Dust Density Waves Research Articles

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.

Development of nonlinearity in a growing self-excited dust-density wave

The development of nonlinearity is observed in a naturally occurring planar dust-density wave. As it propagates through a dusty plasma, the wave grows and harmonics are generated. The amplitudes, wave numbers, and growth rates are measured for the fundamental and its harmonics. The energy in the harmonic modes exhibits a strong exponential increase with diminishing gas pressure, until it levels off at lower gas pressures. The wave numbers and growth rates for the harmonics are near integer multiples of those for the fundamental.

Spatially extended void-free dusty plasmas in a laboratory radio-frequency discharge

Laboratory experiments with thermophoretic levitation of dust are described that aim at the closure of a central dust-free void region. A careful study of the void structure as a function of the discharge and levitation parameters leads to the discovery of an extended parameter region where stable void-free equilibria are found. The void closure is effected by a novel mechanism that involves a self-organized change in the discharge topology, in which the dust cloud becomes surrounded by a toroidal region of plasma production. In this geometry ions are found to stream radially inwards instead of outwards as in clouds with a central void. This change in ion flow is proved by a reversal of the propagation direction of dust-density waves.

Observation of the spatial growth of self-excited dust-density waves

The growth of a naturally occurring dust-density wave (DDW) is experimentally observed using high-speed imaging. This low frequency wave (∼25 Hz) grows in amplitude as it propagates downward through a dusty plasma. The wave’s linear growth rate −ki is measured using a phase-sensitive analysis method. For the conditions studied here, the growth rate increases as gas pressure decreases. At a critical gas pressure, which is observed, a balance between an ion-flow instability and dissipation by neutral gas drag determines a threshold for wave propagation. A linear dispersion relation is derived, taking into account the effects of strong-coupling, to compare to the experiment.

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.

Experimental Investigation of Dust Density Waves and Plasma Glow

Dust density waves (DDWs) are compressional modes that are often excited by subsonic ion flows in dusty plasmas. Previous experiments relying on imaging of only the dust revealed that they can propagate parallel to the ion flow direction or at an oblique angle. An experiment was performed using microgravity conditions on parabolic flights with video imaging of both the dust and the plasma glow. Glow arises from electron-impact excitation of neutral gas atoms, and it serves as a signature of energetic electrons. Averaging over time, it was found that the presence of dust enhances the glow brightness everywhere in the plasma. Resolving the time variation, a spontaneously excited DDW was observed at 3.9 Hz. It was characterized not only by a compression of the dust number density but also by a modulation of the glow intensity. The correlation between the wave and the glow is analyzed by Fourier methods. We found an unexpected phase relation between the plasma glow and the DDW of 118 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> . A glow maximum is followed by a dust density maximum.

The Structure of Self-Excited Dust-Density Waves Under Microgravity

Dusty plasmas under microgravity conditions are a great opportunity to observe dynamical processes in strongly coupled systems. For example, in such systems, self-excited dust-density waves can occur at low gas pressures in extended regions of the discharge. Recently, we have performed a series of measurements in a parallel-plate RF reactor during parabolic flights. It reveals that the waves can appear in two completely different states. One of them yields a high spatial and temporal coherence of the density fluctuations. This feature allows us to utilize scanning video microscopy to obtain information on the structure of the 3-D wave field. Under different experimental conditions, we also found that a wave field with multiple different wavelengths can arise in the dust volume. This results in defects in the wave pattern due to merging wavefronts. We determine their temporal evolution, which can be derived accurately from the phase information.

Synchronization of dust density waves in anodic plasmas

Dust clouds can be confined in anodic plasmas by an equilibrium of the electric field force and the ion drag force. In these dust clouds self-excited dust density waves are observed as soon as the cloud length exceeds typical wavelengths. By superimposing an external modulation signal on the anode bias, the self-excited wave can be synchronized at the modulation frequency when the frequency is close to the natural frequency of the self-excited wave. For very low frequencies, an interaction of the center of mass motion of the cloud with the self-excited wave is found. For intermediate frequencies, close to the natural frequency of the self-excited modes, entrainment is observed, and for slightly higher frequencies a spatial cascade to subharmonic modes is found. At the highest frequencies, an influence of the modulation is still present, the wave is entrained if the modulation frequency is close to a rational factor of the natural frequency, as can be seen by the narrowing of the spectral components.

Trapped particles by large-amplitude waves in two-dimensional Yukawa liquids

In recent experiments of strongly coupled complex plasmas (SCCPs), trapping-like phenomena of micro-sized dust particles were observed during propagation of large-amplitude dust-density waves. A small number of particles were observed being trapped and carried along with the waves. In the present paper, we study the similar trapping phenomena in two-dimensional SCCPs by using Brownian dynamics simulation. The trapping process and its new features arising from strong-coupling effects are investigated. In particular, similarities and differences between trapping in weakly coupled plasmas and SCCPs are discussed in detail.

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.

Dynamics of small dust clouds trapped in a magnetized anodic plasma

Small dust clouds, which are confined in an anodic plasma, are studied with respect to their structure and their response to modulation of the anode bias. The dust cloud is displaced from the center of the discharge by a process similar to the void mechanism in radio-frequency discharges under microgravity. The top layers of the dust cloud are in a crystalline state and the cloud performs a slow rotation about the magnetic field direction. For modulation frequencies below 15Hz, a sloshing and stretching motion in the confining potential well is found. Spontaneously excited dust density waves are observed when the dust cloud exceeds a minimum size. The waves are characterized by sickle-shaped wave fronts. No standing waves were found. The wave dispersion shows an influence of the boundedness of the system in terms of a frequency cutoff.

Dust density waves in a complex plasma layer

Modifications of dust density waves in a complex plasma layer imbedded in a bulk of electron ion plasmas are studied. In view of laboratory experiments, we consider a layer of finite thickness to be able to examine various cases of interest. Different electron and ion populations inside and outside of the complex plasma layer are assumed. It is found that (i) the geometric factor leads to a splitting of the surface dust-density modes in a specific region of wavenumbers and the appearance of a backward surface mode in case of a thin layer (d∼λD); (ii) due to spatial restriction of a complex plasma, the bulk dust-acoustic mode appears to have the lowest cutoff frequency, whose value is strongly dependent on the ratio between the screening length and the thickness of the layer. Observations of the features discussed can be used for complex plasma diagnostics.

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.

Electrostatic modes in collisional complex plasmas under microgravity conditions.

A linear dispersion relation in a highly collisional complex plasma, including ion drift, was derived in the light of recent PKE-Nefedov wave experiment performed under microgravity conditions onboard the International Space Station. Two modifications of dust density waves with wave frequencies larger than the dust-neutral collision frequency were obtained. The relevance to the space observations was analyzed and a comparison of theory and observations was made for two different complex plasma domains formed by small and large microparticles. Good qualitative agreement is found between the measurements and the theoretical dispersion relations. This allows a determination of the basic complex plasma parameters.

Observation of laser-pulse-induced traveling microbubbles in dusty plasma liquids.

We report a direct experimental observation of traveling microbubbles induced by intense laser pulses in strongly coupled dusty plasma liquids. The dense plasma ablated from a suspended dust particle generates a spherical plasma bubble with a low dust density, in the quiescent regime before a transition to self-organized longitudinal dust density waves. It travels downwards at a velocity about 6 cm/sec inside the dust liquid. Dust density fluctuations trailing the bubble are also observed. The bubble generated in the high pressure dissipative regime collapses right after formation.

Dust-acoustic solitary waves and double layers in dusty plasma with variable dust charge and two-temperature ions

With two-temperature ions in dusty plasma, the effects of variable dust charge on small-amplitude dust-acoustic solitons and large-amplitude dust-acoustic solitary waves are investigated. It is found that both compressive and rarefactive solitons (and solitary waves), as well as double layers, exist. The introduced small, but finite, number density of the lower temperature ions provides the possibility of the coexistence of large-amplitude rarefactive with compressive dust-density solitary waves. The amplitudes of those dust-density solitary waves become smaller and the regime of Mach number is extended wider for the variable dust charge situation compared to the constant dust charge situation.

Nature of the intestinal slow-wave frequency gradient.

Nature of the intestinal slow-wave frequency gradient.