Session CO07: Fundamental Plasma Physics: Dusty plasma and multiphase media

Optical Trapping Single Dust Particles in Plasma For In Situ Plasma Diagnostics.

Optical trapping (OT), conventionally called optical tweezers, use photon radiation pressure force and or photophoretic force to levitate, trap, or transport micron-sized particles (solids or droplets) in different media including air, solution, and reactive gaseous. Very recently, this technology has been demonstrated in plasmas, which shows great promise of a new tool using a single dust particle for in situ noninvasive plasma diagnostics, with high spatial resolution. Here we present our recent developments in OT of various types of particles (size, morphology, and material) in rf plasma and weakly magnetized rf plasma. We show a trapped particle can be smoothly transported in the plasmas vertically along the gravitational field or horizontally perpendicular to the electric field. We use a single trapped particle to measure the electric field in the plasma, operating in various situations. We discuss further potentials for in situ, noninvasive, plasma diagnostics at the microscopic level using a micron-sized particle.   

Torsion Energy Dispersion Coefficients

Torsions, also known as spinning particle pairs, have been shown in previous works to act as a local energy source within dusty plasma crystals. Torsion inclusions in plasma crystals also lead to anisotropic energy transfer throughout the surrounding crystal lattice after an initial laser perturbation. In this presentation we describe a technique which allows the energy transmission, absorption, and reflection coefficients to be established. These coefficients are measured as the energy wave due to a laser pulse encounters the torsion. The torsion will be shown to interfere with the energy wave propagation transmitting some of its energy to the crystal lattice behind it, reflecting some back towards the laser impact position and absorbing some as the torsion orbit eccentricity, frequency and size are affected by the initial pulse   

Studying dust behavior in weakly ionized plasmas with magnetic fields via the DRIAD Code.

Complex plasmas are composed of micron-sized dust particles that are suspended in a

gas with low ionization levels. The charging of these dust particles occurs as a result of

interactions with electrons and ions, and is influenced by factors such as the

temperature of the plasma, the density of the gas, and the strength of electric fields.

Magnetic fields also have an impact on the charging of dust particles and their

subsequent behavior but it is not well understood. The effect is contingent upon the

levels of magnetization exhibited by various charged species present in the complex

plasma. Despite the fact that current theories mainly concentrate on dust particles that

are spherical in shape, practical situations—encountered, for instance, in experiments

related to fusion and in astrophysical settings—often entail dust particles with irregular

shapes. In order to bridge this knowledge gap, an examination into the charging

mechanism of dust aggregates is undertaken. More specifically, we compare how

aggregates become charged in scenarios where no magnetic field is present (B = 0 T)

to situations where a magnetic field is present (0 T < B < 3.5 T). Our analysis takes into

account the variation in the flow of electrons and ions towards specific points on the

surface of the aggregate. The manner in which charge is distributed across the

aggregate's surface results in conflicting torques, ultimately influencing the orientation

and movement of dust particles within the plasma medium.   

Intermittent dynamics in rotating Coulomb clusters: from 2D crystals to liquid dust drops

Complex systems often experience intermittent oscillations between quiescent and highly dynamic states. This phenomenon is also observed in dusty plasmas, where micron-sized charged dust particles form crystalline structures due to the competition between a confining potential and screened Coulomb interactions between particles. Here we investigate clusters of 2-100 particles which are made to rotate applying a non-uniform magnetic field perpendicular to the plasma sheath above an electrode in an argon rf plasma. By tuning the pressure and rf power, the clusters can transition between chaotic liquid-like droplet states and ordered rotating crystals. The intermittent dynamics we observe can depend sensitively on the number of particles, and hence, the normal mode frequencies of the particle clusters. We characterize these dynamics by tracking each particle using 3D tomographic imaging, revealing information about their rotation speed, oscillation frequency, kinetic energy, and structural asymmetry. Interestingly, for certain cluster structures, intermittent switching can occur without changing the plasma environment. For small crystalline clusters (less than 20 particles), the melting often occurs at the onset of the symmetric, radial "breathing" mode oscillation (if one exists), which can parametrically couple to the cluster's vertical oscillation. For larger clusters, melting can also occur, but the breathing mode isn't necessary. We further analyze the normal mode spectra of these clusters and the stability of their structure when intermittent melting is observed.   

Using machine learning to turn particles into probes in dusty plasmas

In laboratory dusty plasmas, micron-sized dust particles are often levitated at the edge of a plasma sheath where the electric field balances gravitational forces. In the traditional study of this environment, intrusive, millimeter-sized Langmuir probes are used to measure plasma sheath properties, and particle interaction forces are calculated by either numerical simulation or mode analysis around their equilibrium positions. Here we simultaneously do both of these things by using machine learning to analyze experimental highly dynamic particle trajectories in 3D, thereby revealing previously inaccessible information about particles and the plasma without external probes. We track the motion 10-30 dust particles in 3D using scanning laser sheet tomography in a laboratory RF dusty plasma. The motion of these particles inherently contains information about the ambient plasma environment, including each particle's ion wake in the plasma sheath. We train machine learning (ML) models using this motion, and importantly, the model treats each particle individually with different sizes. In this presentation, I will detail our ML model and its recent discoveries, including a position-dependent plasma charge density, the position and dependence of the particle charge, particle interactions, and plasma wake structure.   

Dynamics and structure of dense dusty plasmas using 3D tomographic imaging

When levitated at the edge of an argon plasma sheath, micron-sized dust particles can form three-dimensional structures when the confinement is sufficiently strong. Depending on the neutral gas pressure and rf power, the particles can form and coexist in different phases: a low temperature crystal or a high temperature liquid. Here we report how a structural transition can occur between a 2D crystal and a 3D, complex liquid phase. Our experiments utilize a permanent magnetic field to enhance confinement, and the complex liquid phase is characterized by strong, non-reciprocal interactions between vertical layers due to an ion wake in the plasma sheath. Particles often form vertical pairs or triplets that resemble strings. This transition between phases can be quantified by the pair correlation function and the distribution of inter-particle distances. Importantly, the 3D tomographic imaging of our system using a rapidly scanning laser sheet allows us to simultaneously characterize the individual position and velocity of thousands of particles, which is important since we find there are strong gradients in particle density and temperature within the sheath.  By coarse-graining the above quantities, more statistical characteristics in the complex fluid phase can be identified and compared to other soft matter systems.   

Atomistic insight into the plasma-catalyzed nucleation of ice grains

Reactive Molecular Dynamics (MD) and Quantum Mechanics (QM) computations are performed to investigate the nucleation and agglomeration processes of ice grains in a weakly-ionized plasma environment such as in the Caltech ice dust experiment [1] and in protoplanetary disks. The MD simulations use the state-of-the-art eReaxFF force field [2] which can describe the particle nature of both valence and free electrons. QM computations of the ground and ionized states of hydrate clusters provide electronic structure and potential energy that benchmark the much larger MD simulations. Findings indicate that solvated free electrons occupy a large volume, either localized on the surface or forming cavities within the water clusters, and attract the dipole moment of OH bonds of nearby water molecules. A methodology has been developed for the QM calibration of the eReaxFF description of solvated electrons in water clusters. Using this method, MD simulations of large spherical and ellipsoidal-shaped grains with solvated electrons are performed. These simulations are then analyzed to determine the impact of electrostatic interactions, surface tension, and binding energy on the shape and chemical composition of amorphous dust grains. The analysis shows that the distribution of free electrons in ice grains is not controlled by electron-electron repulsion because the Coulomb interaction energy between free electrons within the grain is much smaller than the binding energy of a solvated free electron to a cluster of water molecules. This strong binding of solvated free electrons to water molecules suggests that ice grains cannot be considered as being electrically conductive. QM computations on reactants, products, and transition states of hydrate systems reveal that O-, and especially OH-, are the most energetically favorable nucleation sites in a dusty plasma environment. MD simulations of dusty plasmas have been started to investigate the kinetics and nucleation process of ice grains.

[1] A. Nicolov, M. S. Gudipati, P. M. Bellan, The Astrophysical Journal 966 (1) (2024) 66.

[2] M. M. Islam, G. Kolesov, T. Verstraelen, E. Kaxiras, A. C. Van Duin, Journal of chemical theory and computation 12 (8) (2016) 3463–3472.   

Dynamical impact of fractal-shaped ice grains on a cryogenic dusty plasma experiment

Ice grains are formed and contained in a capacitively-coupled plasma with cryogenically cooled electrodes. The grains exhibit an elongated dendritic shape that grows fractally in time with a 2-D fractal dimension of about 1.5. We measure ice grain trajectories in the plasma afterglow using high-speed imaging and use laser-induced fluorescence to measure neutral temperature gradients which cause a thermophoretic force on the grains. We find that the fractal growth significantly affects the dynamics, as the scaling laws of mass, charge, and cross-section depend on the fractal dimension. These cause the thermophoretic and drag forces to dominate gravity  until the grains grow to macroscopic sizes. These insights are expected to impact models of ice dust dynamics in both astrophysical and laboratory contexts.   

Electron photodetachment from plasma-charged substrate surfaces and nanoparticles

The laser-stimulated electron photodetachment (LSPD) has been proposed as a particle charge diagnostics method in nano dusty plasmas. Additionally, the impact of nanoparticle size on photodetachment energy barrier reduction due to grain charging has been predicted [1]. However, the photodetachment yield and cross sections from material surfaces for real gas discharge plasma charging conditions remain unknown. Here, we report on the study of LSPD from both plasma-charged dielectric planar substrates and carbonaceous nanoparticles grown in a plasma medium. Electron photodetachment from planar insulators such as quartz (fused silica), alumina, and h-BN demonstrates that surface charge decay is nonlinear for subsequent laser pulses, and photodetachment yield defined by the initial surface charge density for the same material [2]. The LSPD from dust nanoparticles was carried out utilizing fast Langmuir probe measurements, synchronized with separate laser shot events. The photodetached electron density obtained from the pulsed probe current and specific charge for the nanoparticles was estimated by using the dust particle density measured by the laser light extinction method. Also, the role of the electron detachment from the residual background negative ions due to the defragmentation of C₂H₂ molecules and sputtering of carbonaceous thin film on the cathode has been discussed.   

Intershell and configurational properties of two-dimensional dust crystals in complex plasmas

Intershell dynamics of two-dimensional (2D) charged particle systems have been extensively researched in condensed matter physics. Due to their quantum scales, pure Coulomb crystals are extremely fragile to produce experimentally and are generally studied via simulations and numerical methods. However, complex/dusty plasmas can serve as classical analogs, where charged dust particles can be of micro-meter size, enabling direct visualization of individual particles. In 2D complex plasma, particles interact with screened-Coulomb potential rather than pure Coulomb but exhibit similar shell structures and six-fold hexagonal symmetry. We used experimental methods presented in the reference [1] to obtain 2D finite dust crystals, called N-Clusters, where N can be of any desired value between one to any large number. We further studied the evolution of crystalline intershell structure from N = 1 to 50 with dN = 1. A few N-Clusters were then perturbed with multiple heating and cooling cycles using randomized laser beams to extract configurational entropy and statistically obtained ground and meta-stable states of a specific N-Cluster. We also compared our experiments with published simulation results.

[1] Kumar, R., et al., Rev. Sci. Instrum. 95, 053503 (2024)   

Towards Realistic Simulations of Shocks in 2D Dusty Plasmas

The phenomenon of shocks in 2D dusty plasmas has been a subject of both experimental and simulation studies. The 2D layer of dust particles becomes highly compressed in a layer that propagates at a supersonic speed. While many features of the shock are similar in the experiments and molecular dynamics simulations, there are also significant differences. Phenomena observed in simulations that were never confirmed in experiments include pre-heated particles, oscillatory profiles of the dust number density, and a solid-liquid phase separation in the aftershock region. Conversely, there is a feature seen in the experiments but not in simulations, which is significant out-of-plane particle motion at high densities in the compressed region of the shock, where the dust layer has been observed buckling in experiments. Out-of-plane motion could not be observed in the previous simulations because their particle motion was constrained to a single plane. To improve the fidelity of simulations, we plan new runs that allow out-of-plane motion, limited by a parabolic potential that mimics the vertical confinement of particles in the experiment.   

Afterglow method of selectively removing larger dust particles to prepare a 2D layer in a plasma

Dusty plasma experiments are often performed using a single horizontal layer of dust particles levitated in a capacitively coupled radio-frequency plasma discharge. Imaging such a layer using video microscopy allows experimental investigations of physics such as 2D shocks. In this talk, we demonstrate a method of preparing a 2D layer. Under the same conditions as one of our 2D shock experiments, we introduced 8.69 micron polymer spheres into a 13.5 mTorr argon plasma. To remove the heavier particles, and yield only a single layer of identically sized particles, we modulated the rf plasma, repeatedly turning it off for almost 1 ms of afterglow conditions followed by reigniting the plasma for about 5 or 10 microsec. This modulation method can be performed at the single touch of a button. Unwanted heavier particles selectively land on the lower electrode during the afterglow, and remain there so that they are removed. This process yields a layer of a single size of particles in the plasma.   

Nonlinear Mode Coupling in a 1D Dusty Plasma

We report experimental findings on the excitation and nonlinear interaction of collective modes in a 1D dusty plasma system. The dusty plasma is produced by introducing mono-dispersive Melamine Formaldehyde particles in the background of a capacitively coupled radio frequency (RF) Argon discharge in the Capacitively Coupled Dusty Plasma Experimental (CCDPx) device. A uniquely designed lower electrode, that can dynamically modify the transverse confining potential of the electrode, is used to form a 1D chain of dust particles. Collective oscillations are excited  by applying an oscillatory radiation force on the particles using a tuneable laser. The dynamics of the dust particles are recorded using a high-speed optical imaging system, and the trajectories of the individual particles are tracked using Particle Tracking Velocimetry (PTV) tools. The nature of coupling between the different modes is estimated from kinetic level measurements using bi-spectral and bi-coherence analyses. The experimental findings, covering a wide parameter regime, are then compared with classical molecular dynamics simulations that assume a Yukawa interaction among the particles in order to provide useful physical insights into some of the observed novel features of the nonlinear phenomena.