John K. Meyer

Dusty plasmas: Experiments on nonlinear dust acoustic waves, shocks, and structures

A review is presented of recent experiments performed on the University of Iowa dc discharge dusty plasma device on various aspects of dust acoustic waves. A brief introduction to the physics of dusty plasmas and the dust acoustic wave is first presented. Three experiments are then described: (i) observation and interpretation of large amplitude (nonlinear) dust acoustic waves; (ii) evolution of large amplitude dust acoustic waves into shocks, and comparison to numerical shock solutions of the generalized hydrodynamic equations and (iii) the spontaneous formation of stationary, stable dust structures in a moderately coupled dusty plasma (dust structurization).

Nonlinear dust acoustic waves and shocksa)

We describe experiments on (1) nonlinear dust acoustic waves and (2) dust acoustic shocks performed in a direct current (DC) glow discharge dusty plasma. First, we describe experiments showing nonlinear dust acoustic waves characterized by waveforms of the dust density that are typically sharper in the wave crests and flatter in the wave troughs (compared to sinusoidal waves), indicating the development of wave harmonics. We discuss this behavior in terms of a second-order fluid theory for dust acoustic waves. Second, experimental observations of the propagation and steepening of large-amplitude dust acoustic waves into dust acoustic shock waves are presented. The observed shock wave evolution is compared with numerical calculations based on the Riemann solution of the fully nonlinear fluid equations for dust acoustic waves.

Experimental quiescent drifting dusty plasmas and temporal dust acoustic wave growth

We report on dust acoustic wave growth rate measurements taken in a dc (anode glow) discharge plasma device. By introducing a mesh with a variable bias 12–17 cm from the anode, we developed a technique to produce a drifting dusty plasma. A secondary dust cloud, free of dust acoustic waves, was trapped adjacent to the anode side of the mesh. When the mesh was returned to its floating potential, the secondary cloud was released and streamed towards the anode and primary dust cloud, spontaneously exciting dust acoustic waves. The amplitude growth of the excited dust acoustic waves was measured directly along with the wavelength and Doppler shifted frequency. These measurements were compared to fluid and kinetic dust acoustic wave theories. As the wave growth saturated a transition from linear to nonlinear waves was observed. The merging of the secondary and primary dust clouds was also observed.

Secondary dust density waves excited by nonlinear dust acoustic waves

Secondary dust density waves were observed in conjunction with high amplitude (nd/nd0>2) dust acoustic waves (DAW) that were spontaneously excited in a dc glow discharge dusty plasma in the moderately coupled, Γ∼1, state. The high amplitude dust acoustic waves produced large dust particle oscillations, displacements, and trapping. Secondary dust density waves were excited in the wave troughs of the high amplitude DAWs. The waveforms, amplitudes, wavelengths, and wave speeds of the primary DAWs and the secondary waves were measured. A dust-dust streaming instability is discussed as a possible mechanism for the production of the secondary waves.

Low-frequency electrostatic waves in a magnetized, current-free, heavy negative ion plasma

We report experimental observations of a low-frequency ( ion gyrofre- quency) electrostatic wave mode in a magnetized cylindrical (Q machine) plasma containing positive ions, very few electrons and a relatively large fraction (n−/ne > 10 3 ) of heavy negative ions (m−/m+ ≈ 10), and no magnetic field-aligned current. The waves propagate nearly perpendicular to B with a multiharmonic spectrum. The maximum wave amplitude coincided spatially with the region of largest density gradient suggesting that the waves were excited by a drift instability in a nearly electron-free positive ion-negative ion plasma

Interaction of a biased cylinder with a flowing dusty plasma

Experimental observations of supersonically flowing dusty plasmas and their interaction with an electrically biased circular cylinder are presented. Two methods for producing flowing dusty plasmas are described. The dusty plasma is produced in a DC anode glow discharge plasma. In Configuration I, a secondary dust cloud, initially formed near a biased grid, flowed away from the grid at supersonic speeds when the grid voltage was suddenly changed. In Configuration II, a pencil-like dust beam was produced using a nozzle-like (converging-diverging) electrostatic potential structure. Using Configuration I, the streaming dust encountered a biased cylinder (wire) whose axis was oriented transverse to the dust flow. The flowing dust particles were repelled by the electrostatic field of the negatively charged cylinder, and a dust void was formed around the cylinder. A detached electrohydrodynamic bow shock, akin to the Earths magnetohydrodynamic bow shock, was formed on the upstream side of the cylinder, while an extended teardrop-shaped wake region was formed on the downstream side. Video imaging of the dust stream allowed for observations of the structure and evolution of the bow shock. Configuration II was used to produce a narrow beam of dust particles and observe how the beam was deflected around the biased cylinder. Three multimedia files (movies) of the observed phenomena are provided in the online Supplementary material.

Transient bow shock around a cylinder in a supersonic dusty plasma

Visual observations of the formation of a bow shock in the transient supersonic flow of a dusty plasma incident on a biased cylinder are presented. The bow shock formed when the advancing front of a streaming dust cloud was reflected by the obstacle. After its formation, the density jump of the bow shock increased as it moved upstream of the obstacle. A physical picture for the formation of the electrohydrodynamic bow shock is discussed.

Coupling of non-crossing wave modes in a two-dimensional plasma crystal

We report an experimental observation of the coupling of the transverse vertical and longitudinal in-plane dust-lattice wave modes in a two-dimensional complex plasma crystal in the absence of mode crossing. A new large-diameter rf plasma chamber was used to suspend the plasma crystal. The observations are confirmed with molecular dynamics simulations. The coupling manifests itself in traces of the transverse vertical mode appearing in the measured longitudinal spectra and vice versa. We calculate the expected ratio of the trace to the principal mode with a theoretical analysis of the modes in a crystal with finite temperature and find good agreement with the experiment and simulations.

Evolution of Dust Clouds in Afterglow Plasmas

The observations of the evolution of dust clouds in afterglow plasmas at various neutral pressures are presented. Four cases were studied which showed a large variation in the cloud dynamics. In two cases, the clouds responded by rapidly shedding an outer layer of dust. In another case, the entire cloud exhibited a nearly uniform expansion-Coulomb explosion. In perhaps the most exotic case, the cloud splits into two clouds-Coulomb fission. The results for the case of the Coulomb expansion were compared with a theoretical model that included the effects of neutral drag.

Coulomb fission of a dusty plasma

Experimental observations are presented of the splitting (fission) of a suspension of charged microparticles (dusty plasma) into two fragments when the plasma was suddenly turned off. The triggering mechanism for fissioning of the dust cloud is discussed in terms of a pinching instability driven by the ion drag force.