystal in the microgravity environment. The study of such phenomena in complex plasma crystals under microgravity environment then becomes possible. In this paper, we focus on the continuum spectrum, including both phonon and optic branches of the impurity mode in a 2D system in microgravity environments. The results show the dispersion relation of the longitudinal and transverse impurity oscillation modes and their properties. Considering the macroscopic visibility of complex mesoscopic particle lattices, theoretical and experimental studies on this kind of complex plasma systems will help us further understand the physical nature of a wide range of condensed matters.

Stability of two-dimensional complex plasma monolayers in asymmetric capacitively coupled radio-frequency discharges.

In this article, the stability of a complex plasma monolayer levitating in the sheath of the powered electrode of an asymmetric capacitively coupled radio-frequency argon discharge is studied. Compared to earlier studies, a better integration of the experimental results and theory is achieved by operating with actual experimental control parameters such as the gas pressure and the discharge power. It is shown that for a given microparticle monolayer at a fixed discharge power there exist two threshold pressures: (i) above a specific pressure p_{cryst}, the monolayer always crystallizes; (ii) below a specific pressure p_{MCI}, the crystalline monolayer undergoes the mode-coupling instability and the two-dimensional complex plasma crystal melts. In between p_{MCI} and p_{cryst}, the microparticle monolayer can be either in the fluid phase or the crystal phase: when increasing the pressure from below p_{MCI}, the monolayer remains in the fluid phase until it reaches p_{cryst} at which it recrystallizes; when decreasing the pressure from above p_{cryst}, the monolayer remains in the crystalline phase until it reaches p_{MCI} at which the mode-coupling instability is triggered and the crystal melts. A simple self-consistent sheath model is used to calculate the rf sheath profile, the microparticle charges, and the microparticle resonance frequency as a function of power and background argon pressure. Combined with calculation of the lattice modes the main trends of p_{MCI} as a function of power and background argon pressure are recovered. The threshold of the mode-coupling instability in the crystalline phase is dominated by the crossing of the longitudinal in-plane lattice mode and the out-of plane lattice mode induced by the change of the sheath profile. Ion wakes are shown to have a significant effect too.

DPEx-II: a new dusty plasma device capable of producing large sized DC coulomb crystals

The creation of a spatially extended stable DC complex plasma crystal is a big experimental challenge and a topical area of research in the field of dusty plasmas. In this paper we describe a newly built and commissioned dusty plasma experimental (DPEx-II) device at the Institute for Plasma Research. The device can support the formation of large sized Coulomb crystals in a DC glow discharge plasma. The plasma in this L-shaped table-top glass chamber is produced between a circular anode and a long tray shaped cathode. It is characterized with the help of various electrostatic probes over a range of discharge conditions. The dust particles are introduced by a dust dispenser to form a strongly coupled Coulomb crystal in the cathode sheath region. The unique asymmetric electrode configuration minimizes the heating of dust particles and facilitates the formation of crystalline structures with a maximum achievable dimension of 40 cm × 15 cm using this device. A larger crystal has numerous advantages over smaller ones, such as higher structural homogeneity, fewer defects, lower statistical errors due to finite size effects etc. A host of diagnostics tools are provided to characterize the Coulomb crystal. Results of a few initial experiments aimed at demonstrating the technical capabilities of the device and its potential for future dusty plasma research, are reported.

Thermal conductivity analysis of two-dimensional complex plasma liquids and crystals

A novel homogeneous perturbed non-equilibrium molecular dynamics (HPMD) scheme, proposed by Evan-Gillan, has been employed to calculate the thermal conductivity of two-dimensional (2D) complex plasma liquids and crystals (CPLCs). The thermal conductivity has been reported using an improved HPMD method under the influence of constant external perturbation with different system sizes (N) and combinations of plasma parameters (Γ, κ). The current HPMD scheme provides precise outcomes with fast convergence for small-to-large N effects over a complete range of (Γ, κ). Temperature scaling law is tested for 2D thermal conductivity with appropriate Einstein frequency and found excellent behaviors. New simulations show that the thermal conductivity of CPLCs depends on (Γ, κ) and N and a slightly decreasing behavior is noted for thermal conductivity with increasing Γ and N, but, overall, thermal conductivity becomes constant at intermediate-to-large Γ. The reported thermal conductivity obtained from present HPMD method, in the limit of low equilibrium perturbation, has established a reasonable agreement with that obtained from earlier known 2D numerical and experimental data. It is demonstrated that the present HPMD method is an alternative efficient tool to compute the thermal conductivity of 2D CPLCs and can be a suitable method for complete trends of complex plasmas.

Experimental Approach for Obtaining a Complex (Dusty) Plasma Fluid

An experimental approach is proposed to obtain a complex (dusty) plasma fluid. In contrast to previously employed laser heating of a complex (dusty) plasma crystal, this approach is based on the mode-coupling instability of the crystal and does not require any additional influence. We consider a typical experiment and report results of post-processing, including the Voronoi-cell area distribution and order parameter, justifying a liquid state of the system existing for a long enough time. The results can be useful for studies of phenomena and properties of fluids at different couplings, such as excitation spectra, density–density correlations, diffusion, and relations between them.

Electric field in two-dimensional complex plasma crystal: Ideal lattices

Electric field in two-dimensional complex plasma crystal: Ideal lattices

Experimental studies of two-dimensional complex plasma crystals: waves and instabilities

A review of experimental studies on waves, phonon dispersion relations, and mode-coupling instability in two-dimensional complex plasma crystals is presented. An improved imaging method allowing simultaneous measurements of the three wave modes (compression in-plane, shear in-plane, and out-of-plane) is given. This method is used to evidence the formation of hybrid modes and the triggering of the mode-coupling instability due to wake-mediated interactions. The main stages of the mode-coupling instability are analyzed. In the early stages, synchronization of microparticle motion at the hybrid mode frequency is reported. The spatial orientation of the observed synchronization pattern correlates well with the directions of the maximal increment of the shear-free hybrid mode. When the instability is fully developed, a melting front is formed. The propagation of the melting front has similarities with flame propagation in ordinary reactive matter. Finally, it is experimentally demonstrated that an external mechanical excitation of a stable 2D complex plasma crystal can trigger the mode-coupling instability and lead to the full melting of a two-dimensional complex plasma crystal.

Experimental investigation of test particle induced micro-structural changes in a finite two-dimensional complex plasma crystal

The microstructural changes induced in a two dimensional complex plasma crystal by the movement of a single extra particle existing below it are investigated experimentally using a DC glow discharge device. The finite monolayer crystal consisting of monodispersive melamine formaldehyde particles is created in the background of an Argon plasma. The crystal formation is frequently found to be accompanied by the presence of one or more slightly heavier particles suspended a little below the monolayer. The interplay of one such test particle with the crystal is investigated for two distinct cases—(i) when the particle remains confined (trapped) in the space below the crystal and (ii) when it interacts for a short while with the crystal and then moves out of the vicinity. The trapped particle orbit induces permanent structural changes in the crystal in the form of microcracks, and these changes are identified using a variety of diagnostic analyses. The cracks can be enhanced by energizing the test particle with an incident laser. The passing particle, on the other hand, induces only transient changes in the crystal from which it is able to recover and return to its initial state in a clear demonstration of the viscoelastic property of a strongly coupled system.

Experimental validation of interpolation method for pair correlations in model crystals.

Accurate analysis of pair correlations in condensed matter allows us to establish relations between structures and thermodynamic properties and, thus, is of high importance for a wide range of systems, from solids to colloidal suspensions. Recently, the interpolation method (IM) that describes satisfactorily the shape of pair correlation peaks at short and at long distances has been elaborated theoretically and using molecular dynamics simulations, but it has not been verified experimentally as yet. Here, we test the IM by particle-resolved studies with colloidal suspensions and with complex (dusty) plasmas and demonstrate that, owing to its high accuracy, the IM can be used to experimentally measure parameters that describe interaction between particles in these systems. We used three- and two-dimensional colloidal crystals and monolayer complex (dusty) plasma crystals to explore suitability of the IM in systems with soft to hard-sphere-like repulsion between particles. In addition to the systems with pairwise interactions, if many-body interactions can be mapped to the pairwise ones with some effective (e.g., density-dependent) parameters, the IM could be used to obtain these parameters. The results reliably show that the IM can be effectively used for analysis of pair correlations and interactions in a wide variety of systems and therefore is of broad interest in condensed matter, complex plasma, chemical physics, physical chemistry, materials science, and soft matter.

Melting transition of two-dimensional complex plasma crystal in the DC glow discharge

The formation of self-consistent dust crystal and its melting is a well- known phenomenon in rf generated plasmas but remains challenging in DC glow discharge plasmas. Here, we report the melting of a two- dimensional dusty plasma crystal, suspended in the cathode sheath of a DC glow discharge plasma. The experiments are carried out in a Dusty Plasma Experimental (DPEx) device which consists of a circular powered electrode and an extended grounded cathode tray. A stationary crystal of melamine formaldehyde particles is formed in a background of Argon plasma between a pair of confining strips that are placed on the cathode. The stable structure breaks and transition to a fluid state when the neutral pressure is reduced. The neutral pressure range where this melting transition is observed is an order of magnitude less than what is typically reported in rf discharge plasma. The transition is confirmed by evaluating the variation in different characteristic parameters such as the pair correlation function, Voronoi diagram, local bond order parameter, defect fraction and dust kinetic temperature as a function of background neutral pressure. For instance, the average bond order parameter (Ψ6) reduces from 0.87 to 0.4 when the neutral pressure is reduced from 15 to 11 Pa. The asymmetry of the electrode is a special feature that is believed to enable the formation of the plasma crystal.

Tracking and Linking of Microparticle Trajectories During Mode-Coupling Induced Melting in a Two-Dimensional Complex Plasma Crystal.

In this article, a strategy to track microparticles and link their trajectories adapted to the study of the melting of a quasi two-dimensional complex plasma crystal induced by the mode-coupling instability is presented. Because of the three-dimensional nature of the microparticle motions and the inhomogeneities of the illuminating laser light sheet, the scattered light intensity can change significantly between two frames, making the detection of the microparticles and the linking of their trajectories quite challenging. Thanks to a two-pass noise removal process based on Gaussian blurring of the original frames using two different kernel widths, the signal-to-noise ratio was increased to a level that allowed a better intensity thresholding of different regions of the images and, therefore, the tracking of the poorly illuminated microparticles. Then, by predicting the positions of the microparticles based on their previous positions, long particle trajectories could be reconstructed, allowing accurate measurement of the evolution of the microparticle energies and the evolution of the monolayer properties.

Экспериментальные исследования двумерных комплексных плазменных кристаллов: волны и неустойчивости

Экспериментальные исследования двумерных комплексных плазменных кристаллов: волны и неустойчивости

New radio-frequency setup for studying large 2D complex plasma crystals

Complex plasma crystals are popular model systems where various plasma-specific or generic phenomena can be studied at the level of individual particles. Addressing the growing need for larger two-dimensional (2D) plasma crystals, a new plasma setup was built at the DLR Institute of Materials Physics in Space. The setup allows obtaining larger than before, highly ordered 2D plasma crystals and exploring new parameter ranges. It is based on a relatively large (90 cm in diameter) vacuum chamber where a capacitively coupled radio-frequency discharge is used to levitate polymer microparticles. The discharge is created between the lower rf electrode and the grounded chamber walls, the particles levitate in the plasma (pre)sheath above the electrode and are observed by video microscopy through the large top glass window and through the side windows. The first observations of plasma crystals in the new setup are reported.

Mode-coupling instability in a single-layer complex plasma crystal: Strong damping regime

Mode-coupling instability (MCI) in a single-layer complex plasma crystal was studied experimentally in the regime of strong neutral gas damping. To trigger MCI, the discharge power was reduced at constant gas pressure. Surprisingly, at the onset of MCI the mean interparticle spacing became larger and the normalized neutral gas damping rate marginally increased. A steady-state regime where MCI occurred but was suppressed by the neutral gas friction was observed. Suppressed MCI caused heating up of the plasma crystal but did not lead to its melting. A theoretical model is proposed which describes well our experimental observations.

Full melting of a two-dimensional complex plasma crystal triggered by localized pulsed laser heating.

The full melting of a two-dimensional plasma crystal was induced in a principally stable monolayer by localized laser stimulation. Two distinct behaviors of the crystal after laser stimulation were observed depending on the amount of injected energy: (i) below a well-defined threshold, the laser melted area recrystallized; (ii) above the threshold, it expanded outwards in a similar fashion to mode-coupling instability-induced melting, rapidly destroying the crystalline order of the whole complex plasma monolayer. The reported experimental observations are due to the fluid mode-coupling instability, which can pump energy into the particle monolayer at a rate surpassing the heat transport and damping rates in the energetic localized melted spot, resulting in its further growth. This behavior exhibits remarkable similarities with impulsive spot heating in ordinary reactive matter.

Electric field in two-dimensional complex plasma crystal: Simulated lattices

We focus on molecular dynamics simulated two-dimensional complex plasma crystals. We use rigid walls as a confinement force and produce square and rectangular crystals. We report various types of two-row crystals. The narrow and long crystals are likely to be used as wigglers; therefore, we simulate such crystals. Also, we analyze the electric fields of simulated crystals. A bit change in lattice parameters can change the internal structures of crystals and their electric fields notably. These parameters are the number of grains, grains charge, length, and width of the crystal. With the help of electric fields, we show the details of crystal structures.

Transport properties of disordered two‐dimensional complex plasma crystal

In this study, we numerically investigate the transport properties of a two‐dimensional (2D) complex plasma crystal using diffusion of coplanar dust lattice waves. In the limit where the Hamiltonian interactions can be decoupled from the non‐Hamiltonian effects, we identify two distinct types of wave transport: Anderson‐type delocalization and long‐distance excitation. We use a recently developed spectral approach to evaluate the contribution of the Anderson problem and compare it to the results of the simulation. The benefit of our approach to transport problems is twofold. First, we employ a highly tuneable macroscopic hexagonal crystal, which exhibits many‐body interactions and allows for the investigation of transport properties at the kinetic level. Second, the analysis of the transport problem in 2D is provided using an innovative spectral approach, which avoids the use of scaling and boundary conditions. The comparison between the analytically predicted and numerically observed wave dynamics allows for the study of important characteristics of this open system. In our simulations, we observe long‐distance lattice excitation, which occurs around lattice defects even when the initial perturbation does not spread from the centre to the exterior of the crystal. In the decoupled Hamiltonian regime, this many‐body effect can be attributed to the dust lattice interaction with the plasma environment.

Coupling of Noncrossing Wave Modes in a Two-Dimensional Plasma Crystal.

We report an experimental observation of the coupling of the transverse vertical and longitudinal in-plane dust-lattice wave modes in a two-dimensional complex plasma crystal in the absence of mode crossing. A new large-diameter rf plasma chamber was used to suspend the plasma crystal. The observations are confirmed with molecular dynamics simulations. The coupling manifests itself in traces of the transverse vertical mode appearing in the measured longitudinal spectra and vice versa. We calculate the expected ratio of the trace to the principal mode with a theoretical analysis of the modes in a crystal with finite temperature and find good agreement with the experiment and simulations.

Dynamics of spinning particle pairs in a single-layer complex plasma crystal.

Spontaneous formation of spinning pairs of particles, or torsions, is studied in a single-layer complex plasma crystal by reducing the discharge power at constant neutral gas pressure. At higher gas pressures, torsions spontaneously form below a certain power threshold. Further reduction of the discharge power leads to the formation of multiple torsions. However, at lower gas pressures the torsion formation is preceded by mode-coupling instability (MCI). The crystal dynamics are studied with the help of the fluctuation spectra of crystal particles' in-plane velocities. Surprisingly, the spectra of the crystal with torsions and MCI are rather similar and contain hot spots at similar locations on the (k,ω) plane, despite very different appearances of the respective particle trajectories. The torsion rotation speed is close (slightly below) to the maximum frequency of the in-plane compressional mode. When multiple torsions form, their rotation speeds are distributed in a narrow range slightly below the maximum frequency.

Spontaneous formation and spin of particle pairs in a single-layer complex plasma crystal

In an experiment with a single-layer plasma crystal, spontaneous pairing of particles was observed upon a sudden reduction of the discharge power. The pairs were oriented vertically with the upper particle above the crystal layer and the lower particle beneath it, the pair size was about 0.2 mm. The pairs were spinning around their vertical axis with the upper particle leading and the lower one following it; the rotation speed was 10–13 Hz. Spinning particle pairs disturbed the plasma crystal through interaction with their neighbors. Upon further reduction of the discharge power, the spinning pairs proliferated in the plasma crystal and eventually it melted. The experiment was performed with micron-size polymer particles suspended in the radio-frequency (rf) argon plasma at a pressure of 157 mtorr. We propose a theoretical model of a spinning particle pair based on the plasma wake effect. Spinning particle pairs can be used as a diagnostic tool for plasma wakes or as a generic model of a 2D system of vortices.