"In this earlier literature, the mode was often called the dust acoustic wave. However, recent experiments have shown that much more complicated situations can occur, where the propagation direction of the waves is not strictly parallel to the local ion flow , . The authors also presented a model , which explains the direction of these obliquely propagating DDWs and its dependence on the ion local flow velocity. "
[Show abstract][Hide abstract] ABSTRACT: 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>o</sup>. A glow maximum is followed by a dust density maximum.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
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