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Convection is a phenomenon that often occurs in the presence of temperature gradients. In micro-gravity, free convection can not occur due to the lack of buoyancy. However, during parabolic flights we observed convections of microparticles in a gas discharge within the cylindrical plasma chamber of the setup PK-4. The microparticles and the plasma were exposed to a thermal gradient caused by a heating ring attached to the outer wall of the plasma chamber, which is made of glass. Because of the thermophoretic force caused by the thermal gradient the microparticles were stopped in front of the heater. There, the cloud convections and dust waves were observed. Analysis by tracking the microparticles’ trajectories showed that the vortices were induced by thermal creep, a gas flow that commonly occurs in gases with low pressures at inhomogeneously heated solid interfaces. This effect has driven a gas convection which in turn caused the convection of the microparticle cloud.

The future of complex plasma research under microgravity condition, in particular on the International Space Station ISS, is discussed. First, the importance of this research and the benefit of microgravity investigations are summarized. Next, the key knowledge gaps, which could be topics of future microgravity research are identified. Here not only fundamental aspects are proposed but also important applications for lunar exploration as well as artificial intelligence technology are discussed. Finally, short, middle and long-term recommendations for complex plasma research under microgravity are given.

In a plasma containing micrometer-size dust particles, nonlinear wave synchronization was investigated experimentally under microgravity conditions on board the International Space Station (ISS). These dust particles were confined into a 3-D cloud, in the vicinity of a diffuse edge of the plasma, which was generated by an inductively coupled radio frequency (RF) glow discharge. A cross-sectional slab of the cloud was imaged using a video camera. The dust-density fluctuations in the slab were characterized using the video image data. A steady and long-lived dust acoustic wave (DAW) was observed to be spontaneously generated in the cloud; it propagated through the dust cloud, which had a gradually varying density distribution. Two kinds of spectral analyses of the wave motion were performed, using Fourier transforms and Hilbert transforms, respectively; these revealed two distinctive spatial domains in the cloud, termed frequency clusters. Within each cluster, waves were found to oscillate at a dominant frequency that remained constant, manifesting mutual synchronization throughout the cluster. Across the two clusters, the dominant frequency exhibited a step-wise change, with a frequency ratio of 2:1, which is consistent with phase-lock conditions for a harmonic synchronization state.

The scientific equipment «Plasmakrystall–4» («PK–4») is designed to study complex (dusty) plasma under microgravity conditions aboard the International Space Station (ISS) and is a joint project of the European Space Agency (ESA) and Roscosmos. Scientific equipment «PK–4» is integrated into «European physiological modules» (EPM) rack, in the European laboratory module Columbus. Experiment control — automated, software-interactive, or manual from an on-board laptop and/or from a terminal in the ground control center. A low-pressure direct current discharge in noble gases in a glass tube is used to create a plasma at scientific equipment «PK–4». Microparticles of a given size are injected into the discharge to obtain a complex plasma. Two digital video cameras allow to trace individual microparticles inside the tube in phase space, which makes a complex plasma to be a good model for studying classical phenomena in condensed matter at the kinetic level. To monitor the plasma conditions, an integrated spectrometer and another video camera are used allowing to observe the plasma's own emission at different wavelengths. To study the reaction of microparticles to external forces, they can be exposed to radiation from a powerful laser, a gas stream, and also to thermophoretic force, i.e., by producing a given temperature gradient. Key words: complex plasmas, microparticles, soft matter, laser manipulation, microgravity, viscosity measurements, gas discharges, plasma diagnostics.

Drug resistant bacteria, prions and nosocomial infections underline the need of more effective sterilizing technologies. The cold plasma technology is expected to bring a benefit in this context. Six different plasma sources, based on printed circuit boards, were evaluated fourfold. This include measurements of the power consumption, the ignition behavior by an ICCD-camera and ozone formation by absorption spectroscopy at 254 nm. To evaluate the biocidal effect, four bacterial test series were performed with Escherichia coli. The entirety of the tests analyze the plasma inactivation process from the input parameters to the desired biocidal effect. The discharge current and time resolved ignition behaviors indicated a simultaneous formation of filaments at the beginning of the negative half-cycle. The dynamics of the ozone production showed a saturated exponential growth upon a maximum value of 435 ppm. Additionally, the microbiological test series unveiled differences between the plasma source concepts. A total reduction rate of Log-4 within a minute was achievable. An air flow through slits within the plasma sources destabilized the plasma. Minor changes of the electrode geometry changed all measured parameters. Hence, to develop a pathogen inactivating plasma source, these results recommend a comb-shaped electrode design, which is laminated on a dielectric.

Microparticle suspensions in a polarity-switched discharge plasma of the Plasmakristall-4 facility on board the International Space Station exhibit string-like order. As pointed out in [Phys. Rev. Research 2, 033314 (2020)2643-156410.1103/PhysRevResearch.2.033314], the string-order is subject to evolution on the timescale of minutes at constant gas pressure and constant parameters of polarity switching. We perform a detailed analysis of this evolution using the pair correlations and length spectrum of the string-like clusters (SLCs). Average exponential decay rate of the SLC length spectrum is used as a measure of string order. The analysis shows that the improvement of the string-like order is accompanied by the decrease of the thickness of the microparticle suspension, microparticle number density, and total amount of microparticles in the field of view. This suggests that the observed long-term evolution of the string-like order is caused by the redistribution of the microparticles, which significantly modifies the plasma conditions.

The use of cold atmospheric plasmas (CAP) to sterilize sensitive surfaces is an interesting new field of applied plasma physics. Motivated by the shortages of face masks and safety clothing at the beginning of the corona pandemic, we conducted studies on the sterilization of FF3 face masks with CAP and the resulting material effects. Therefore, the bactericidal and sporicidal efficacy of CAP afterglow sterilization of FFP3 mask material was investigated by inoculating fabric samples with test germs Escherichia coli (E. coli) and Bacillus atrophaeus (B. atrophaeus) and subsequent CAP afterglow treatment in a surface-micro-discharge (SMD) plasma device. In addition, a detailed analysis of the changes in long-term plasma treated (15h) mask material and its individual components - ethylene vinyl acetate (EVA) and polypropylene (PP) - was carried out using surface analysis methods such as laser microscopy, contact angle measurements, X-ray photoelectron spectroscopy (XPS) as well as fabric permeability and resistance measurements. The experiments showed that E. coli and B. atrophaeus could both be effectively inactivated by plasma treatment in nitrogen mode (12 kVpp, 5 kHz). For B. atrophaeus inactivation of more than 4-log was achieved after 30 minutes. E. coli population could be reduced by 5-log within one minute of CAP treatment and after five minutes a complete inactivation (> 6-log) was achieved. Material analysis showed that long-term (> 5 h) plasma treatment affects the electrostatic properties of the fabric. From this it can be deduced that the plasma treatment of FFP3 face masks with the CAP afterglow of an SMD device effectively inactivates microorganisms on the fabric. FFP3 masks can be plasma decontaminated and reused multiple times but only to a limited extent, as otherwise the permeability levels no longer meet the DIN EN 149 specifications., 15 pages, 9 Figures

Time-dependent shear motion of dust particles in a three-dimensional (3-D) strongly coupled dusty plasma under microgravity conditions was investigated using the European Space Agency–Roscosmos facility Plasma-Kristall 4 (PK-4) on the International Space Station (ISS). The dust particles, which were negatively charged, were trapped in a glow discharge plasma powered by a dc voltage that switched its polarity periodically. They self-organized themselves into a structure resembling a cold liquid. A manipulation laser beam of a circular cross section pushed a stream of particles, moving them through the surrounding sample region; its power was modulated using a rectangular pulse, causing an impulsive unsteady flow. At the onset of the impulsive motion, the flow was observed as it grew in space and time. Later, during the steady manipulation, we find the thickness of the flow region fluctuated significantly over time.

The self-organization of dust grains into stable filamentary dust structures (or "chains") largely depends on dynamic interactions between the individual charged dust grains and the complex electric potential arising from the distribution of charges within the local plasma environment. Recent studies have shown that the positive column of the gas discharge plasma in the Plasmakristall-4 (PK-4) experiment onboard the International Space Station (ISS) supports the presence of fast-moving ionization waves, which lead to variations of plasma parameters by up to an order of magnitude from the average background values. The highly-variable environment resulting from ionization waves may have interesting implications for the dynamics and self-organization of dust particles, particularly concerning the formation and stability of dust chains. Here we investigate the electric potential surrounding dust chains in the PK-4 by employing a molecular dynamics model of the dust and ions with boundary conditions supplied by a Particle-in-Cell with Monte Carlo collisions (PIC-MCC) simulation of the ionization waves. The model is used to examine the effects of the plasma conditions within different regions of the ionization wave and compare the resulting dust structure to that obtained by employing the time-averaged plasma conditions. Comparison between simulated dust chains and experimental data from the PK-4 shows that the time-averaged plasma conditions do not accurately reproduce observed results for dust behavior, indicating that more careful treatment of plasma conditions in the presence of ionization waves is required. It is further shown that commonly used analytic forms of the electric potential do not accurately describe the electric potential near charged dust grains under these plasma conditions., Comment: 15 pages, 14 figures

We report measurements of the time-averaged surface floating potential of magnetic and non-magnetic spherical probes (or large dust particles) immersed in a magnetized capacitively coupled discharge. In this study, the size of the spherical probes is taken greater than the Debye length. The surface potential of a spherical probe first increases, i.e. becomes more negative at low magnetic field ( $B < 0.05\ \textrm {T}$ ), attains a maximum value and decreases with further increase of the magnetic field strength ( $B > 0.05\ \textrm {T}$ ). The rate of change of the surface potential in the presence of a $B$ -field mainly depends on the background plasma and types of material of the objects. The results show that the surface potential of the magnetic sphere is higher (more negative) compared with the non-magnetic spherical probe. Hence, the smaller magnetic sphere collects more negative charges on its surface than a bigger non-magnetic sphere in a magnetized plasma. The different sized spherical probes have nearly the same surface potential above a threshold magnetic field ( $B > 0.03\ \textrm {T}$ ), implying a smaller role of size dependence on the surface potential of spherical objects. The variation of the surface potential of the spherical probes is understood on the basis of a modification of the collection currents to their surface due to charge confinement and cross-field diffusion in the presence of an external magnetic field.

Shear flow in a three-dimensional complex plasma was experimentally studied in microgravity conditions using Plasmakristall-4 (PK-4) instrument on board the International Space Station (ISS). The shear flow was created in an extended suspension of microparticles by applying the radiation pressure force of the manipulation-laser beam. Individual particle trajectories in the flow were analyzed and from these, using the Navier-Stokes equation, an upper estimate of the complex plasma's kinematic viscosity was calculated in the range of $0.2$--$6.7~{\rm mm^2/s}$. This estimate is much lower than previously reported in ground-based experiments with 3D complex plasmas. Possible reasons of this difference are discussed., 5 pages, 4 figures

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.

The collective dynamics of annulus dusty plasma formed between a co-centric conducting (non-conducting) disk and ring configuration is studied in a strongly magnetized radio-frequency (rf) discharge. A superconducting electromagnet is used to introduce a homogeneous magnetic field to the dusty plasma medium. In absence of the magnetic field, dust grains exhibit thermal motion around their equilibrium position. The dust grains start to rotate in anticlockwise direction with increasing magnetic field (B $>$ 0.02 T), and the constant value of the angular frequency at various strengths of magnetic field confirms the rigid body rotation. The angular frequency of dust grains linearly increases up to a threshold magnetic field (B $>$ 0.6 T) and after that its value remains nearly constant in a certain range of magnetic field. Further increase in magnetic field (B $>$ 1 T) lowers the angular frequency. Low value of angular frequency is expected by reducing the width of annulus dusty plasma or the input rf power. The azimuthal ion drag force due to the magnetic field is assumed to be the energy source which drives the rotational motion. The resultant radial electric field in the presence of magnetic field determines the direction of rotation. The variation of floating (plasma) potential across the annular region at given magnetic field explains the rotational properties of the annulus dusty plasma in the presence of magnetic field., 10,9

International audience; The transition from old space to new space along with increasing commercialization has a major impact on space flight, in general, and on electric propulsion (EP) by ion thrusters, in particular. Ion thrusters are nowadays used as primary propulsion systems in space. This article describes how these changes related to new space affect various aspects that are important for the development of EP systems. Starting with a historical overview of the development of space flight and of the technology of EP systems, a number of important missions with EP and the underlying technologies are presented. The focus of our discussion is the technology of the radio frequency ion thruster as a prominent member of the gridded ion engine family. Based on this discussion, we give an overview of important research topics such as the search for alternative propellants, the development of reliable neutralizer concepts based on novel insert materials, as well as promising neutralizer-free propulsion concepts. In addition, aspects of thruster modeling and requirements for test facilities are discussed. Furthermore, we address aspects of space electronics with regard to the development of highly efficient electronic components as well as aspects of electromagnetic compatibility and radiation hardness. This article concludes with a presentation of the interaction of EP systems with the spacecraft.

The crystallization of a 3-D complex plasma under gravity conditions at different gas pressures is investigated. Three different methods are applied to determine the phase state of the complex plasma: the pair correlation, the nonspherical distribution (which uses the geometric properties of the Voronoi cells of each particle), and an improvement of the scalar product of the local bond-order parameter (SPBOP). All criteria are in accordance with that the complex plasma is in solid at low and in liquid state at high pressure.

We report observation of the dust ionization waves (DIWs) excited by an external oscillating electric field on the Plasma Kristall-4 facility under microgravity conditions. It is shown that at the smallest excitation amplitude, the waves are linear, and the dispersion relation can be deduced from the experimental data. The microparticle oscillations are represented as a superposition of two longitudinal waves propagating in the opposite directions. In the investigated range of excitation frequency, the wavenumber is not directly proportional to the frequency, and the phase velocity is almost proportional to the frequency. We propose an interpretation of DIW assuming that the microparticle effect on the recombination rate rather than the microparticle subsystem compressibility is responsible for the wave propagation. The calculated phase velocity of DIW is compatible with the experimental one.

Tunable diode laser induced fluorescence (TDLIF) measurements are discussed and quantitatively evaluated for nitrogen admixtures in argon plasma under the influence of a strong magnetic field. TDLIF measurements were used to evaluate light-transport properties in a strongly magnetized optically thick argon/nitrogen plasma under different pressure conditions. Therefore, a coupled system of rate balance equations was constructed to describe laser pumping of individual magnetic sub-levels of $\mathrm{2p_8}$ state through frequency-separated sub-transitions originating from $\mathrm{1s_4}$ magnetic sub-levels. The density distribution (alignment) of $\mathrm{2p_8}$ multiplet was described by balancing laser pumping with losses including radiative decay, transfer of excitation between the neighboring sub-levels in the $\mathrm{2p_8}$ multiplet driven by neutral collisions (argon and nitrogen) and quenching due to electron and neutral collisions. Resulting $\mathrm{2p_8}$ magnetic sub-level densities were then used to model polarization dependent fluorescence, considering self-absorption, which could be directly compared with polarization-resolved TDLIF measurements. This enables to estimate the disalignment rate constant for the $\mathrm{2p_8}$ state due to collisions by molecular nitrogen. A comparison to molecular theory description is given providing satisfactory agreement. The presented measurement method and model can help to describe optical emission of argon and argon-nitrogen admixtures in magnetized conditions and provides a basis for further description of optical emission spectra in magnetized plasmas., Comment: 15 pages , 8 figures

A new quantitative evaluation of tunable diode laser induced fluorescence (TDLIF) measurements in magnetized plasma is presented in this article, taking into account Zeeman splitting of energetic levels as well as inter- and intra-multiplet mixing, defining the density distribution (alignment) of the excited 2 p 8 multiplet of argon. TDLIF measurements were used to evaluate light-transport properties in a strongly magnetized optically thick argon plasma under different pressure conditions. Therefore, a coupled system of rate balance equations was constructed to describe laser pumping of individual magnetic sub-levels of the 2 p 8 state through frequency-separated sub-transitions originating from 1 s 4 magnetic sub-levels. The density distribution of the 2 p 8 multiplet was described by balancing laser pumping with losses, including radiative decay, transfer of excitation between the neighboring levels within the 2 p 8 multiplet driven by neutral collisions, and quenching due to electron and neutral collisions. Resulting 2 p 8 magnetic sub-level densities were then used to model polarization dependent fluorescence, considering self-absorption, which could be directly compared with measured polarization-resolved TDLIF measurements. The achieved results enable to obtain unique solutions for the 1 s 4 and 1 s 5 magnetic sub-level densities which were found to be in good agreement with the densities obtained by laser absorption measurements. It is shown that polarization resolved TDLIF measurements in magnetized plasma conditions have strong pressure dependence. The effective disalignment rate constant which redistributes the 2 p 8 sub-levels among each other has to be considered for a correct description of the TDLIF. This rate is dependent on the neutral gas density and a specific rate coefficient. With the presented method, 1s state densities involved in the TDLIF can be determined without any absolute intensity calibration in an optically thick plasma. Additionally, the presented measurement method and model can help to further understand and improve the description of optical emission of argon based on individual sub-transition descriptions under magnetized conditions.

Dust acoustic waves were investigated in an experiment under microgravity conditions, using the European Space Agency-Roscosmos facility PK-4 on the International Space Station. A three-dimensional dust cloud was confined in a glow-discharge plasma powered by a radio-frequency coil in low-pressure neon gas. Low-frequency dust acoustic waves (DAWs) were spontaneously excited due to the flowing ions in the plasma. Dust motion was imaged using video cameras, yielding the position for individual dust particles. Besides these particle-level measurements, the dust was also measured as a fluid continuum to obtain its velocity and density fluctuations. A space–time diagram of the dust-fluid velocity reveals that the waves changed properties as they propagated across the dust cloud, in the direction of the ion flow. Density fluctuations were characterized by a wave spectrum, indicating the emergence of collective vibration modes in a broad range of wave numbers. The decay of the modes was characterized by density correlation functions. Fitting the correlation functions to an analytical model yields an experimental measurement of the dispersion relation for DAWs, which is compared to that of linear theory.

By applying an external electric AC field to a dusty plasma, the micro-particles arrange in strings or chains caused by the wake potential between the micro-particles due to the ion drift in the plasma. In analogy to electrorheological fluids, such a system is called electrorheological plasma. In contrast, in the case of an external DC field, no string formation has been observed. Therefore, turning gradually the AC field into a DC field, a phase transition from a string-like electrorheological to an isotropic dusty plasma is expected. A molecular dynamics (MD) simulation assuming a superthermal ion drift is performed for studying this phase transition. A superthermal ion drift in an external electric field could be realized in a future microgravity experiment with complex plasmas proposed for the ISS. In this case, the MD simulation presented here predicts a crossover transition from a string fluid to an isotropic system if the AC field is turned into a DC field. The details of the transition depend on the gas pressure, particle size, and particle density. A microscopic explanation due to particle collisions for the phase transition could be identified from the simulation.

This paper reports on the dynamics of a 3-dimensional dusty plasma in a strong magnetic field. An electrostatic potential well created by a conducting or non-conducting ring in the rf discharge confines the charged dust particles. In the absence of the magnetic field, dust grains exhibit a thermal motion about their equilibrium position. As the magnetic field crosses a threshold value (B > 0.02 T), the edge particles start to rotate and form a vortex in the vertical plane. At the same time, the central region particles either exhibit thermal motion or E → × B → motion in the horizontal plane. At B > 0.15 T, the central region dust grains start to rotate in the opposite direction resulting in a pair of counter-rotating vortices in the vertical plane. The characteristics of the vortex pair change with increasing the strength of the magnetic field (B ∼ 0.8 T). At B > 0.8 T, the dust grains exhibit very complex motion in the rotating torus. The angular frequency variation of rotating particles indicates a differential or sheared dust rotation in a vortex. The angular frequency increases with increasing the magnetic field from 0.05 T to 0.8 T. The ion drag force and dust charge gradient along with the E-field are considered as possible energy sources for driving the edge vortex flow and central region vortex motion, respectively. The directions of rotation also confirm the different energy sources responsible for the vortex motion.

The charge of microparticles immersed in the dc discharge of the Plasmakristall-4 experimental facility has been estimated using the particle velocities from experiments performed on Earth and under microgravity conditions on the International Space Station. The theoretical model used for these estimates is based on the balance of the forces acting on a single particle in the discharge. The model takes into account the radial dependence of the discharge parameters and describes reasonably well the experimental measurements., Comment: 6 pages, 5 figures, published in Phys. Plasmas

This paper reports experiments on self$-$excited dust acoustic waves (DAWs) and its propagation characteristics in a magnetized rf discharge plasma. The DAWs are spontaneously excited in dusty plasma after adding more particles in the confining potential well and found to propagate in the direction of streaming ions. The spontaneous excitation of such low-frequency modes is possible due to the instabilities associated with streaming ions through the dust grain medium. The background E-field and neutral pressure determine the stability of excited DAWs. The characteristics of DAWs strongly depend on the strength of external magnetic field. The magnetic field of strength B $$ 0.05 T and get completely damped at higher magnetic field B $\sim$ 0.13 T. After lowering the power and pressure to 3 W and 23 Pa respectively, the excited DAWs in the absence of B are slightly unstable. In this case, the magnetic field only stabilizes and modifies the propagation characteristics of DAWs while the strength of B is increased up to 0.1 T or even higher. The modification of the sheath electric field where particles are confined in the presence of the external magnetic field is the main cause of the modification and damping of the DAWs in a magnetized rf discharge plasma., Comment: 9,6

Potential in biomedical-related applications by atmospheric pressure plasma-treated water gradually increased recently. In order to enhance the generation of reactive species in atmospheric pressure plasma in regards to liquid treatment, this study aims to investigate the effect of external axial magnetic field on a helium atmospheric pressure plasma jet (APPJ) and plasma-irradiated water. Magnetic field strength up to 2.0 T was generated by an electromagnet in Helmholtz configuration. The dielectric barrier discharge-based APPJ is driven by 4.4 slm helium and sinusoidal 30 kHz, 10 k V p p signals. Plasma was mainly characterized by optical emission where vibrational and rotational temperature was estimated by nitrogen second positive system, and electron temperature and density by emission intensity line ratio method. Representative reactive species of nitrite, nitrate, and hydrogen peroxide concentration in the plasma-treated water were also quantified. Results of vibrational temperature and electron density showed a first drop then rise trend which partially corresponds to the reactive species in the plasma-treated water. No significant changes were found in rotational temperature, while electron temperature monotonically increased with the magnetic field.

We report on the observation of the self-excited dust density waves in the dc discharge complex plasma. The experiments were performed under microgravity conditions in the Plasmakristall-4 facility on board the International Space Station. In the experiment, the microparticle cloud was first trapped in an inductively coupled plasma, then released to drift for some seconds in a dc discharge with constant current. After that the discharge polarity was reversed. DC plasma containing a drifting microparticle cloud was found to be strongly non-uniform in terms of microparticle drift velocity and plasma emission in accord with [Zobnin et.al., Phys. Plasmas 25, 033702 (2018)]. In addition to that, non-uniformity in the self-excited wave pattern was observed: In the front edge of the microparticle cloud (defined as head), the waves had larger phase velocity than in the rear edge (defined as tail). Also, after the polarity reversal, the wave pattern exhibited several bifurcations: Between each of the two old wave crests, a new wave crest has formed. These bifurcations, however, occurred only in the head of the microparticle cloud. We show that spatial variations of electric field inside the drifting cloud play an important role in the formation of the wave pattern. Comparison of the theoretical estimations and measurements demonstrate the significant impact of the electric field on the phase velocity of the wave. The same theoretical approach applied to the instability growth rate, showed agreement between estimated and measured values., 7 pages, 4 figures

Three-dimensional plasma crystals are often described as Yukawa systems for which a phase transition between the crystal structures fcc and bcc has been predicted. However, experimental investigations of this transition are missing. We use a fast scanning video camera to record the crystallization process of 70 000 microparticles and investigate the existence of the fcc-bcc phase transition at neutral gas pressures of 30, 40, and 50 Pa. To analyze the crystal, robust phase diagrams with the help of a machine learning algorithm are calculated. This work shows that the phase transition can be investigated experimentally and makes a comparison with numerical results of Yukawa systems. The phase transition is analyzed in dependence on the screening parameter and structural order. We suggest that the transition is an effect of gravitational compression of the plasma crystal. Experimental investigations of the fcc-bcc phase transition will provide an opportunity to estimate the coupling strength $\mathrm{\ensuremath{\Gamma}}$ by comparison with numerical results of Yukawa systems.

The velocity distribution function of dust particles immersed in a plasma was investigated under microgravity conditions. A three-dimensional (3D) cloud of polymer microspheres was suspended in a neon plasma, in the PK-4 instrument onboard the International Space Station (ISS). These dust particles were tracked using video microscopy in a cross section of the 3D dust cloud. The velocity distribution function (VDF) is found to have a non-Maxwellian shape with high-energy tails; it is fit well by a combination of low-energy Maxwellian core and a high-energy non-Gaussian Kappa-distribution halo. Similar non-Maxwellian VDFs are typically observed in space plasmas.

In this study, we investigated the bacterial biofilm reduction by combined treatment of atmospheric pressure plasma (APP) and chelating agents. Many of hospital acquired infections (HAI) are related to biofilm infections. APP and chelating agents were reported to be effective in bacterial biofilm inactivation and eradication. It is believed that chelating agents disrupt the biofilm formation through bonding metal ions, while APP inactivates bacteria mainly through reactive species [1–3]. In our study, we used a surface micro-discharge (SMD) driven by sinusoidal power input of 2 kHz and peak to peak voltage of 9 kV to treat Escherichia coli (E. coli), Enterococcus faecalis (E. faecalis), and Staphylococcus capitis (S. capitis) biofilm on 316 L stainless steel and hopkins rod lens glass plates in combination with chelating agents of trisodium citrate (TSC), ethylenediaminetetraacetic acid (EDTA), egtazic acid (EGTA), and alizarin. Bacterial biofilm reduction was measured by means of colony count assay and BacTiter-Glo cell viability assay. The results of colony count assay showed that combined treatment of EDTA and TSC with plasma has synergistic effects on all three bacterial biofilms, while EGTA only on E. coli and none for alizarin. Experiments of BacTiter-Glo cell viability assay indicate that EDTA, EGTA, and TSC has synergistic effects on all three biofilms while also none for alizarin.

Machine learning is one of the most popular fields in computer science and has a vast number of applications. In this work we will propose a method that will use a neural network to locally identify crystal structures in a mixed phase Yukawa system consisting of fcc, hcp, and bcc clusters and disordered particles similar to plasma crystals. We compare our approach to already used methods and show that the quality of identification increases significantly. The technique works very well for highly disturbed lattices and shows a flexible and robust way to classify crystalline structures that can be used by only providing particle positions. This leads to insights into highly disturbed crystalline structures.

In this work, existing methods for plasma crystal analysis are investigated using artificial and simulated calibration data, which reproduce a multiphase system consisting of fcc, hcp, and bcc crystal data. Disturbances of the artificial data including Gaussian noise, stretching, and randomly missing particles are used to investigate the methods thoroughly. A popular method, called bond order parameter, has been repeatedly criticized as a structure analysis tool and will be improved with the help of a recent development. The method is called the bcc-sensitive Minkowski structure metric. It enhances robustness and consistency, while remaining compatible with previous bond-order-based results. Also, a promising method rooted in the molecular dynamics community is tested, yielding detailed insight of bond-order-specific drawbacks. With this investigation, the state of three-dimensional plasma crystal analysis will be significantly improved.

New complex-plasma facility, Plasmakristall-4 (PK-4), has been recently commissioned on board the International Space Station. In complex plasmas, the subsystem of μm-sized microparticles immersed in low-pressure weakly ionized gas-discharge plasmas becomes strongly coupled due to the high (103–104 e) electric charge on the microparticle surface. The microparticle subsystem of complex plasmas is available for the observation at the kinetic level, which makes complex plasmas appropriate for particle-resolved modeling of classical condensed matter phenomena. The main purpose of PK-4 is the investigation of flowing complex plasmas. To generate plasma, PK-4 makes use of a classical dc discharge in a glass tube, whose polarity can be switched with the frequency of the order of 100 Hz. This frequency is high enough not to be felt by the relatively heavy microparticles. The duty cycle of the polarity switching can be also varied allowing to vary the drift velocity of the microparticles and (when necessary) to trap them. The facility is equipped with two videocameras and illumination laser for the microparticle imaging, kaleidoscopic plasma glow observation system and minispectrometer for plasma diagnostics and various microparticle manipulation devices (e.g., powerful manipulation laser). Scientific experiments are programmed in the form of scripts written with the help of specially developed C scripting language libraries. PK-4 is mainly operated from the ground (control center CADMOS in Toulouse, France) with the support of the space station crew. Data recorded during the experiments are later on delivered to the ground on the removable hard disk drives and distributed to participating scientists for the detailed analysis.

Template matching algorithms represent a viable tool to locate particles in optical images. A crucial factor of the performance of these methods is the choice of the similarity measure. Recently, it was shown in [Gao and Helgeson, Opt. Express 22 (2014)] that the correlation coefficient (CC) leads to good results. Here, we introduce the mutual information (MI) as a nonlinear similarity measure and compare the performance of the MI and the CC for different noise scenarios. It turns out that the mutual information leads to superior results in the case of signal dependent noise. We propose a novel approach to estimate the velocity of particles which is applicable in imaging scenarios where the particles appear elongated due to their movement. By designing a bank of anisotropic templates supposed to fit the elongation of the particles we are able to reliably estimate their velocity and direction of motion out of a single image.

Complex plasmas are low-temperature plasmas containing micron-sized particles (microparticles) such as dust grains. These are present in astrophysical systems (comets, molecular clouds, et al.) and in technological applications (microchip production by plasma etching, deposition of solar cells, et al.). Complex plasmas are also of interest in basic science because these are often used as models for many other strongly coupled many-body systems in solid state, fluid, or plasma physics. Since gravity has a strong influence on the microparticle component, experiments under microgravity (parabolic flights, sounding rockets, International Space Station (ISS)) are performed. Interaction between microparticles depends on plasma parameters such as ion density or ion temperature. Also, the presence of microparticles may change the properties of background plasma. Therefore, the background plasma needs to be characterized to provide adequate interpretation of the microgravity experiments. For this purpose a dedicated high-speed diagnostic system has been set up.

The addition of angular momenta can be reduced to elementary coupling processes of spin-$\frac{1}{2}$-particles. In this way, a method is developed which allows for a non-recursive, simultaneous computation of all Clebsch-Gordan coefficients concerning the addition of two angular momenta. The relevant equations can be interpreted easily, analogously to simple probabilistic considerations. They provide an improved understanding of the addition of angular momenta as well as a practicable evaluation of Clebsch-Gordan coefficients in an easier way than within the well-known methods., 13 pages, REVTEX

Experiments with complex plasmas in a dc discharge using the PK-4 facility will be discussed, focusing on the formation of strings corresponding to an electrorheological fluid observed in recent parabolic flight experiments.

Conditions for formation of the double-layer structure at the interface of a complex plasma/electron-ion plasma due to the presence of strongly coupled microparticles is theoretically investigated in a weakly collisional discharge. The result is of consequence for all problems involving the coexistence of different plasma regions, the sharp plasma boundaries, void formation, etc.

The influence of a dust cloud on the structure of the positive column of a direct current gas discharge in a cylindrical glass tube under milligravity conditions has been studied both experimentally and numerically. The discharge was produced in neon at 60 Pa in a glass tube with a diameter of 30 mm at a discharge current 1 mA. Spherical monodisperse melamine formaldehyde dust particles with a diameter of 6.86 μm were injected into the positive column and formed there a uniform dust cloud with a maximum diameter of 14.4 mm. The shape of the cloud and the dust particle number density were measured. The cloud was stationary in the radial direction and slowly drifted in the axial direction. It was found that in the presence of the dust cloud, the intensity of the neon spectral line with a wavelength by 585.25 nm emitted by the discharge plasma increased by 2.3 times and 2 striations appeared on the anode side of the cloud. A numerical simulation of the discharge was performed using the 2D (quasi-3D) nonlocal...

Ultrarelativistic electron-positron plasmas can be produced in high-intensity laser fields and play a role in various astrophysical situations. Their properties can be calculated using QED at finite temperature. Here perturbative QED at finite temperature is used for calculating various important properties, such as the equation of state, dispersion relations of collective plasma modes of photons and electrons, Debye screening, damping rates, mean free paths, collision times, transport coefficients, and particle production rates, of ultrarelativistic electron-positron plasmas. In particular, the focus is on electron-positron plasmas produced with ultrastrong lasers.