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We previously reported the observation of a hydrodynamic-type interchange instability occurring sporadically at the curved interface between two dust clouds of different densities, in a three-dimensional dusty plasma [Pacha et al., Phys. Plasmas 19, 014501 (2012)]. The mechanism for the instability was not conclusively identified. Further analysis of the video images has been performed, showing that the instability is associated with the appearance of large amplitude dust acoustic waves or shocks. Evidence of a dust acoustic shock at the boundary, which could drive a Richtmyer–Meshkov-type instability, will be presented. We also discuss the possibility, suggested by Avinash and Sen [Phys. Plasmas 22, 083707 (2015)], that the observed instability is due to a Rayleigh–Taylor instability driven by a component of the gravitational acceleration, which is perpendicular to the curved boundary.

Electrostatic flow-driven instabilities have been predicted in the presheath region of low-temperature plasmas for both ion and electron rich sheaths. The ion-acoustic fluctuations that arise from these instabilities can influence transport through an enhanced collision rate. In this work, ion fluctuation spectra were measured using laser-induced fluorescence (LIF) in the presheath region for both sheath polarities. The non-invasive nature of LIF compared to that of probes is particularly valuable for these measurements because the sheath and presheath around a probe may generate their own flow-driven instabilities, which obscure the measurement. Measurements were made using a recently implemented field programmable gate array-based system able to measure ion fluctuation spectra up to 1 MHz using a two-point correlation function technique. Narrow bandwidth ion fluctuations were observed near 0.46 f pi (460 kHz) for both sheath polarities, where f pi is the ion plasma frequency. The observed fluctuations were significantly stronger in the electron sheath case and were observed over a larger volume of plasma. Fluctuations were measured in the ion sheath case at locations far enough from the sheath that linear theory predicts stability, suggesting that ion-acoustic instabilities generated in the relatively small unstable region near the boundary reflect from the sheath and perturb a much larger volume of the plasma. This is expected to affect probes with both ion and electron rich sheaths, meaning any dc biased probe may effectively act as an ion-acoustic wave antenna. These measurements are consistent with the recent theory and particle-in-cell results.

Dusty plasmas have been studied in argon, radio frequency (rf) glow discharge plasmas with magnetic fields up to 1.5 T. Plasmas were formed between a powered bottom electrode and a grounded top electrode in which an aluminum wire mesh is fixed in the center. This work will examine the experimental conditions at which imposed, ordered structures form within the dusty plasma. These imposed, ordered structures, as reported in earlier works, are characterized by the alignment of dust particles to the spatial structure of the wire mesh in the top electrode and occur at high magnetic fields (B ≥ 1 T) and low neutral pressure (P ≤ 100 mTorr). A pair of parameters will be established to characterize the degree to which the dust particles are affected by these imposed ordered structures on both long and short time scales. Using these parameters, it will be shown that a correlation exists between the onset of imposed, ordered structures and the magnetization of ions within the plasma.Dusty plasmas have been studied in argon, radio frequency (rf) glow discharge plasmas with magnetic fields up to 1.5 T. Plasmas were formed between a powered bottom electrode and a grounded top electrode in which an aluminum wire mesh is fixed in the center. This work will examine the experimental conditions at which imposed, ordered structures form within the dusty plasma. These imposed, ordered structures, as reported in earlier works, are characterized by the alignment of dust particles to the spatial structure of the wire mesh in the top electrode and occur at high magnetic fields (B ≥ 1 T) and low neutral pressure (P ≤ 100 mTorr). A pair of parameters will be established to characterize the degree to which the dust particles are affected by these imposed ordered structures on both long and short time scales. Using these parameters, it will be shown that a correlation exists between the onset of imposed...

An experimental study of the formation of voids (dust-free regions) around negatively biased probes in a dusty plasma is described. Stable voids are maintained by the balance of electric and ion drag forces on the dust particles. A theoretical model is proposed to explain how the size of the void scales with the probe bias potential.

The shielding of dust particles by each other in a dusty plasma is studied taking into account the effects of dust charging. It is shown that the assumption of a Boltzmann response for dust species is not appropriate under these circumstances. Further, it is shown that in the close-packing limit, dust grains screen each other by charge reduction, and an expression is obtained for this shielding scale length.

The effect of negatively charged dust on the Kelvin–Helmholtz (parallel velocity shear) instability is investigated experimentally in a magnetized cesium plasma. The dust generally has a stabilizing effect on the instability, although, in some cases, the addition of negatively charged dust into the plasma results in a slight increase in the instability fluctuation amplitude. The results are in general agreement with theoretical predictions.

The effect of the magnetic field configuration on the formation of ion acoustic shocks has been investigated experimentally and through numerical solutions of the fluid equations. The tendency for compressive pulses to steepen into shocks is enhanced if the pulses travel into a configuration of converging magnetic field geometry.

An experimental investigation of the effect of negatively charged dust on ion acoustic shock formation in a Q machine is described. Ion acoustic compressional pulses were observed to steepen as they traveled through a dusty plasma if the percentage of the negative charge in the plasma on the dust grains was ≳75%.

The interaction of an electrically floating metal object with a dusty plasma formed in a dc discharge was investigated experimentally. Three cases were studied in which (a) the object was at rest in the dusty plasma, or was moved through the dusty plasma with a speed that was either (b) on the order of the dust-acoustic speed or (c) much faster than the dust-acoustic speed. When the object was at rest, a dust-free cavity with a boundary with a sharp dust density gradient was formed around the object. The size of the cavity was largely determined by shielding of the object by the plasma ions. A similar (although dynamic) behavior was observed in the moderate speed case (case b). When the object was moved very rapidly (case c) the dust grains did not have time to respond, and the object mechanically removed the grains, leaving behind a narrow channel which quickly filled in.

Theoretical and experimental studies of low-frequency electrostatic waves in plasmas containing negatively charged dust grains are described. The presence of charged dust is shown to modify the properties of ion-acoustic waves and electrostatic ion-cyclotron waves through the quasineutrality condition even though the dust grains do not participate in the wave dynamics. If the dust dynamics is included in the analysis, new “dust modes” appear—dust acoustic and dust cyclotron modes. The results of laboratory experiments dealing with dust ion acoustic (DIA) waves and electrostatic dust ion cyclotron (EDIC) waves are shown. These modes are more easily excited in a plasma containing negatively charged dust. Finally, observations of dust acoustic (DA) waves are presented and measurements of the dispersion relation are compared with one obtained from fluid theory.

An experimental investigation of dust acoustic (DA) waves in a dc glow discharge plasma is described. The glow discharge is formed between a 3 cm anode disk and the grounded walls of a 60 cm diameter vacuum chamber which is filled with nitrogen gas at a pressure of about 100 mTorr. Dust located on a tray in the chamber is attracted into the plasma where it is trapped electrostatically. The dust acoustic waves were produced by applying a modulation signal (5–40 Hz) to the anode. The wavelength of the DA waves was measured from single frame video images of scattered light from the dust grains. The measured dispersion relation is compared with theoretical predictions.

What effect does a dielectric material surrounding a small positively biased electrode have on the ion flow and sheath structure near the electrode? Measurements of the ion velocity distribution function and plasma potential near positively biased electrodes were made using laser-induced fluorescence and an emissive probe. The results were compared with 2D particle-in-cell simulations. Both measurements and simulations showed that when the positive electrode was surrounded by the dielectric material, ions were accelerated toward the electrode to approximately 0.5 times the ion sound speed before being deflected radially by the electron sheath potential barrier of the electrode. The axial potential profile in this case contained a virtual cathode. In comparison, when the dielectric material was removed from around the electrode, both the ion flow and virtual cathode depth near the electrode were dramatically reduced. These measurements suggest that the ion presheath from the dielectric material surrounding...

Negatively charged, micron‐sized dust grains have been electrostatically confined in the high‐potential region of an anode double layer formed in a single‐ended Q‐machine plasma. The levitated dust grains are large enough to be visually observed in the scattered light from the plasma source. The various forces on a dust grain in a plasma, i.e., gravitational, electric, ion drag, and neutral drag are estimated based on the measured plasma and dust parameters. The possibility of observing a Coulomb lattice in such a setup is discussed.

The Magnetized Dusty Plasma Experiment at Auburn University has been operational for over one year. In that time, a number of experiments have been performed at magnetic fields up to B = 2.5 T to explore the interaction between magnetized plasmas and charged, micron-sized dust particles. This paper reports on the initial results from studies of: (a) the formation of imposed, ordered structures, (b) the properties of dust wave waves in a rotating frame, and (c) the generation of plasma filaments.

The effect of magnetic field-aligned ion flow with a transverse flow velocity gradient (parallel velocity shear) on the excitation of the electrostatic ion cyclotron (EIC) instability in narrow current channels (width

The properties of double layers produced in current filaments have been studied. The experiments were performed in a magnetized triple plasma device in which the diameter of the central plasma column could be varied. The scaling of the double layer potential drop Vdl with the parameter jd2, where j is the current density in the double layer and d is the double layer thickness, was determined for several values of the current filament radius R0. For relatively large values of R0, the one‐dimensional (Langmuir) scaling was obeyed, Vdl∼(jd2)2/3. When R0 was decreased, a departure from the strictly one‐dimensional scaling was observed, with Vdl becoming less and less dependent on the parameter jd2.

Dusty plasmas have been studied in argon, radio frequency (rf) glow discharge plasmas at magnetic fields up to 2.5 T where the electrons and ions are strongly magnetized. Plasmas are generated between two parallel plate electrodes where the lower, powered electrode is solid and the upper electrode supports a dual mesh consisting of #24 brass and #30 aluminum wire cloth. In this experiment, we study the formation of imposed ordered structures and particle dynamics as a function of magnetic field. Through observations of trapped particles and the quasi-discrete (i.e., “hopping”) motion of particles between the trapping locations, it is possible to make a preliminary estimate of the potential structure that confines the particles to a grid structure in the plasma. This information is used to gain insight into the formation of the imposed grid pattern of the dust particles in the plasma.

Dusty plasmas have been studied in argon, rf glow discharge plasmas at magnetic fields up to 2 T, where the electrons and ions are strongly magnetized. In this experiment, plasmas are generated between two parallel plate electrodes where the lower, powered electrode is solid and the upper, electrically floating electrode supports a semi-transparent, titanium mesh. We report on the formation of an ordered dusty plasma, where the dust particles form a spatial structure that is aligned to the mesh. We discuss possible mechanisms that may lead to the formation of the “dust grid” and point out potential implications and applications of these observations.

A laboratory observation of the dust‐acoustic instability is reported. The results are compared with available theories.

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.

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.

Observations of the Taylor instability in a laboratory dusty plasma are presented. The dust cloud, formed in a dc argon glow-discharge plasma, is stratified into regions of high and low dust densities. The instability was triggered by a spontaneous intrusion of the low density dust fluid into the high density dust fluid at the interface. The instability in the dust fluid was phenomenologically similar to the hydrodynamic Taylor instability that occurs when a light fluid is accelerated into a heavy fluid.

Using a fluid treatment, it is shown that electrostatic waves with angular frequencies ∼ωci (ion cyclotron frequency) propagating at large angles to the ambient magnetic field can be excited in a magnetized plasma by perpendicular shear in the magnetic field aligned plasma flow. The role of density gradients in determining the stability of shear-driven electrostatic ion-cyclotron (EIC) modes is also considered. These shear-driven modes may provide an explanation for observations of high frequency waves in the ionosphere.

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.

We report observations of an instability that occurs in a dc glow discharge (anodic plasma) dusty plasma when a floating plate with a 5 mm aperture is placed in front of the anode. The instability is characterized by periodic quenching and reignition of the discharge. When the discharge is quenched, the dust is ejected from the aperture at speeds on the order of the dust-acoustic speed. The ejected dust remains partially charged and retracts back to the aperture when the discharge spontaneously redevelops.

Dust acoustic waves (DAWs) are spontaneously excited in dusty plasmas produced in dc and rf discharge plasmas over a wide range of plasma and dust conditions. A common feature of these plasmas is the presence of an ion drift relative to the dust, which is driven by an electric field, Eo in the discharge. Using a three fluid model of the DAWs, including the zero order electric field and collisions of all species with the background neutral gas (pressure Po), DAW stability curves were obtained in the Eo-Po plane, for various dust and wave parameters. The (Eo,Po) data points from several experiments in which DAWs have been observed are also shown in comparison with the theoretical stability boundaries. This analysis supports the conclusion that the DAWs are excited by an ion-dust streaming instability.

We point out a deficiency in our previous analytic calculation of the ion drag force for conditions of the experiment by Nosenko et al. [Phys. Plasmas 14, 103702 (2007)]. An inaccurate approximation is corrected and the ion drag force is recalculated. The improved model yields better overall agreement with the experimental results as compared to the original calculation.

The diffraction of dust acoustic (DA) waves around a long dielectric rod is observed using video imaging methods. The DA waves are spontaneously excited in a dusty plasma produced in a direct current glow discharge plasma. The rod acquires a negative charge that produces a coaxial dust void around it. The diameter of the void is the effective size of the “obstacle” encountered by the waves. The wavelength of the DA waves is approximately the size of the void. The observations are considered in relation to the classical problem of the diffraction of sound waves from a circular cylinder, a problem first analyzed by Lord Rayleigh [Theory of Sound, 2nd ed. (MacMillan, London, 1896)].

Ion-dust streaming instability in a plasma containing dust grains with large thermal speeds is considered using kinetic theory. The results are compared with experimental measurements of the dispersion relation of dust acoustic waves performed at the University of Iowa and Auburn University.