Jonathon R. Heinrich

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.

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.

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.

Flow of Dusty Plasma Around an Obstacle

Single frame video images of dusty plasmas flowing around a conducting wire are presented in this paper. The images were obtained by laser illumination of the dust suspension with the reflected light recorded using a fast video camera. The images from two different configurations are shown. The first image records the formation of a bow shock formed when a supersonic dust cloud impinges on a thin-wire biased to repel the negatively charged dust. The second image shows the deflection of a thin stream of dust particles around a negatively charged wire.

Dynamics of positive probes in underdense, strongly magnetized, E×B drifting plasma: Particle-in-cell simulations

Electron trapping, electron heating, space-charge wings, wake eddies, and current collection by a positive probe in E×B drifting plasma were studied in three-dimensional electromagnetic particle-in-cell simulations. In these simulations, electrons and ions were magnetized with respect to the probe and the plasma was underdense (ωpe<ωce). A large drift velocity (Mach 4.5 with respect to the ion acoustic speed) between the plasma and probe was created with background electric and magnetic fields. Four distinct regions developed in the presences of the positive probe: a quasi-trapped electron region, an electron-depletion wing, an ion-rich wing, and a wake region. We report on the observations of strong electron heating mechanisms, space-charge wings, ion cyclotron charge-density eddies in the wake, electron acceleration due to a magnetic presheath, and the current-voltage relationship.

Dynamics of a positive probe in ExB drifting plasma: 3-D electromagnetic particle-in-cell simulations

Summary form only given. We present the recent results of a series of electromagnetic particle-in-cell simulations, taken over a broad range of parameter space, that modeled a positive probe in E×B drifting plasma. These results address two fundamental problems: momentum coupling in magnetized flowing plasma and the plasma/sheath dynamics of a positive probe in E×B drifting plasma. In these simulations we observed rarefied electron wings develop along the magnetic field in the wake of the positive probe and a quasi-trapped electron region. Depending on the parameter regime, either electrostatic or electromagnetic waves formed in the probes wake (directly downstream of the probe or along the rarefied electron wings), similar to predictions by Stenzel and Urrutia1. Additionally, we report on sheath dynamics of a positive probe in E×B drifting plasma, which were clearly resolved and indicated multiple electron heating processes. These electron heating processes included electron heating due to the quasi-trapped electron mode as well as electron heating due to wave-particle interactions in the ram region of the probe and along the electron rarefied wings. These electron heating processes significantly affect the overall current collected by the probe2.

Nonlinear Dust Acoustic Waves, Shocks and Stationary Structures in a DC Glow Discharge Dusty Plasma

The dust acoustic wave (DAW) is a very low frequency (tens of Hz) dust density wave in which the dust particles participate in the wave dynamics. The early experimental observations of DAWs showed that the wave was self‐excited by a modest relative ion drift and grew to very high amplitudes (∼100%). In the first part of this paper we describe experiments showing the self‐steepening of nonlinear DAWs into dust acoustic shock waves. In the second part we present observations of self‐organized, stationary (i.e., non‐propagating), stable, dust density structures formed in a DC glow discharge dusty plasma.