Dust Density Waves Research Articles

Shivalik Plasma Device-I, a glow discharge device to study the collective dynamics of dusty plasma

Using a uniquely configured glow discharge-based Shivalik Plasma Device-I, we demonstrate a variety of collective phenomena in dusty plasma away from the glow discharge region. The cylindrical glass device produces plasma using parallel disc-shaped electrodes with a smaller anode size than the cathode. The dust microparticles are initially sprinkled over the grounded cathode. These particles acquire a significant negative charge upon plasma formation, resulting in their levitation due to the balance between the Coulomb force and gravity. The new device supports the levitation of a big-sized (10 × 8 × 5 cm3) three-dimensional dust cloud over the glass surface. It contrasts the dusty plasma formations in-between electrodes reported earlier. As the discharge voltage varies from high to low, the dust cloud travels from over the glass surface to between the electrodes. A complex interplay of dust void over the cathode, a sharp density gradient, and gravity lead to self-excitation of collective dust phenomena. It includes dust density waves (phase velocity, vph ∼ 4 cm/s), dust cloud oscillation (frequency, f = 5 Hz), sheared flow (flow velocity, vf ∼ 1 cm/s), and multiple-sized dust vortices. These dust vortices provided an excellent platform for studying turbulent mixing phenomena. The power spectrum analysis agreed with two-dimensional Kolmogorov power-law scaling. This is an ideal dusty plasma apparatus where we can create or move the dust cloud to a location of choice from the glass surface to in-between the electrodes and excite one among many collective dust dynamics.

Propagation characteristics of dust density waves in an external magnetic field

Propagation of dust density waves (DDW) is theoretically investigated in a collisional plasma in the presence of an external magnetic field. The magnetic field is kept constant in a direction perpendicular to the direction of streaming ions, which is an important requirement for the self-excitation of these waves. Standard stability analysis is done to predict the conditions under which ion-drift instability may drive the DDW unstable. It is found that a minimum dust density is required to excite the wave. There exists a lower cut-off value of ion streaming velocity for the excitation of DDW which may change with the applied magnetic field and neutral gas pressure. The wave is found to behave complexly with the applied magnetic field and gets damped beyond a critical value. The results of this paper may be helpful in understanding the behavior of low-frequency dust modes as well as the background plasma both in laboratory and astrophysical environments.

Decoupling of dust cloud and embedding plasma for high electron depletion in nanodusty plasmas

In recent years nanoparticles have become key technological products, e.g., as coatings with tunable optical gap in third generation solar cells, as nanocrystals for photonic applications, and as pharmaceutical nanocarriers. In particle sources, that use reactive, nanodusty plasmas, a high dust density changes the properties of the dusty plasma compared to a dust free plasma considerably, as electron depletion leads to a reduced number of free electrons. This is called the Havnes effect and was central for the understanding of the famous spokes in Saturns rings. We see here, that it is also important for technological applications. Using self excited dust density waves (DDW) as a diagnostic tool, we completely characterize an argon discharge with embedded amorphous hydrocarbon nanoparticles of different size and density. The results show, that electron depletion governs the charge of dust grains, while the size of the particles has only a weak influence. The ion density and electric potential profile are almost independent of both, dust size as well as dust density. This suggests, that the rf generated plasma and the dust cloud seem to coexist and coupling of both is weak.

Spatial distribution of dust density wave properties in fluid complex plasmas.

Complex plasmas consist of microparticles embedded in a low-temperature plasma and allow investigating various effects by tracing the motion of these microparticles. Dust density waves appear in complex plasmas as self-excited acoustic waves in the microparticle fluid at low neutral gas pressures. Here we show that various properties of these waves depend on the position of the microparticle cloud with respect to the plasma sheath and explain this finding in terms of the underlying ion-drift instability. These results may be helpful in better understanding the propagation of dust density waves in complex plasmas and beyond, for instance, in astrophysical dusty plasmas.

Growth and treatment of hydrogenated amorphous carbon nanoparticles in a low‐pressure plasma

AbstractA parallel‐plate, low‐pressure plasma for fundamental nanodusty plasma research is used to grow hydrogenated amorphous carbon nanoparticles using an argon‐acetylene gas mixture. The particles stay confined in the volume of the argon plasma after turning off the gas flow and the effects of prolonged treatment with noble gas (Ar) and reactive gas mixtures (Ar/, Ar/, or Ar/) are investigated using in situ infrared absorption spectroscopy. Additionally, ex situ scanning electron microscopy imaging of extracted nanoparticles is used to analyze their size and surface morphology. In 45 min of argon treatment, a size increase of about 50% is seen together with a decrease in bonds and an increase in C═O bonds, indicating incorporation of oxygen from gas impurities into the particle material. All reactive gas mixtures lead to the expected etching of the nanoparticle material without any exchange reactions between gas‐phase deuterium and surface‐bonded hydrogen atoms. These results are important for in situ studies of nanoparticle clouds such as dust density wave diagnostics, but they also provide fundamental information about plasma interaction with a‐C:H material.

Spatiotemporal evolution of a self-excited dust density wave in a nanodusty plasma under strong Havnes effect

A broad-spectrum self-excited dust density wave is experimentally studied in a vertically extended nanodusty plasma consisting of in situ grown carbonaceous nanometer sized particles. The nanodusty plasma having high particle density (of the order of 1012–1013 m−3) is created with vertical extension up to (40±0.1) cm and radial extension up to (5±0.1) cm. The propagation of the self-excited dust density wave under strong Havnes effect is examined over a large axial distance (19±0.1) cm. Time-resolved Hilbert transformation and Fast Fourier transformation techniques are used to study the spatiotemporal evolution of frequency and wavenumbers along three directions from the dust void, viz., axial, radial, and oblique. The propagation is found to be inhomogeneous throughout the dust cloud. The phase velocity of the wave is estimated to be quite low and decreasing along the direction of propagation. This effect is attributed to the strong reduction of particle charge due to a high Havnes parameter along the propagation direction. By the estimation of average particle charge, ion density, and the finite electric field throughout the nanodust cloud, a quantitative analysis of the void formation in nanodusty plasma is presented. New insights are also made regarding wave merging phenomena using time-resolved Hilbert transformation.

A nanodusty plasma experiment to create extended dust clouds using reactive argon acetylene plasmas

A large volume 3D dust cloud containing in situ grown nanometer-sized particles is produced in a newly developed versatile table-top experimental device. Carbonaceous nanoparticles having almost uniform size throughout the dust cloud are grown using capacitively coupled rf discharge in Ar–C2H2 gas mixture with a low precursor gas flow rate (∼2 sccm) and minimal rf power (∼1 W). The vertical and radial extensions of the dust cloud are 40 cm and 5 cm, respectively. The pure Ar plasma in the setup is characterized by measuring the discharge parameters as well as plasma parameters under different discharge conditions. The average particle size and its temporal growth profile are determined by analyzing the scanning electron microscope images of the particles. The dust density measured using the laser extinction method is found to be of the order of 1016–1012 m−3 for the discharge duration of 2–10 min. A spontaneous dust density wave is also observed in the dust cloud.

Simulations of ion–dust streaming instability in a highly collisional plasma

The excitation of low frequency dust acoustic (or dust density) waves in a dusty plasma can be driven by the flow of ions relative to dust. We consider the nonlinear development of the ion–dust streaming instability in a highly collisional plasma, where the ion and dust collision frequencies are a significant fraction of their corresponding plasma frequencies. This collisional parameter regime may be relevant to dusty plasma experiments under microgravity or ground-based conditions with high gas pressure. One-dimensional particle-in-cell simulations are presented, which take into account collisions of ions and dust with neutrals, and a background electric field that drives the ion flow. Ion flow speeds of the order of a few times thermal are considered. Waveforms of the dust density are found to have broad troughs and sharp crests in the nonlinear phase. The results are compared with the nonlinear development of the ion–dust streaming instability in a plasma with low collisionality.

Dust-density waves in radio-frequency discharges under magnetic fields

Experiments on self-excited dust-density waves under various magnetic fields have been performed. For that purpose, different dust clouds of micrometer-sized dust particles were trapped in the sheath of a radio frequency discharge. The self-excited dust-density waves were studied for magnetic field strengths ranging from 0 mT to about 2 T. It was observed that the waves are very coherent at the lowest fields (B < 20 mT). At medium fields (20 mT < B<300 mT), the waves seem to feature a complex competition between different wave modes before, at even higher fields, the waves become more coherent again. At the highest fields (above 1 T), the wave activity is diminished. The corresponding wave frequencies and wavenumbers have been derived. From the comparison of the measured wave properties and a model dispersion relation, the ion density and the dust charge are extracted. Both quantities show only little variation with magnetic field strength.

Suppression of a spontaneous dust density wave by modulation of ion streaming

Synchronization of a self-excited dust density wave has been experimentally investigated in a strongly coupled dusty plasma. A dust density wave of frequency ∼78 Hz is spontaneously generated from the dust void boundary due to the ion streaming instability. The electric field in the dust void region is measured, and the electric field force and ion drag force on the dust particles at the void boundary are estimated to explain the mechanism of spontaneous dust density wave excitation. Synchronization occurring through the suppression mechanism is observed by modulating the ion streaming by applying an external sinusoidal signal to the dust void. At sufficiently high modulation amplitude, the onset of period-doubling bifurcation is observed. Fast Fourier transform spectral analysis is done using time-series data obtained from high-speed video imaging. The van der Pol equation with a force term is used to correlate the observed suppression phenomena.

Measurement of gas temperatures in dust-free and dusty argon discharges

Gas temperatures in an argon low-power capacitively-coupled rf discharge were measured by means of tunable diode laser absorption spectroscopy on 1s5 metastable states. It was shown that the uncertainty of the measurements in dust-free plasma could be minimized to 1 K. In the presence of dust, due to instabilities in the dust subsystem (e.g. dust density waves), the uncertainty of the temperature measurements increased several times. Nevertheless, an evidence of the gas temperature increase in the presence of dust was detected. This result suggests that the microparticle suspensions, immersed in the low-pressure plasmas, significantly modify the temperature fields of the neutral gas.

Excitation of low-frequency dust density waves in flowing complex plasmas

We study low frequency dust density waves excited in the microparticle streams of the Plasmakristall-4 chamber under microgravity conditions on board the International Space Station. Dispersion relations and instability conditions are analyzed for the dust density modes propagating along the axial electric field and in the reversed direction, revealing some important implications for the dust diagnostics. The results clearly point out to the Havnes parameter as a key factor for the excitation of the dust density modes.

Dust density waves in a dc flowing complex plasma with discharge polarity reversal

We report on the observation of the self-excited dust density waves in the dc discharge complex plasma. The experiments were performed under microgravity conditions in the Plasmakristall-4 facility on board the International Space Station. In the experiment, the microparticle cloud was first trapped in an inductively coupled plasma and then released to drift for some seconds in a dc discharge with constant current. After that, the discharge polarity was reversed. DC plasma containing a drifting microparticle cloud was found to be strongly non-uniform in terms of microparticle drift velocity and plasma emission in accordance with [Zobnin et al., Phys. Plasmas 25, 033702 (2018)]. In addition to that, non-uniformity in the self-excited wave pattern was observed: In the front edge of the microparticle cloud (defined as head), the waves had larger phase velocity than in the rear edge (defined as tail). Also, after the polarity reversal, the wave pattern exhibited several bifurcations: Between each of the two old wave crests, a new wave crest has formed. These bifurcations, however, occurred only in the head of the microparticle cloud. We show that spatial variations of electric field inside the drifting cloud play an important role in the formation of the wave pattern. Comparison of the theoretical estimations and measurements demonstrate the significant impact of the electric field on the phase velocity of the wave. The same theoretical approach applied to the instability growth rate showed agreement between estimated and measured values.

Probing a dusty magnetized plasma with self-excited dust-density waves.

A cloud of nanodust particles is created in a reactive argon-acetylene plasma. It is then transformed into a dusty magnetized argon plasma. Plasma parameters are obtained with the dust-density wave diagnostic introduced by Tadsen etal. [Phys. Plasmas 22, 113701 (2015)10.1063/1.4934927]. A change from an open to a cylindrically enclosed nanodust cloud, which was observed earlier, can now be explained by a stronger electric confinement if a vertical magnetic field is present. Using two-dimensional extinction measurements and the inverse Abel transform to determine the dust density, a redistribution of the dust with increasing magnetic induction is found. The dust-density profile changes from being peaked around the central void to being peaked at an outer torus ring resulting in a hollow profile. As the plasma parameters cannot explain this behavior, we propose a rotation of the nanodust cloud in the magnetized plasma as the origin of the modified profile.

Observation of metallic sphere–complex plasma interactions in microgravity

The PK-3 Plus laboratory on board the International Space Station is used to study the interaction between metallic spheres and a complex plasma. We show that the metallic spheres significantly affect both the local plasma environment and the microparticle dynamics. The spheres charge under the influence of the plasma and repel the microparticles, forming cavities surrounding the spheres. The size of the cavity around a sphere is used to study the force balance acting on microparticles at the cavity edge. We show that the ion drag force and pressure force from other microparticles balances with the electric force acting from the sphere to within 20%. At intermediate distances from the sphere surface, the interaction between the microparticles and the metallic spheres is attractive due to the drag force stemming from the ions which are moving towards the highly charged spheres. The spheres thus strongly affect the plasma fluxes. This modification of the plasma flux can lead to an effective surface tension acting on the microparticles, and to the excitation of dust-density waves near the spheres, as the local electric field crosses a threshold.

On the amplitude of dust-density waves in inhomogeneous dusty plasmas

Density waves in a cloud of submicron dust particles confined in an rf plasma are examined with respect to their amplitude. The plasma shows a strong inhomogeneity in its densities and the dust particle charge. An ion density gradient appears due to ambipolar diffusion and leads to a dust charge gradient provided by charge depletion. Using dust-density wave frequency and wave number as a diagnostic for the plasma parameters, it is found that the wave amplitude increases or decreases as the wave travels into regions with decreasing or increasing particle charge. The experimental amplitude profile is similar to the theoretical curve predicted by a formula from Singh and Rao [Phys. Plasmas 6, 3157–3162 (1999)].

Simulations of the dust acoustic instability in a collisional plasma with warm dust

Dust acoustic (or dust density) waves have been observed in many laboratory dusty plasmas. These low-frequency waves involve the dynamics of highly charged and massive dust grains, and can be excited by the flow of ions relative to dust. In this paper, we consider the nonlinear development of the dust acoustic instability, excited by thermal ion flow, in a collisional plasma containing dust with high kinetic temperature (warm dust). It is shown that under certain conditions there may be a long-wavelength secondary instability in the nonlinear stage as dust gets heated by the waves. The characteristics of the nonlinear development are considered as a function of the relative charge density of the dust. Application to possible experimental parameters is discussed.

On Dust Wave Instabilities in Collisional Magnetized Plasmas

The physics of dusty plasmas in high magnetic field is an area of current interest, spurred by the development of experiments in this area. Dust acoustic (or dust density) waves have been observed in many laboratory dusty plasmas where there is either no or weak magnetic field. These waves may be excited by the flow of ions relative to dust. The effects of high magnetic field on the excitation of dust waves have been considered in the theoretical literature, including instabilities driven by ions streaming either along or across the magnetic field. We review and extend prior theory work on such instabilities, exploring their behavior as the magnetization of the ions is varied, while the electrons are strongly magnetized and the dust is unmagnetized. It is shown that when the ions are magnetized, there can be structure in the unstable wavenumber spectrum under certain conditions. Application to possible experimental parameters is discussed.

Stereoscopic imaging of dusty plasmas

The fundamentals of stereoscopy and their application to dusty plasmas are described. It is shown that stereoscopic methods allow us to measure the three-dimensional particle positions and trajectories with high spatial and temporal resolution. The underlying technical implications are presented and requirements and limitations are discussed. The stereoscopic method is demonstrated for dust particles in dust-density waves under microgravity conditions.

Self-excited dust-acoustic waves in an electron-depleted nanodusty plasma

A dust density wave field is observed in a cloud of nanodust particles confined in a radio frequency plasma. Simultaneous measurements of the dust properties, grain size and density, as well as the wave parameters, frequency and wave number, allow for an estimate of the ion density, ion drift velocity, and the dust charge using a hybrid model for the wave dispersion. It appears that the charge on the dust grains in the cloud is drastically reduced to tens of elementary charges compared with isolated dust particles in a plasma. The charge is much higher at the cloud's periphery, i.e., towards the void in the plasma center and also towards the outer edge of the cloud.