Nonlinear Wave Synchronization in a Dusty Plasma Under Microgravity on the International Space Station (ISS)

Abstract

In a plasma containing micrometer-size dust particles, nonlinear wave synchronization was investigated experimentally under microgravity conditions on board the International Space Station (ISS). These dust particles were confined into a 3-D cloud, in the vicinity of a diffuse edge of the plasma, which was generated by an inductively coupled radio frequency (RF) glow discharge. A cross-sectional slab of the cloud was imaged using a video camera. The dust-density fluctuations in the slab were characterized using the video image data. A steady and longlived dust acoustic wave (DAW) was observed to be spontaneously generated in the cloud; it propagated through the dust cloud, which had a gradually varying density distribution. Two kinds of spectral analyses of the wave motion were performed, using Fourier transforms and Hilbert transforms, respectively; these revealed two distinctive spatial domains in the cloud, termed frequency clusters. Within each cluster, waves were found to oscillate at a dominant frequency that remained constant, manifesting mutual synchronization throughout the cluster. Across the two clusters, the dominant frequency exhibited a step-wise change, with a frequency ratio of 2:1, which is consistent with phase-lock conditions for a harmonic synchronization state.

Experiment and model for a Stokes layer in a strongly coupled dusty plasma

“…Besides providing an intuitive way of visualizing a Stokes layer, these diagrams also allow an easy identification of features such as the depth of penetration and the wavelength. We are motivated to use space-time diagrams because they are familiar in the dusty plasma literature, not for Stokes layers, but for other purposes [40,67,68,[81][82][83][84][85][86][87][88][89], including the characterization of density waves [90][91][92][93][94][95]. Some of these authors have used the terms periodograms or periodgrams for such plots.…”

“…The latter force is used in our experiment, as it was in previous experiments involving a shear flow. Some of those previous experiments used a steady shear flow [38,[52][53][54][55][56][57][58][59][60][61][62][63][64][65][66] while in others the shear was suddenly switched on [67,68]. There have also been some experiments with a periodically modulated shear [38,53], but they were analyzed for purposes other than the identification of a Stokes layer.…”