Session CP12: Poster Session: Fundamental Plasmas: Dusty Plasmas (2:00pm - 5:00pm)

Energy Dissipation in a Microgravity Complex Plasma Cloud

Complex plasmas under microgravity conditions allow the study of interparticle forces that are masked by gravity. To overcome the gravitational influence, we use the dc glow discharge Plasma Kristall-4 (PK-4) microgravity laboratory on the International Space Station (ISS). Dust particles injected into PK-4 flow along an axial electric field until stopped by the application of a periodic oscillation of the electric field. We seek to understand the redistribution of kinetic energy of the dust particles at the onset of this periodic oscillation. This presentation will focus on comparing data obtained using the ground-based science reference module and the ISS microgravity experiment. Initial results show a substantive difference in the shape of the velocity distribution functions between ground and flight data. While the distribution function from ground data is reasonably well described using a Maxwellian distribution, the distributions from flight data show evidence of extended, skewed tails and other non-Maxwellian features. This presentation will focus on the modification of the velocity distribution shape and the subsequent determination of the thermal properties of the dust component of the plasma system.   

Measurement of plasma conditions inside the sheath using dust grains

Various types of ordered, dust particle structures can be formed experimentally inside a glass box when it is placed on the lower, powered electrode of a GEC rf reference cell. In this case, the number of particles and the resulting confinement provided by the box can be adjusted by changing the power and/or pressure of the system. The interaction between the dust particles and the ions streaming toward the negatively charged lower electrode creates an ion wake field downstream of the grains, thereby altering the charge on the grains, the local electric field and the ion flow speed. Experimentally, these quantities are difficult to measure independently. This study will present a molecular dynamics simulation of this system and examine how it can be used to study the interaction between the ion wake fields and the dust grains. It will also be shown that the results allow the plasma conditions inside the glass box, including the electric field, the ion flow speed and the dust charge to be determined, even for changing electron density.   

Modeling Ionization Wave Effects on Dust Chain Formation

Recently it has been observed that the PK-4 BU ground-based system replicating the PK-4 experiment on the International Space Station exhibits microsecond time scale inhomogeneities. These inhomogeneous features are related to ionization waves moving through the positive column. Initial numerical simulations found that the plasma parameters within these waves can grow to several times larger than the background average values. This is important since previous works have focused on the formation of dust chains levitated within the electric field in the plasma sheath in a stable, homogeneous plasma. Results from a numerical model of the environment within the PK-4 experiment will be reported in this talk. These results allow examination of the various plasma conditions within the observed microsecond time scale inhomogeneities and the effect these conditions have on the formation of flow-aligned dust chains. The numerical model employed incorporates both dust and ion motion in an N-body simulation, where the boundary conditions are determined from plasma parameters derived using a 2D particle-in-cell with Monte Carlo collisions (PIC/MCC) simulation.   

Pulsed shear motion in a three-dimensional dusty plasma under microgravity conditions

Shear motion of dust particles in a strongly-coupled dusty plasma under microgravity conditions was investigated using the European Space Agency's facility PK-4 on the International Space Station. The particles were trapped in a plasma powered by a DC voltage that switches its polarity periodically. They self-organized themselves into a structure resembling a solid or a cold liquid. A manipulation laser beam pushed a slab of particles to move through the surrounding sample region. The power of the laser was modulated, causing a sudden onset of pulsed shear motion among the particles. We tracked particle motion using video cameras. Results including the spatial and temporal variations of dust fluid velocity will be shown.   

Correlation and Spectrum of Dust Acoustic Waves in a Plasma Under Microgravity Conditions

Dust acoustic waves under microgravity conditions were investigated in an experiment using the European Space Agency's facility PK-4 on the International Space Station. A dust cloud was confined in a neon plasma powered by a radio-frequency coil. Using video cameras, waves were observed to be spontaneously excited. The density fluctuations associated with the waves were characterized by calculating a microscopic particle density. The fluctuation spectrum indicates the emergence of collective modes in a broad range of wave numbers. The persistence of the coherence of the modes was characterized by density correlation functions. An experimental measurement of the wave's dispersion relation was also obtained.   

Overview of CASPER's on-ground PK-4 laboratory system

The PlasmaKristal-4 (PK-4) laboratory on the International Space Station (ISS) continues to produce a wealth of dusty plasma data across research areas such as dusty plasma waves, chains, vorticity, phase cloud behavior, striations and ionization waves. The majority of analogs that exist for this device are ground versions of the flight model used in PK-4 ISS experiments, which by default restricts their operation. The experimental group at CASPER has assembled a PK-4 analog (the PK-4 BU) that has now been producing data for two years. The PK-4 BU was specifically designed to allow accessibility to all system components, including electronic circuits, cameras and diagnostic instrumentation. This provides great flexibility that expedites the setup required for a given experiment and allows researchers more control over system operating parameters. This combination of flexibility and accessibility have allowed CASPER researchers and collaborators to explore experimental data regimes bringing a new understanding of dusty plasma characteristics in microgravity.   

Lunar Swirl Formation with Irregular Shaped Dusty Plasma Medium

On the lunar surface, features with high albedos aptly named Lunar Swirls, have been observed near anomalous lunar magnetic fields. Although the formation mechanism for Lunar Swirls remains in question, numerical models suggest that complex plasmas interacting with local magnetic fields may be accountable for these lunar features via dust transport and weathering of regolith. Identifying the actual relationship between dusty plasma and these lunar magnetic fields would lend insight into the formation of lunar swirls. At Baylor University, a GEC RF Reference Cell and an Inductively Coupled Plasma Generator (IPG-B) are being used to experimentally study the interaction between dusty plasma and simulated lunar magnetic fields on non-conducting surfaces similar to crustal magnetic abnormalities affected by solar winds. This poster considers the formation of Lunar Swirls experimentally using dipole magnets and irregular dust particle sizes from a lunar regolith-like source and compares them to results using uniform dust particles. Results are then extrapolated to map the swirl patterns on the lunar surface in accordance with known magnetic field lines. Furthermore, the kinematics of dust transport in a simulated direct solar wind environment are further examined.   

Determining Electrostatic Field of Microcavities in Lunar Dust Grains

The lunar regolith covering the surface of the moon has a component of very fine, jagged dust particles. The Apollo astronauts found that lunar dust obscured visors and instrument readouts, degraded seals, and abraded materials. The dust is difficult to remove from spacesuit material, and dust in the lunar habitat poses a hazard to astronaut's health. Thus an understanding of the transport of lunar dust is important for future lunar missions. Dust grains in the regolith become charged through exposure to the solar wind, photoemission and secondary electron emission. Differential charging due to the stochastic charging processes and recollection of photoelectrons in micro cavities can lead to to electrostatic fields large enough to loft grains from the surface, allowing their transport. Here we implement a numerical model to resolve the charging of grains on the lunar surface and determine local variation in the electrostatic field. Grains are lofted when the electrostatic force exceeds gravity and cohesive forces between grains. The resultant predicted frequency of lofting events and size distribution of lofted grains will be calibrated and verified by experiments conducted with various shapes and sizes of dust grains, material properties, and charging conditions.   

Lofting of Lunar Simulant in a Dusty Plasma Environment

First observed by the Surveyor missions and later the Apollo astronauts, a glow above the lunar horizon created by the refraction of light through a cloud of dust particles can be seen just before the sun rises into view over the silhouette of the moon. The physics behind this ``Lunar Horizon Glow'' is still not well understood and is critical to determining the role that dust will play in the daily environment of astronauts during extended stays on the lunar surface. To date, prior modeling and experimental work suggests that one possible mechanism behind the glow is lunar regolith lofted from the surface of the moon due to photoelectric charging effects. This poster will seek to further previous studies by examining dust lofting for both lunar simulant and spherical dust particles under varying experimental conditions employing a GEC RF reference cell and an inductively-heated plasma wind tunnel IPG6-B. Together these devices provide a wide variety of charging environments, allowing examination of the charging and lofting of lunar simulant due to a stream of ionized particles. The data produced will be compared with previous experimental and numerical results.~   

Status of New Caltech Water-Ice Dusty Plasma Experiment

A new water ice-dusty plasma experiment under construction will provide a significant upgrade of the existing Caltech water-ice dusty plasma experiment. The upgrade has a larger vacuum chamber with many more ports and, in contrast to the liquid nitrogen cooling used in the previous experiment, each RF electrode will now be cooled by a cryocooler to enable lower and controlled temperatures. As in the previous experiment, water vapor injected into the chamber via an adjustable leak valve will spontaneously nucleate to form quickly growing ice grains having size and shape depending on gas species, RF power, ambient pressure, and ambient temperature. The previous experiment showed that at low background pressures, the ice grains are elongated, fractal-shaped, and that they levitate with alignment along the RF field. The new experiment will investigate how grain properties depend on ambient conditions, especially much colder background gas temperatures. Assembly is 40{\%} complete at time of writing with the lower electrode system (heater, diode, RF connector, insulator, guard, plasma-facing electrode) installed on the lower cryocooler which is mounted on a welded SS bellows to provide an adjustable separation between the RF electrodes.   

Do Dust Grains in a Dusty Plasma Collide with Each Other?

Astrophysical models of micron size dust grains immersed in a weakly ionized gas typically assume that upon collision dust grains can stick together or fragment. Fragmentation has never been observed in lab dusty plasma experiments; sticking has sometimes been inferred but no movie capturing sticking exists. Because the neutral mean free path ($l_{nn}\sim 0.5$ mm) exceeds the dust radius $r_{d}$, dust drag is given by the Epstein formula $ m_{d}du/dt=-u/\tau $ where $\tau =$ $3m_{d}/4\pi \rho _{g}r_{d}^{2}v_{Tg}\ $ and $g$ refers to gas. It is found that the stopping distance $u_{0}\tau $ is much less than the dust-dust mean free path $l_{dd}$ for astrophysical parameters but may or may not be so for lab plasmas. Being charged, the grains mutually repel so they cannot diffuse towards each other after stopping as the repulsive electrostatic energy greatly exceeds the dust grain thermal kinetic energy. If two dust grains on a collision course stop before traveling a mean free path they will not be able to touch and stick together.   

Validity of Moments of the Dynamic Structure Factor in a Dusty Plasma

The dynamic structure factor, $S(k,\omega)$, can be used in scattering diagnostics of tokamak and ionospheric plasmas to measure plasma parameters and in theory of strongly coupled plasmas. The dynamic structure factor reveals time and length scales for two types of motion: wave-like and random thermal. Theoretical models that describe $S(k, \omega)$ depend on the so-called frequency moments. We plan to address the validity of these moments for an experiment and a simulation.* The physics of strongly coupled plasmas can be studied experimentally using a dusty plasma. An advantage of dusty plasmas is that the motion of polymer microspheres can be analyzed using video microscopy. An experiment is planned, with a 2D monolayer of microspheres levitated in an RF plasma, to measure $S(k, \omega)$. We use laser heating to maintain the monolayer at a constant temperature, under liquid-like conditions with no shear flow. The motion of microspheres is recorded by a scientific video camera. Image analysis yields the particle positions, which are the inputs for calculating $S(k, \omega)$. For comparison to this experiment, we are also performing a 2D molecular dynamics simulation.   

Experimental Measurement of Viscoelasticity in a 2D Dusty Plasma Using Modulated Shear Flows

Viscoelasticity is a property of strongly coupled plasmas that describes their dissipative (viscous) and energy storing (elastic) properties. Dusty plasmas are well suited for experiments to measure this effect. The large particle size results in large charges and therefore strong coupling, and it allows the particles to be tracked easily using a video camera. We present results for an experiment performed to measure viscoelasticity using a dusty plasma produced in a modified Gaseous Electronics Conference (GEC) chamber with capacitively coupled RF power. Polymer microspheres are levitated and form a 2D cloud that conforms the strongly coupled component of the plasma. Laser heating is used to maintain a steady temperature in the suspension. Two additional laser beams are directed onto the cloud in a counter-propagating configuration to apply a shear on the dust cloud. The amplitudes of these two beams are sinusoidally modulated at frequencies $\omega $ to investigate the behavior of the particle cloud at different time scales. The microspheres are imaged from above by a high-speed camera. The particle positions and velocities are obtained from the videos and used as the inputs for calculating the time-dependent shear stress Pxy(t) and shear rate $\gamma $(t). Fourier analysis is performed, and results are reported as a frequency-dependent complex viscosity $\eta (\omega )$, defined as the ratio of Pxy($\omega )$ over $\gamma (\omega )$.   

Molecular Dynamics Simulations of the interaction of an electron beam with a plasma crystal

The kinetic effects on the dust particles in a plasma crystal locally irradiated by a narrow, pulsed electron beam (EB) with energies from 10 – 15 keV have recently been presented. [C.M. Ticoş, et. al., Phys. Plasmas, Phys. Plasmas 26, 043702 (2019)., C.M. Ticoş, et. al., Plasma Phys. Control. Fusion 62, 025003 (2020)] These studies have revealed that the EB pushes the dust particles in the irradiation zone, leading to both laminar and turbulent flow. Further, these studies have examined interaction of the EB is described in terms of the electron penetration depth, deposited energy and heating of the MPs, as well as resulting motion of the dust that has been irradiate by the electron beam. In this, we report on molecular dynamic simulations of this interaction. These simulations reproduce many of the observed experimental results and provide new insight into the interaction of the dust grains in a plasma crystal and an externally applied electron beam.   

Current Research Activities in the Magnetized Plasma Research Laboratory

The Magnetized Plasma Research Laboratory at Auburn University explores a wide range of low temperature plasma phenomena in plasmas and complex/dusty plasmas. The centerpiece of the laboratory is the Magnetized Dusty Plasma Experiment (MDPX) device - a highly flexible, high magnetic field (up to 4 T) research instrument with a mission to serve as an open access, multi-user facility for the dusty plasma, basic plasma, and fusion plasma research communities. Other instruments include the ALEXIS magnetized linear plasma device and a wide variety of ``tabletop'' scale devices. In the last year, we have performed a variety of new studies of particle growth at high magnetic field, pattern formation in both the background plasma and the dusty plasma at high magnetic field, and the impact of charged dust on the propagation of driven low frequency, electrostatic fluctuations in magnetized plasma. This presentation will summarize results from these studies.   

Formation of imposed patterns in a strongly magnetized plasma: A numerical Approach

The formation of imposed patterns due to placing a metal wire mesh in the bulk of a strongly magnetized ($B \ge $\textit{1 T}) plasma is investigated numerically. A 3D fluid model is developed to self-consistently solve the plasma fluid equation along with the Poisson's equation. Simulations using this model are able to qualitatively reproduce experimental observations. It is shown through these simulations that, due to the presence of the wire mesh in the bulk of the magnetized plasma, an organized pattern appears in the plasma potential. The emergence of this spatial pattern in the potential is due to the effect of the magnetic field on the cross-field transport of the electrons and ions. The potential structure is extended in the plasma along the applied magnetic field. It is proposed that this process is responsible for the formation of gridding phenomenon in magnetized dusty plasma experiments. This work is supported with funding from the U.S. Department of Energy, NASA, and the National Science Foundation (Physics Division and EPSCoR Office).   

Controlled Photo-Discharging of Dust in a Complex Plasma

Photoelectric charging of dust particles in a plasma has previously been investigated in very low pressure experiments in the past and is the primary charging mechanism in astronomical dusty plasmas. However, photoelectric charging of dust in low-temperature laboratory plasmas is difficult due to the high work functions and poor photoelectron yields of most conventional dust materials (like silica) and the substantially larger particle (ion/electron) fluxes onto the dust in laboratory LTPs compared to space plasmas. A new experiment at Auburn University utilizes lanthanum boride (LaB6) powder and a high intensity UV source to investigate whether controlled photo-discharging of dust particles can be achieved in a weakly ionized, argon, DC glow discharge and whether it can be accomplished with minimal perturbation to the background plasma. Data extracted from dust videos and Langmuir probe data are presented with theoretical models of charging effects.   

Dust charge reversal in an afterglow plasma

Dusty plasmas contain small particles of solid matter, which are usually charged negatively. However, we have discovered a condition where the polarity reverses. Although they are negatively charged while the plasma is powered steadily, the particles switch polarity and gain a large positive charge a few msec after switching off the plasma power. In the afterglow, electrons are largely absent, and ions collect on the particle, giving it a positive charge. This discovery was made using a radiofrequency glow-discharge plasma. Polymer microspheres, of 8.7-micron diameter, were levitated in a horizontal sheath above a lower electrode powered at 13 MHz. Due to capacitive coupling, the lower electrode sustained a negative potential, even after the RF power was extinguished. Viewing from the side, while suddenly switching off the RF power, we observed the particles fall. They had a constant acceleration greater than that of gravity alone. This increased acceleration can only be explained if the dust particles acquire positive charges in the first few msec of their fall, so that they are accelerated toward the negatively biased electrode.   

Filamentation in Capacitively Coupled Magnetized Plasmas.

Recent experiments at the Magnetized Dusty Plasma Experiment (MDPX) at Auburn University have observed the formation of filamentary structures in capacitively-coupled, rf generated plasmas at high magnetic field (B $\ge $ 0.5 T). These plasma filaments, when viewed from the side, appear as bright vertical columns aligned parallel to the magnetic field that can either be stable or mobile structures, depending upon the experimental conditions in the plasma. In this work, the MDPX device is used to study the threshold conditions for filamentation formation under a variety of RF power, pressure, and applied magnetic field conditions. At low pressures (p \textless 10 mTorr), low RF power (P \textless 5 W), and high magnetic field, a regime of individual filaments with rotating spiral arms has been identified. This presentation will focus on how those properties of the filaments (size distribution, number, rotation velocity, intensity/density, etc.) vary with the magnetic field. Ultimately, we seek to understand how the filaments scale with fundamental length scales in the plasmas e.g., ion/electron gyroradii and the collision mean free path.