Brownian-like motion of a single dust grain in a radio-frequency plasma discharge comparison of experiments and simulations

Bronwnian-like motion of a single dust-grain in a radio frequency plasma has been studied by different research groups. The rise of the particles temperature above “room temperature” is attributed to e.g. random fluctuations of the particle charge and fluctuations of the electrical field. Additional disturbance might occur due to gas density variations, temporal variation of the particles mass and particle interaction with the illuminating laser light. In addition, a nonoptimal frame rate of the optical diagnostic system and pixel locking can lead to an incorrect estimation of the particle kinetic temperature. Our experiments are conducted in a weakly ionized radio-frequency gas discharge at a low neutral gas pressure and power. A single micron sized spherical particle is trapped in a harmonic-like potential trap in the sheath of the lower driven electrode [1]. Its twodimensional planar motion is recorded with a long-distance microscope and a high-resolution camera. From the measured particle positions we derive the probability density function, the velocity autocorrelation function and the mean squared displacement. We obtain a particle kinetic temperature above 350 K, a neutral gas damping time of about 0.5 sec and a resonance frequency of 1-2 Hz. Anisotropic oscillation of the particle occurs, leading to angle dependent temperatures along the x and y direction in the plane of the recorded images, which can be explained by the presence of an asymmetric horizontal potential trap. Experimental observations are compared with our simulations using md simulations and the Ornstein-Uhlenbeck stochastic process

Three-dimensional structure of a string-fluid complex plasma

Three-dimensional structure of complex (dusty) plasmas was investigated under long-term microgravity conditions in the International-Space-Station-based Plasmakristall-4 facility. The microparticle suspensions were confined in a polarity-switched dc discharge. The experimental results were compared to the results of the molecular dynamics simulations with the interparticle interaction potential represented as a superposition of isotropic Yukawa and anisotropic quadrupole terms. Both simulated and experimental data exhibited qualitatively similar structural features indicating the bulk liquid-like order with the inclusion of solid-like strings aligned with the axial electric field. Individual strings were identified and their size spectrum was calculated. The decay rate of the size spectrum was found to decrease with the enhancement of string-like structural features

Image Registration with Particles, Examplified with the Complex Plasma Laboratory PK-4 on Board the International Space Station

Often, in complex plasmas and beyond, images of particles are recorded with a side-by-side camera setup. These images ideally need to be joined to create a large combined image. This is, for instance, the case in the PK-4 Laboratory on board the International Space Station (the next generation of complex plasma laboratories in space). It enables observations of microparticles embedded in an elongated low temperature DC plasma tube. The microparticles acquire charges from the surrounding plasma and interact strongly with each other. A sheet of laser light illuminates the microparticles, and two cameras record the motion of the microparticles inside this laser sheet. The fields of view of these cameras slightly overlap. In this article, we present two methods to combine the associated image pairs into one image, namely the SimpleElastix toolkit based on comparing the mutual information and a method based on detecting the particle positions. We found that the method based on particle positions performs slightly better than that based on the mutual information, and conclude with recommendations for other researchers wanting to solve a related problem

Influence of Reduced Soot Emission and Increased Water Vapor Content on Contrail Climate Impact

Contrails have a significant impact on climate and contribute to global warming. Their formation is primarily caused by the emission of water vapor and soot particles from aircraft engines. When water vapor is released into the cold and humid upper atmosphere, it can rapidly condensate on soot particles and form ice crystals. Reducing carbon and soot emissions from aviation is critical in mitigating the climate impact of air travel. Hydrogen presents a compelling alternative to traditional jet fuels; it emits no carbon when burned in an engine, although it does produce more water vapor, which could intensify contrail formation. In our study, we explore the effect of reducing soot emissions and increasing water vapor in the engine exhaust as a first step towards using hydrogen as fuel. The Contrail Cirrus Prediction (CoCiP) model was used to simulate contrail properties and estimate their climate impact. We focus on Europe with air traffic data from Eurocontrol for the entire year 2019. Specific aircraft data were obtained from the BADA3 database, soot emissions stem from a state-of-the-art method to scale ICAO emissions to cruise conditions and weather data from ECMWF-IFS forecast. For our investigations we reduce the engine soot emissions for kerosene by up to 99% for the complete fleet. Then for each flight and aircraft, we calculate the optical thickness and radiative forcing from contrails in Europe in 2019 based on ECMWF weather information and quantify the relative changes for a stepwise reduction in soot emissions of the fleet. In a second step we increase the water vapor emissions in relation to the expected change for future hydrogen combustion. Results from the simulations are presented and the potential impact for future novel engine technologies and fuels are discussed

Laser-stimulated Melting of a Two-Dimensional Complex Plasma Crystal

The melting of a two-dimensional plasma crystal was induced in a principally stable monolayer by laser-stimulated localized melting. Depending on the energy amount injected by the laser, the melted spot expanded outwards in a similar fashion to the mode-coupling instability (MCI) induced melting or recrystallized. As fluid MCI always exists in a melted monolayer, if the spot exceeded a critical size, the fluid MCI growth rate surpassed the damping rate and MCI-like melting was then observed. This behavior exhibits remarkable similarities with impulsive spot heating and thermal explosion in ordinary matter

Crystallization in three-dimensional complex plasmas

Complex plasmas are low-temperature plasmas in which microparticles are embedded. The microparticles are illuminated with a laser sheet, and their movement is traced in time and space using high speed digital cameras. This allows studying their dynamics on the most basic level, that of the individual particles. Moving the laser sheet through the system makes three-dimensional investigations possible by recording several slices in succession in a tomographic procedure. The microparticles in the plasma get strongly charged by collecting unequal amounts of ions and electrons from the surrounding plasma. They acquire mean charges of several thousands of electrons. Thus, they strongly interact with each other. When the interaction strength is much larger than the microparticles' kinetic temperature, the microparticles arrange in ordered structures, and a 'plasma crystal' forms. Under gravity conditions, these crystals are stressed by the strong forces required to counteract gravity, and plasma-specific instabilities easily induce melting. Under microgravity, however, the microparticles are suspended in the bulk of the plasma, where the ion fluxes are small, and the fluid-solid phase transition is realized via a generic mechanism that is common to a wide range of materials. Here, we give an overview over crystallization experiments in complex plasmas with an emphasis on data recorded with the PK-3 Plus Laboratory that was hosted on board the International Space Station until 2013. This laboratory was very versatile, but its main goal was to study crystallization, with 32 experiments dedicated to this topic. Crystallization or melting were typically induced by changing the neutral gas pressure, but could also be caused by changes in the particles charges, or, in the case of melting, by shaking a plasma crystal with an electric field. This last method was applied to induce crystallization fronts by melting a preformed plasma crystal incompletely and studying the recrystallization process. We study the three- dimensional propagation of these crystallization fronts: By performing repeated short scans through the system, we find the three-dimensional position of the fronts and determine their propagation velocity. We use both conventional analysis of the microparticle dynamics and novel techniques developed by C. Dietz et al. to accurately identify the crystalline and fluid regions

Particle charge in PK-4 dc discharge from ground and microgravity experiments

Particle charge in PK-4 dc discharge from ground and microgravity experiments T. Antonova1, S.A. Khrapak1, M. Pustylnik1, M. Rubin-Zuzic1, H.M. Thomas1, A.M. Lipaev2, A.D. Usachev2, V.I. Molotkov2, M.H. Thoma3 1Institut fur Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), Weßling, GERMANY 2Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow, RUSSIA 3I. Physikalisches Institut, Justus-Liebig Universität, Gießen, GERMANY The complex plasma facility Plasmakristall-4 (PK-4) was installed in the Columbus module of the International Space Station (ISS) in November 2014. This is an experimental laboratory developed to provide a range of various experiments in the direct current (dc) or/and radiofrequency (rf) low temperature gas discharge. It allows to use different manipulation technics (e.g. laser manipulation, thermal and electrical disturbances, etc) [1]. Because of the gravity force the positions of microparticles in discharge on ground differ from those under microgravity conditions. The comparison of both cases gives the possibility to resolve discharge parameters as well as main microparticle characteristics in radial direction of the discharge tube. The aim of the current work is to estimate the radial distribution of the particle charge within the discharge tube from the measurements of the particle drift velocity in PK-4 set-up. The experiments have been performed in the Flight Model (FM) onboard ISS as well as in Science Reference Model 1 (SRM 1) of PK-4 in ground based laboratory, which is functionally identical to the FM. The pressure ranged from 20 to 100 Pa in argon and neon gases with the variation of the discharge current from 0.5 to 1.5 mA. The particles of three different diameters of 1.3, 2.5 and 3.4 μm have been injected into the chamber. They were illuminated by the laser beam and their motion was filmed by video cameras with 35 fr/sec and 14,2 μm/pixel resolution. The velocities have been estimated by measuring the velocity of the whole particle cloud as well as from the intensity slope on the so-called space-time diagram. The experimental data from ISS show that under microgravity conditions the velocities of microparticles are always lower than those measured on ground, as it already has been observed in parabolic flight experiments [2]. The difference is more pronounced in the lower pressure range (20-30 Pa). Drift velocities from experimental data have been compared with the results of analytical model, which yielded the estimation of the particle charge for chosen experimental conditions on ground and under microgravity. In the developed model variations of the discharge parameters in radial direction of the discharge tube have been taken into account. The experimentally measured and theoretically estimated particle velocities as well charges show different pressure behavior in argon and neon gases. All authors greatly acknowledge the joint ESA-Roscosmos “Experiment Plasmakristall-4” onboard the International Space Station. This work was also partially supported by DLR Grants Nos. 50WM1441 and 50WM1442. REFERENCES [1] M. Y. Pustylnik, M. A. Fink, V. Nosenko et all, "Plasmakristall-4: New complex (dusty) plasma laboratory on board the International Space Station", Review of scientific instruments 87, 093505 (2016) [2] S. A. Khrapak, M. H. Thoma, M. Chaudhuri, et all, "Particle flows in a dc discharge in laboratory and microgravity conditions", Physical Review E 87, 063109 (2013

Observation of metallic sphere - complex plasma interactions in microgravity

The PK-3 Plus laboratory [1, 2] on board the International Space Station is used to study the interaction between metallic spheres and a complex plasma. We show that the spheres significantly affect both the local plasma environment and the microparticles. The spheres charge under the influence of the plasma and repel the microparticles, forming cavities surrounding the spheres. At intermediate distances from the sphere surface, however, the interaction between the microparticles and the spheres is attractive. The spheres affect the plasma fluxes, which can lead to the excitation of dust-acoustic waves near the spheres

A second look at void closure in complex plasmas

Complex Plasmas are small micrometer sized particles injected into a low temperature rf-plasma. The particles are getting charged by electron and ion fluxes and form systems with gaseous, liquid and solid properties. Normally complex plasmas are compressed to 2D systems in laboratory conditions while they form a 3D cloud in micro-gravity with a particle free region in the center which is called void. The void can be suppressed by gas flow or additional electric fields, however, both ways add stress to the system. Another way is the reduction of rf-power, which was successfully used on the ISS before. The comparison of simulations and emission patterns reveal a lot of open questions, which were targeted by experiments of the 29. DLR parabolic flight campaign. The absence of a void in 1D self-consistent simulations indicates that the void is caused by 2D effects or more difficult geometries. In the experiments we could close and reopen the void by decreasing and increasing the rf-power. Other effects, such as particle mixtures in the former void region and dust density waves across the particle cloud, were observed