Magnetizing a complex plasma without a magnetic field.

We propose and demonstrate a concept that mimics the magnetization of the heavy dust particles in a complex plasma while leaving the properties of the light species practically unaffected. It makes use of the frictional coupling between a complex plasma and the neutral gas, which allows us to transfer angular momentum from a rotating gas column to a well-controlled rotation of the dust cloud. This induces a Coriolis force that acts exactly as the Lorentz force in a magnetic field. Experimental normal mode measurements for a small dust cluster with four particles show excellent agreement with theoretical predictions for a magnetized plasma.

Particle chains in a dilute dusty plasma with subsonic ion flow.

Chains of charged dust particles are observed aligned with a subsonic ion flow. These chains are found in dilute regions, near the midplane of a parallel-plate radio-frequency plasma under microgravity conditions. The argon ion flow speed near these chains was estimated to be of order 10(2) m/s, corresponding to an ion acoustic Mach number M<0.1. The chains were observed to be stable in both the longitudinal and transverse directions. This stability suggests that there is a transverse restoring force. The transverse components of the ion-drag force or electrostatic wake-field forces could provide such a stabilizing effect. The chain appears to terminate with a final dust particle that is located in a dilute region; this observation suggests a possible attractive force in the longitudinal direction in the presence of a subsonic ion flow.

Chain of coupled van der Pol oscillators as model system for density waves in dusty plasmas.

Recent investigations of dust-density waves in a dusty plasma under microgravity conditions [K. O. Menzel et al., Phys. Rev. Lett. 104, 235002 (2010)] showed that the wave field consists of distinct regions of different frequencies. These so-called frequency clusters are known from simulations of chains of mutually coupled van der Pol oscillators. The behavior of distinct oscillators adjacent to the cluster boundaries were studied numerically. The interaction of these oscillators leads to periodic frequency pulling, a typical feature of driven van der Pol oscillators that is also observed in our experiments.

Living archaeology: artefacts of specific nest site fidelity in wild chimpanzees.

Savanna chimpanzees are known to re-use areas of the landscape for sleep, and patterns of chimpanzee sleeping site re-use are proposed as a referential model for early hominin archaeological site formation. We recorded the prevalence of deformed but healed branches and remnants of dead branches found around fresh nests at the savanna site of Issa in Ugalla, Tanzania. These old nest scars were found in 79% of 112 beds. We also randomly selected potential nesting locations for a subset of 32 beds within the same trees, and found nest scars in only 19% of these "control" locations. We then monitored 275 nests for up to 19 months for decay, regeneration of new branches, and re-use. Of these 275 nest locations, 24% were re-used within the first nine months of monitoring, and most re-use occurred when the nest had already decayed and was not easily visible from the ground. After 18 months, the proportion of specific nest positions re-used increased to 48%. This fidelity is likely a result of the creation of ideally-shaped support structures and supple new growth for mattress material with successive use of nest locations. We propose that specific nest site re-use may not be a direct product of environmental determination, but a result of "niche construction" through formation of good building sites within trees. Environmental modification through construction behaviour may have influenced both chimpanzee and early hominin ranging, and thus leaves behind recognisable patterns of artefact deposition across the landscape.

Cavity dynamics and particle alignment in the wake of a supersonic projectile penetrating a dusty plasma.

The penetration of a projectile into a strongly coupled dusty plasma was studied in a radio-frequency discharge under microgravity conditions. A supersonic projectile produces an elongated dust-free cavity in its wake. The dynamics of the cavity is analyzed and compared with Langevin dynamics simulations. Besides a three-dimensional Mach cone structure, the simulation shows that the cavity dynamics can be subdivided into three phases: An opening phase with fixed time scale, a closing phase, whose duration is affected by the projectile speed and, finally, a phase of particle realignment in the target cloud, which persists for a long time after the closure of the cavity.

Frequency clusters and defect structures in nonlinear dust-density waves under microgravity conditions.

Density waves in a dusty plasma emerge spontaneously at low gas pressures and high dust densities. These acousticlike wave modes were studied in a radio-frequency discharge under microgravity conditions. The complex three-dimensional wave pattern shows a spatially varying wavelength that leads to bifurcations, i.e., topological defects, where wave fronts split or merge. The calculation of instantaneous wave attributes from the spatiotemporal evolution of the dust density allows a precise analysis of those structures. Investigations of the spatial frequency distribution inside the wave field revealed that the wave frequency decreases from the bulk to the edge of the cloud in terms of frequency jumps. Between those jumps, regions of almost constant frequency appear. The formation of frequency clusters is strongly correlated with defects that occur exclusively at the cluster boundaries. It is shown that the nonlinearity of the waves has a significant influence on the topology of the wave pattern.

Spatial frequency clustering in nonlinear dust-density waves.

Self-excited density waves were studied in a strongly coupled dusty plasma of a radio-frequency discharge under microgravity conditions. The spatiotemporal evolution of the complicated three-dimensional wave field was investigated and analyzed for two different situations. The reconstructed instantaneous phase information of the wave field revealed a partial synchronization within multiple distinct domains. The boundaries of these regions coincide with the locations of topological defects.

Probability of metastable configurations in spherical three-dimensional Yukawa crystals.

Recently the occurrence probabilities of ground and metastable states of three-dimensional Yukawa clusters with 27 and 31 particles have been analyzed in dusty plasma experiments [D. Block, Phys. Plasmas 15, 040701 (2008)]. There it was found that, in many cases, the ground state appeared substantially less frequently than excited states. Here we analyze this question theoretically by means of molecular dynamics (MD) and Monte Carlo simulations and an analytical method based on the canonical partition function. We confirm that metastable states can occur with a significantly higher probability than the ground state. The results strongly depend on the screening parameter of the Yukawa interaction and the damping coefficient used in the MD simulations. The analytical method allows one to gain insight into the mechanisms being responsible for the occurrence probabilities of metastable states in strongly correlated finite systems.

Existence and vanishing of the breathing mode in strongly correlated finite systems.

One of the fundamental eigenmodes of finite interacting systems is the mode of uniform radial expansion and contraction-the breathing mode (BM). Here we show in a general way that this mode exists only under special conditions: (i) for harmonically trapped systems with interaction potentials of the form 1/rgamma (gamma in R not equal 0) or log(r), or (ii) for some systems with special symmetry such as single-shell systems forming platonic bodies. Deviations from the BM are demonstrated for two examples: clusters interacting with a Lennard-Jones potential and parabolically trapped systems with Yukawa repulsion. We also show that vanishing of the BM leads to the occurrence of multiple monopole oscillations which is of importance for experiments.

Obliquely propagating dust-density waves.

Self-excited dust-density waves are experimentally studied in a dusty plasma under microgravity. Two types of waves are observed: a mode inside the dust volume propagating in the direction of the ion flow and another mode propagating obliquely at the boundary between the dusty plasma and the space charge sheath. The dominance of oblique modes can be described in the frame of a fluid model. It is shown that the results fom the fluid model agree remarkably well with a kinetic electrostatic model of Rosenberg [J. Vac. Sci. Technol. A 14, 631 (1996)]. In the experiment, the instability is quenched by increasing the gas pressure or decreasing the dust density. The critical pressure and dust density are well described by the models.

Heat transport in a two-dimensional complex (dusty) plasma at melting conditions.

The heat transport in a two-dimensional complex (dusty) plasma undergoing a phase transition was studied experimentally. A single layer of highly charged polymer microspheres was suspended in a plasma sheath. A part of this lattice was heated by two counterpropagating focused laser beams that moved rapidly around in the lattice and provided short intense random kicks to the particles. Above a threshold, the lattice locally melted. The spatial profiles of the particle kinetic temperature were analyzed to find a thermal conductivity, which did not depend on temperature.

Ground state of a confined Yukawa plasma including correlation effects.

The ground state of an externally confined one-component Yukawa plasma is derived analytically using the local density approximation (LDA). In particular, the radial density profile is computed. The results are compared with the recently obtained mean-field (MF) density profile [Henning et al., Phys. Rev. E 74, 056403 (2006)]. While the MF results are more accurate for weak screening, the LDA with correlations included yields the proper description for large screening. By comparison with first-principles simulations for three-dimensional spherical Yukawa crystals, we demonstrate that the two approximations complement each other. Together they accurately describe the density profile in the full range of screening parameters.

Langmuir probe system for dusty plasmas under microgravity.

This article describes a fully automated 2D-scanning Langmuir probe system for dusty plasmas under microgravity. The design combines necessary features such as random sampling, radio frequency compensation, and a compact mechanical design. The various aspects of the probe implementation and the contamination problem in the dusty plasma environment are discussed and the functionality of the system is demonstrated by measurements performed on parabolic flights.

Obliquely propagating dust-density plasma waves in the presence of an ion beam.

Self-excited dust-density waves are experimentally studied in a dusty plasma under microgravity. Two types of waves are observed: a mode inside the dust volume propagating in the direction of the ion flow and a new mode propagating obliquely at the boundary between the dusty plasma and the space-charge sheath. A model for dust-density waves propagating at an arbitrary angle with respect to the ion-flow direction is presented, which explains the preference for oblique or parallel modes as a function of ion velocity.

Cutoff wave number for shear waves in a two-dimensional Yukawa system (dusty plasma).

The cutoff wave number for shear waves in a liquid-state strongly coupled plasma was measured experimentally. The phonon spectra of random particle motion were measured at various temperatures in a monolayer dusty plasma, where microspheres interact with a Yukawa potential. In the liquid state of this particle suspension, shear waves were detected only for wavelengths smaller than 20 to 40 Wigner-Seitz radii, depending on the Coulomb coupling parameter. The temperature of the suspension was controlled using a laser-heating method.

Structural properties of screened Coulomb balls.

Small three-dimensional strongly coupled charged particles in a spherical confinement potential arrange themselves in a nested shell structure. By means of experiments, computer simulations, and theoretical analysis, the sensitivity of their structural properties to the type of interparticle forces is explored. While the normalized shell radii are found to be independent of shielding, the shell occupation numbers are sensitive to screening and are quantitatively explained by an isotropic Yukawa model.

Ground state of a confined Yukawa plasma.

The ground state of an externally confined one-component Yukawa plasma is derived analytically. In particular, the radial density profile is computed. The results agree very well with computer simulations of three-dimensional spherical Coulomb crystals. We conclude in presenting an exact equation for the density distribution for a confinement potential of arbitrary geometry.

Dust-free regions around Langmuir probes in complex plasmas under microgravity.

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.

Compressional and shear wakes in a two-dimensional dusty plasma crystal.

Wakes composed of compressional and shear waves were studied experimentally in a two-dimensional screened-Coulomb crystal. Highly charged microspheres suspended in a plasma settled in a horizontal monolayer and arranged in a triangular lattice with a repulsive interparticle potential. Wakes were excited by a moving spot of Ar+ laser light. Depending on the laser spot speed, compressional waves formed a Mach cone and multiple lateral or transverse wakes, similar to ship wakes on the water surface, due to a combination of acoustic and dispersive properties. Shear waves, however, formed only a Mach cone, due to their nearly acoustic, i.e., dispersionless character. The experimental results show agreement with a recently developed theory and with molecular dynamics simulations, which assume a binary Yukawa interparticle potential. A generally useful method is presented for calculating the real part of the dispersion relation of the compressional waves based on the analysis of the spatial structure of a phonon wake. Fitting the resulting dispersion relation provides an independent measure of the interparticle potential, parametrized by the screening parameter kappa and particle charge Q.

Experiments and molecular-dynamics simulation of elastic waves in a plasma crystal radiated from a small dipole source.

The radiation of elastic waves from a localized source is observed experimentally in a two-dimensional plasma crystal. An initial shear stress applied by a laser forms a small dipole source. The emerging complex wave pattern is shown to consist of outgoing compressional and shear wave pulses. Subsequent structures are identified as inward-going waves due to the finite size of the source region, which reappear on the opposite side. The compressional wave forms a trailing wave train due to strong dispersion, while the nondispersive shear wave evolves into a vortex-antivortex pair on either side. The experiments are compared with a molecular-dynamics simulation.