Session NO03: Fundamental Plasmas: Dusty Plasmas

Reversing the dust charge polarity in a plasma afterglow

The charge of a dust grain is normally negative in a laboratory plasma, but we have found that when the plasma power is extinguished, the grain's charge can reverse and become positive. This charge reversal was observed in our experiments with polymer microspheres in a capacitively coupled RF plasma, and the positive charging is attributed to the dust collecting ions that stream in the presence of a DC electric field, in a cathode sheath where there is a deficit of electrons, in the afterglow. In a different afterglow experiment, we obtained the opposite outcome, with negatively charged dust. The latter result was obtained by reversing the lower electrode’s polarity during the early afterglow to make that electrode anodic, while electrons were still present.   

Forming Ordered Structures with Charged Dust in the Presence of Ion Wakes

Charged dust grains have been experimentally observed to form ordered structures in complex ('dusty') plasma, including single chains that align with the ion flow direction, zigzags or double chains, and helices formed by multiple parallel chains surrounding a single axis. The self-organization of dust grains into these structures requires a balance between confinement forces and interparticle interactions among the charged species present in a dusty plasma. The formation of ion wakes (regions of high charge density downstream from the dust grains) alters the interaction between charged dust grains, while the relative positions of charged dust within a structure affect the formation of the ion wakes. As a result, understanding ion wake morphology during the transitions from one stable dust structure to another is a critical component for describing the self-organization of dust grains. In order to develop a model capable of predicting the formation of various structures, the present study systematically analyzes the formation of ion wakes in the presence of various orientations of stationary dust using a molecular dynamics simulation that models ion and dust dynamics and dust charging. The results are then used to determine the variation in radial and axial confinement forces needed to promote the self-organization and transition between these charged dust structures.   

Rotating plasma crystals and their transitions to dynamic non-crystalline states

Complex systems often experience oscillations between quiescent and highly dynamic states. This phenomenon can also be found in dusty plasmas or complex plasmas, which are composed of weakly ionized gas and micron-size charged dust particles. When levitated in a plasma sheath, dust particles can form crystalline structures (plasma crystals). Here we investigate rotating plasma crystals with 5-20 particles by applying a non-uniform magnetic field perpendicular to the sheath above a driven electrode in an argon rf plasma. By tuning the pressure and rf power, the system can undergo transitions between chaotic gas-like states and ordered rotating crystal states. Interestingly, we observe reentrant behavior: as the neutral gas pressure increases from below 1 Pa to above 4 Pa, the plasma crystals undergoes a transition from a rotating crystal state to a dynamic gas-like state, and then back to a rotating crystal state again. This behavior correlates with regimes of small and large damping. The two rotating crystal states exhibit significant differences in terms of crystal structure, angular velocity, and the frequency of small oscillations, all likely due to variations in confinement potential, Debye length, and particle charge. Lastly, we show how this transition can occur spontaneously at constant pressure and power, and depends sensitively on the heterogeneity of particle sizes.   

Global Heating with Torsions in Plasma Crystals

Complex plasmas are composed of nanometer to millimeter-sized grains suspended in a quasi-neutral plasma. Dust grains of the same size and material will self-assemble into monolayer hexagonal cells, known as plasma crystals, under appropriate radial and vertical confinement, through system power and pressure manipulation. Torsions are a feature unique to plasma crystals where two particles rotate as a quasi-pair outside the monolayer after formation, through decreases in power or pressure [1-3]. Torsions funnel energy from the ion wakefield into the crystal lattice through coulomb interparticle interactions. A gaseous electronics conference radio frequency (GEC RF) cell at Baylor University is being utilized to observe the global effects of torsions on plasma crystals. Observing torsions and their long-range effects will aid in the understanding of crystal temperature tolerances and particle movement. This talk will discuss torsion short and long-range heating effects within the dust grain lattice using experimentally collected data employing high-speed cameras, laser fans, and particle tracking software.   

Torsion Induced Anisotropic Plasma Crystal Energy Transfer

Torsions are three-dimensional orbital inclusions in two-dimensional complex plasma crystals which occur when two dust particles vertically leave the plane of the crystal (typically one above and one below). These two particles orbit horizontally within a force cage formed by the remaining crystal lattice. Torsion inclusions give researchers a means by which to examine the 2 ½ dimensional nature of plasma crystals resulting from ion wakefield interactions and the resultant energy transfer mechanisms. In this talk we demonstrate how the inclusion of a torsion within the crystalline lattice leads to anisotropic propagation of energy from vertical laser perturbation. Experiments were performed in a Modified Gaseous Electronic Conference (GEC) rf reference cell utilizing a collimated Verdi laser for crystal perturbation.   

Determining confining and interaction forces on dust in a 2D structure

The behavior of micron sized dust particles in complex plasmas can give insight into plasma conditions that are otherwise difficult to probe and determine. The dust particles tend to charge negatively and interact with the positive ions flowing towards a negatively charged surface, creating an ion wakefield that influences the charge and interparticle forces between dust grains. This interaction contributes to the self-assembly of the dust into ordered structures. These structures can further be influenced by changing the confining forces acting on the system. Strong horizontal confinement can be provided by the charged walls of a glass box placed on the lower electrode of a modified Gaseous Electronics Conference (GEC) cell, which allows vertical structures to form in the overlapping sheaths. Small adjustments to the system power alter the horizontal confinement causing a 1D chain to transition into two side-by-side chains (a 2D zig-zag structure). To investigate the confinement forces, a molecular dynamics simulation has been utilized to model the motion of both the ions and dust in this configuration. An iterative method is then employed to determine a position-dependent electric field that gives a minimal force-balance of the dust grains, which will lead to a stable structure.   

Infer force laws in many-body dusty plasma systems

Scientific laws describing natural systems may be more complex than our intuition suggests, and thus how we discover laws must change. Machine learning (ML) models handle large quantities of data, but their structure should match the underlying physical constraints to provide useful insight. Here we demonstrate a ML model where the physics can be built in, step by step, to infer forces and learn new laws from experimental data in dusty plasmas. Trained solely on the 3D experimental particle trajectories, our model accounts for the inherent symmetries and varying number of non-identical particles, and learns the effective non-reciprocal force law governing their motion, and extracts the mass and charge of each particle. The model validates itself by predicting the masses of each particle in two independent ways with excellent agreement. Furthermore, we discovered a new scaling law in our system that a particle's charge is proportional to its mass to the power of 0.74, different from 1/3 predicted by OML theory. This implies that two particles of different sizes at the same position in the plasma sheath have a different floating potential. Outside of dusty plasmas, these results guide new routes of discovery using physics-tailored ML in many-body systems.   

Self-consistent calculations of the electric charge, ion drag force, and the drift velocity of spherical dust grains immersed in collisional ion flows using Langevin Dynamics and comparisons against canonical experimental data

We present a trajectory simulation-based modeling approach to capture the interactions between ions and charged grains in dusty or complex plasmas (gas discharges containing dust grains). Our study is motivated by the need for a self-consistent and experimentally validated modelling approach for accurately calculating the ion drag force and grain charge that are important in the grain collective behavior in plasmas. We implement Langevin Dynamics in a computationally efficient predictor-corrector approach to capture multiscale ion and grain dynamics. Predictions of grain velocity, grain charge, and ion drag force are compared with prior measurements to assess our approach. The comparisons reveal excellent agreement within <!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>±20%  between predicted and measured grain velocities (Phys. Plasmas 12(9): 093503) for <!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>0.64, 1.25 μm  grains at <!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>~20-120 Pa . Comparisons with the measured grain charge (Phys. Rev. E 72(1): 016406) under similar conditions reveal agreement to within <!--[if gte msEquation 12]>~20%  as well. Measurements of the ion drag force (Phys. Plasmas 11(12): 5690 and IEEE Trans. Plasma Sci. 32(2): 582) are used to assess the viability of the presented approach to calculate the ion drag force experienced by grains exposed to ion beams of well-defined energy. Excellent agreement between calculations and measurements is obtained for beam energies >10 eV and the overprediction below 10 eV is attributed to the neglect of charge exchange collisions in our modeling. Along with critical assessments of our approach, suggestions for future experimental design to probe charging of and momentum transfer onto grains that capture the effect of ion and grain number concentration are outlined.    

Red border for the photo-detachment from charged metallic and dielectric nanoparticles in plasmas

The Schottky effect is predicted to decrease the energy of photons required for electron photo-detachment from metal particles negatively charged in plasma [1]. The same effect should be manifested for charged dielectric particles as well. As an example, spherical nanoparticles made from SiO₂ and Al₂O₃in the size ranging from 100 nm to 2 μm, immersed in weakly ionized nonequilibrium nitrogen plasma are considered. Our theoretical analysis has shown that the critical wavelength of the laser at which the electron photo-detachment starts (so-called the red border of the photoelectric effect) depends not only on affinity energy, but also on the charge of nanoparticles, their size and dielectric constant. The theory predicts that the smaller the size of the nanoparticles, the stronger the shift of the red border. The cross-section for the electron photo-detachment is also discussed with the Schottky effect considered. The obtained results are important for measurements of the charge and size of dust nanoparticles using a laser-stimulated photo-detachment (LSPD) technique [2]. Such measurements can provide valuable information required for modeling dusty plasmas and plasmas for the synthesis and processing of nanomaterials.  The presented theory provides guidance for the selection of the LSPD laser wavelength and analysis of the measured results.

1. M.N. Shneider, Y. Raitses and S. Yatom,  J. Phys. D: Appl. Phys. 56 29LT01 (2023)

2. T. J. A. Staps et al.,  J. Phys. D: Appl. Phys. 55, 08LT01  (2022)   

Trapping of wave in a flowing dusty plasma

We report on experimental observations of trapping of waves in a flowing dusty plasma. The experiments are performed in an inverted Π-shaped dusty plasma experimental device in which the dusty plasma is created in a DC glow discharge argon plasma using micrometersized kaolin particles. Two copper wires are installed radially on the cathode, which serve to generate the flow in the dust fluid as well as to confine the waves. The dust fluid is initially made to flow over both the wires by altering the sheath potential of one of these wires, and as a result, the wave gets excited and propagates in the downstream direction. The wave gets trapped in between the wires when their separation is below a critical value of ∼ 2 cm. For a long time (of the order of a few seconds), the trapped-wave structure retains its identity. The amplitude of the wave crests and the distance between them remain constant with the dust fluid flow velocities. A numerical solution of the forced Korteweg-de Vries equation with two source terms as well as molecular dynamic simulations reproduce our experimental findings in a qualitative manner.   

Growth and phase of ice grains formed in a cryogenically cooled RF plasma

In the Water-Ice Dusty Plasma Experiment at Caltech, microscopic grains of ice are spontaneously created within a cryogenically-cooled RF-powered weakly ionized plasma. FTIR absorption spectroscopy shows that the ice phase can be either crystalline or amorphous. This spectroscopy is then used to examine how the ice phase depends on background gas temperature, plasma parameters, and time. The FTIR absorption spectrum also enables measurement of optical constants which can then be used to determine the characteristic grain size at which electromagnetic radiation scatters from the ice grains. These spectroscopic results indicating grain size are compared to direct microscope imaging of the dust grains, linking the microstructure to macroscopic behavior.   

Pulsed nanosecond discharge development in liquids and nano-voids formation

The dynamics of pulsed nanosecond discharge development in liquid water was investigated experimentally. High-voltage pulses with durations of 20 and 60 ns and amplitudes of 6-60 kV were used for discharge initiation. It is shown that the dynamics of discharge formation in water consists of two phases. The first phase is connected with electrostriction compression of the media near the needle tip and the formation of a rarefaction wave in the surrounding liquid. The second phase (the discharge phase) has a pronounced start delay, which depends on the voltage of the high-voltage electrode. Thus, at low voltages, the pulse length is insufficient for the initiation of discharge, and the process consists of the first phase only, i.e., the formation of an electrostriction rarefaction wave. At higher voltages, the discharge start delay time decreases rapidly, and discharge commences simultaneously with the formation of hydrodynamic perturbations by the electrostriction forces present in the media. Shadowgraphic laser visualization of the process demonstrates the transition from a pure hydrodynamic density perturbation in the rarefaction wave to a developed streamer-leader process with a strong energy release in the channels and the formation of strong shock waves around the channels.   

Buckling of Two-Dimensional Dusty Plasma Under Shock Compression

A shock was created in a two-dimensional (2D) dusty plasma consisting of monodisperse microparticles suspended in a sheath of radio-frequency discharge. The shock was mechanically excited by moving a thin wire parallel to the 2D microparticle cloud at supersonic speed. The out-of-plane motion of the microparticles, known as buckling, was observed in the region of shock compression. The conditions for the onset of the buckling were investigated.   

Molecular dynamics simulation of a dusty plasma under external electric and magnetic fields

Being a collection of charged particles (ions, electrons, and dust particles), the dynamics of a dusty plasma can be controlled by the presence of exter- nal electric and magnetic fields. The presence of external electric or magnetic fields is likely to introduce anisotropy in the shielding lengths and the related interactions potentials. Depending on the strength of the external magnetic field, three regimes can be identified: (i) only electrons magnetized, (ii) elec- trons and ions magnetized, and (iii) electrons, ions, and dust particles magne- tized. Here we discuss two types of molecular dynamics simulations: one where electron magnetization leads to anisotropic ion-ion interaction and another one where ion response to a polarity-switched electric field leads to anisotropic dust- dust interaction. The former simulation uses initial conditions relevant to the MDPX experiment at Auburn, while the latter models conditions available at the Plasmakristall-4 facility on the International Space Station. We argue that in both cases, the interaction potentials can be reasonably approximated by a time-independent anisotropic Yukawa potential, i.e., one where the Debye shielding length varies with space.