Imaging of the Dust Acoustic Wave to Explore Synchronization

Abstract

Abstract-A self-excited dust acoustic wave is synchronized by sinusoidal modulation of the ion density, and the wave is imaged by planar laser-light scattering. Space-time diagrams based on the images reveal how the nonlinearity of the plasma's response causes the wave's frequency to be synchronized to a multiple of 0.5, 1, 2, or 3 of the modulation frequency. Space-time diagrams also reveal wave front merging as is observed for a wide range of modulation frequencies.Index Terms-Dusty plasmas, plasma waves. T HE dust acoustic wave (DAW) is a plasma wave that an experimenter can observe with the naked eye. This wave is analogous to an ion acoustic wave and propagates in a dusty plasma containing highly charged particles of solid matter (dust) in addition to electrons, ions, and neutral gas atoms. Compressions and rarefactions of the dust particles have such a high amplitude, and they propagate so slowly, that they are easy to view and image by laser light scattering. Due to the low charge-to-mass ratio of the dust particles, the wave frequency is typically only a few tens of hertz. The wave is self-excited by an ion-streaming instability [1].We performed an experiment to observe nonlinear phenomena, which occur because the wave grows to a large amplitude. We observed wave synchronization where the wave's frequency changes in response to an external modulation of the plasma conditions [2]- [4]. We study the same synchronization phenomenon as in [4], but here we rely on space-time diagrams, prepared as in [5], as our analysis method.An argon glow discharge plasma was ignited by applying a 13.56-MHz radio frequency voltage with 57 V peak-to-peak amplitude to a horizontal lower electrode [ Fig. 1(a)]. Dust particles, which were 4.8-µm melamine formaldehyde spheres, were dropped into the plasma using a dispenser with a single hole. The dust particles were negatively charged by collecting more electrons than ions, and they levitated under the action of naturally occurring electric fields. To vertically elongate the dust cloud, we used a glass box that enhanced the horizontal component of the electric field. The vertical component of this electric field drove a downward ion flow, which was the energy source for exciting the DAW. The 120-mTorr gas pressure was low enough that gas damping did not prevent the DAW from growing to large amplitudes with significant nonlinearities. The natural frequency of the wave, meaning its frequency without any modulation present, was 20.5 Hz. Our primary diagnostic was planar laser-light scattering for imaging the dust cloud [6]. The intensity of the scattered light was proportional to the dust number density n d , because the dust cloud was optically thin. A vertical cross section of the cloud was illuminated with a sheet of 532-nm laser light. A movie was recorded at 256 frames/s using a Phantom v5.2 camera. A still image is shown in Fig. 1(b). We analyzed the intensity within a region of interest (ROI) [ Fig. 1(b)]. The wavefronts were planar in the ROI, so that we can average the int...

Characterization of three-dimensional structure using images

“…The advantage of doing so is that the instrumentation is simplified; one can use a common 2D imaging configuration. 7 For micron-size particles, such a two-dimensional imaging configuration is sketched in Fig. 1(a) with a single camera.…”