Session UO4: Basic and Low Temperature Plasmas: Plasma Sheath, Dusty Plasmas, and Pattern Formation

Measurement of ion-acoustic fluctuations using high frequency laser-induced fluorescence in the presheath of a biased electrode

Ion-acoustic fluctuations driven by strong flows in the presheath are observed using a high frequency laser-induced fluorescence (LIF) diagnostic. Ion fluctuation spectra are resolved spatially through the presheath region of a positively and negatively biased electrode. Measurements are made using a recently developed field programmable gate array (FPGA)-based LIF system. The system can measure ion fluctuation spectra up to 1 MHz using a correlation function method. Ion-acoustic fluctuations are observed near 500 kHz, about half of the ion plasma frequency, throughout the presheath and for positively and negatively biased electrodes. The fluctuation power is observed to increase significantly when the electrode is biased above the plasma potential. However, the fluctuation power does not vary greatly with distance from the electrode. These observations are consistent with a recent theory that predicts the presence of a long-range electron presheath, in which the fast electron flow enhances ion-acoustic fluctuations.   

Dynamic Sheath Formation and sub-THz Radiation Emission from Laser-Metal Interactions

We are investigating secondary radiation from a short pulse laser with mJ energy incident on a metal surface. The electrons absorb energy from the laser pulse, increasing in temperature and resulting in some electrons at the tail of the Fermi-Dirac distribution function to overcome the work function barrier. The resultant electron motion at the surface contributes to a surface current, which in turn sources secondary radiation emission. The traditional thermionic emission picture breaks down due to Coulombic effects and therefore a modified emission model is presented. Previous precedence exists for the modified thermionic emission models for laser-metal interactions of a similar nature, such as one presented by Rife et., al [ J. Opt. Soc. Am. B, Vol. 10, p.1424 (1993) ]. Surface currents generated by such a mechanism are modeled with a Particle-In-Cell (PIC) simulation and their implication on the sourcing of microwave radiation is discussed. The predictions of our model in its relation to recent experiments is also discussed.   

\textbf{Using dust grains to investigate plasma conditions in the sheath}

Plasma conditions within a plasma sheath or a narrow trench on the powered electrode are difficult to measure, since traditional probes can alter the plasma on small spatial scales. Micron-sized dust grains have been found to be both non-perturbative and very sensitive to changing plasma conditions allowing them to be used to investigate dust dynamics and equilibrium dust structures. Unfortunately, the charge on the grain and the electric field(s) in the plasma are difficult to measure independently. For example, the streaming ions accelerated by the electric field inside a glass box placed on the lower powered electrode of a GEC rf reference cell can affect the overall charge on the surface of the grains within the box. In addition, the positive ion wake field formed downstream of the grains changes the interaction between the grains. In this study, a molecular dynamics simulation is used to consistently model the interaction between the ions and dust grains, while determining the dust charge for grains within the glass box. Comparison to experimental observations of dust structures formed inside the glass box allows plasma characteristics such as the wake potential, the electric field, and variations in the electron and ion density within the sheath to be identified.   

The Synthesis of Novel Materials via Dielectric Barrier Discharge Plasma Reactors

This work describes recent ARL research on the production of novel materials via several prototype atmospheric dielectric barrier discharge (DBD) plasma reactors. A bottom-up approach was exploited to synthesize novel carbon-containing deposits via plasma-assisted chemical vapor deposition. Issues regarding local variations in morphology, uniformity and thickness were identified and correlated to reactor configuration and design. A top-down approach was recently investigated to produce aluminum nanoparticles (nAl). Commercial nAl were plasma-treated to reduce the native oxide shell, followed by mixing in an iodic solution (HIO$_{\mathrm{3}})$ to coat the nAl with an oxidizing salt, aluminum iodate hexahydrate (AIH). Preliminary results suggest that the reactor design with stronger arcing led to increased energy release from the plasma-treated nAl-AIH, presumably due to more efficient arc-induced surface modification and improved nAl-HIO$_{\mathrm{3}}$ interactions. This paper underscores the importance of recognizing not only the benefits, but also the challenges of applying plasma techniques to novel material production.   

Numerical computation of the enhancement factor for the coagulation of silicon nanoparticles in low-temperature silane-argon plasmas

The coagulation enhancement factor was calculated numerically for nanoparticles of different sizes and charges under typical conditions in low-temperature argon-silane plasmas. We computed the electrostatic interaction between nanoparticles using a rigorous formulation in terms of the multipole coefficient expansion\footnote{Lindgren, E. B., et al. Phys. Chem. Chem. Phys. 18, 5883–5895 (2016).}. It is shown that coagulation is enhanced for neutral particles. Besides, the short-range force between nanoparticles of the same charge can become attractive. Furthermore, the potential calculated from the multipolar coefficients is compared to a more straightforward approximated analytical form. The second part of the results relates to the evolution of the size and charge distributions when the nanoparticles follow the processes of charging, coagulation, nucleation, and surface growth. The model could be easily extended to particles of other materials or in a 1D / 2D model configuration.   

A fully kinetic model of dust particle charging by contact with a thermal plasma

Dusty plasmas, which contain dust particles immersed in a thermal plasma, are important in a wide range of areas including fusion, planetary formation, hypersonic flight, and plasma machining. The dust particles accumulate charge from collisions by the surrounding plasma particles in a manner that is still not well understood. Orbital Motion Limited (OML) theory is a common explanatory tool but it ignores key aspects of the charging process. We have modeled dusty plasmas using a new, fully kinetic simulation based on the particle-in-cell code, LSP [D. Welch and D. Rose, Comp. Phys. Comm. \textbf{164}, 183-188 (2004)]. We find the equilibrium charge of the dust particle exceeds that predicted by OML by as much as a factor of four in the regime where the dust particle is smaller than the Debye length. Our simulations also capture aspects of the time evolution of the system that OML does not. We discuss these results and their origins and compare to various models and computational approaches.   

Structural Formation within Dusty Plasma

Complex plasmas have proven a versatile analog for the study of self-ordered systems, particularly those where structuring is determined by the particles' kinetic energy, local and global confinement and the interparticle forces created by the interaction of the dust particles with the plasma environment. Unfortunately, given the magnitude of these forces gravity often masks the underlying physics involved. In order to clarify this role, PK-4 data collected under microgravity on the International Space Station (ISS) has been compared to PK-4 data collected under gravity using the PK-4 BU analogue at Baylor University. The PK-4 BU analogue provides extended diagnostic capabilities allowing examination of dust systems trapped employing DC polarity switching and a RF field with a movable electrode. This talk will examine the physics underlying the formation of various dust particle structures (single particle(s), extended particle chains, hexagonal cylindrical structures) as observed in the PK-4 (ISS) and the PK-4 BU system. Data from the PK-4 ISS will be directly compared to data collected from the PK-4 BU as well as to numerical simulations of the dusty plasma system in order to examine dust charging, linear and non-linear interactions and interparticle interaction forces.   

Machine-learning Analysis of the Structure of a Dust Cloud trapped in DC Plasmas

In the PK-4 dusty plasma experiment onboard the International Space Station (ISS), the dust cloud can form unique structures, such as multiple chains along the direction of the DC electric field. An identical experimental setup on earth (PK-4 BU) provides the ability to manipulate and investigate these structures under similar experimental conditions, but under the influence of gravity. In this research, the 3D structure of dust clouds formed in the PK-4 BU is investigated where dust clouds are trapped by rapidly switching the polarity of the DC electric field. Vertical slices of the trapped cloud are imaged by scanning a vertically-fanned laser beam with a width of $\approx $ 150 $\mu $m. The 3D structure of the clouds is then reproduced from the two-dimensional (2D) pictures obtained from the scan. In this manner, crystallization as well as phase transitions of the 3D dust cloud can be recognized and analyzed using a machine learning based approach. These results will be compared to data from the PK-4 ISS as well as to simulations of dust structures in environments similar to those found in the experiments. Dispersion relations obtained from the dust particle motion will be used to probe the inter-chain interactions.   

Non-linear structure formation in strongly coupled dusty plasma flow past an obstacle

A novel device has been designed to study a dusty plasma fluid flow past an obstacle (a dust void) immersed (created) in a stationary dusty plasma medium. In most experiments, dust flows are induced either by neutral gas flow variation or by change in electric potential. We have studied the transition from laminar to turbulent flow dynamics behind the dust void by controlling the dust flow speed. Experiment is performed in an RF discharge plasma mixed with dust grains of 5 micron diameter. The dusty plasma flow is sustained for a few seconds and the velocity is controllable in a variable range $\sim0-30 cm/sec$. The Reynolds number associated with the flow is measured from the kinematic viscosity parameter of the medium. We report on the first observation of a counter rotating vortex pair behind a dust void when the Reynolds number is $\sim130$1) generates bow shock in the upstream region of the obstacle.   

Synchronization of dust density wave by ion streaming modulation in nanodusty plasma

Synchronization of the self-excited dust density waves (DDWs) by ion streaming modulation is investigated in a rf discharge plasma. Plasma is produced in a glass cylinder of diameter 2.8 cm and length 15 cm using rf discharge (13.56 MHz, 5 to 15 W) at a pressure of 0.015 mbar of argon. Carbon nano powder of average diameter 50 nm has been introduced into the argon plasma. In presence of an ion streaming with drift velocity larger than the ion thermal speed, self-excited DDWs are observed in dusty plasma. Here, we have observed spontaneous DDWs of frequency 78 to 100 Hz originating from the void boundary above the live electrode. Dust cloud is illuminated by laser light scattering and dust dynamics are recorded using a high speed camera. The ions streaming in the outward direction from the dust void, is modulated by applying an external sinusoidal signal with frequency close to the self-excited DDW frequency into a grid. Above a threshold modulation voltage, we observe the synchronization of the DDW through the mechanism of mode suppression. The time series data are obtained from the optical pixel intensity profiles of image frames and the FFT spectrums are analyzed to interpret the observed phenomena. Typical plasma parameters are measured by a Langmuir probe.   

Using Dust Dynamics to Diagnose Evolving Plasma Conditions

Micron-sized dust grains have been successfully employed as non-perturbative probes to measure variations in plasma conditions on small spatial scales, such as those found in plasma sheaths. Within a sheath, ions are accelerated towards the charged boundary, and this ion flow creates a positively-charged spatial region downstream of the dust grain, called the ion wake. The ion wake in turn modifies the interaction potential between the charged grains and can contribute to the stability of the dust structures formed under specific plasma conditions. A multi-scale numerical model of the dust-plasma interactions is compared with experimental data which allows determination of quantities such as the charge on individual grains, the electric field within the region, the ion density, and the ion flow velocity. This work is supported by NASA grant 1571701, and NSF grants 1740203, 1707215.   

Pattern formation and filamentation in low-pressure, low-temperature magnetized plasmas: A descriptive model

Self-organization is a commonly observed phenomenon in a wide variety of plasma systems. One of the most recent examples of this phenomenon is the observation of filamentary structures and their associated pattern formation in low-pressure capacitively coupled rf glow discharged plasmas that are exposed to high magnetic fields. We define `filamentary structure' as distinct, localized regions within a plasma that appears brighter than the surrounding plasma and that extend parallel to the magnetic field lines. Despite several experimental investigations of the phenomenon, the underlying physics that describes the initial formation and long persistence (up to several seconds) of the filaments remains poorly understood. In this presentation, a model is presented to describe self-organization in magnetized plasmas based on the results from 3D numerical simulations that self-consistently solve the plasma fluid equations along with the Poisson's equation. The formation of these structures is thought to be mainly due to discrepancy between the fluxes of electrons and ions across and parallel to the magnetic field. Additional evidence is presented that suggests that filament formation is also affected by plasma-surface interactions at the boundaries of the plasma.   

The Effect of Particles on Standing Shockwaves Regulating Spark Discharges in Volcanic Eruptions

Near-vent discharges emit in the VHF spectrum without lower frequencies seen in meteorological lightning (Behnke et al. 2018. JGR). This suggests the discharges are cut-off before they can form leaders associated with lower frequency emission. Experiments generating discharges in a particle-laden gas jet formed during the decompression of a shocktube filled with ash have identified a standing shockwave regulating the breakdown process (Mendez-Harper et al. 2018. GRL). Particles charge through collisions, triboelectrically, in the high pressure conduit. Once they pass the vent and enter the rarefaction region the pressure and the Paschen breakdown voltage rapidly drop enabling breakdown. The breakdown is cutoff by the rapid increase in pressure beyond the stationary Mach disk. While Mach disk formation is well understood for gas flows through a vent/nozzle, the effect of particles has not been explored. We have adapted a granular compressible hydrodynamics model (Houim, Oran 2016. JFM) to the volcanic shocktube problem. Elucidating the relationship between mass eruption rate, particle size distribution, and spark discharge behavior could result in novel radiofrequency measurement techniques for the hard to diagnose near-vent region of volcanos.