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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 main results of a completed cycle of complex studies of a strongly coupled dusty plasma conducted in the unique, experimental Plasma Crystal-3 Plus space laboratory that operated on the International Space Station are discussed. The experiments on the physics of phenomena in the dusty plasma are analyzed. A new state of dusty plasma, the electrorheological plasma, in which there is a transition from an isotropic plasma-dusty liquid to an anisotropic state, is studied. The study of the interpenetration of particles streams of various diameters, a nonequilibrium transition, is described. Experimental studies of the liquid–crystal phase transition in a three-dimensional dusty plasma system in which it is established that the dust component is compressed and crystallized upon a decrease in the neutral gas pressure are discussed. The nearly free motion of large particles in the volume of a plasma crystal, a nonviscous, irrotational flow about a large particle by a “liquid” of small dust particles, is analyzed. An experimental study of the dynamics of the crystallization of three-dimensional plasma-dust systems at a constant argon pressure under the effect of a low-frequency, alternating electric field and without its influence under microgravity conditions is described. The formation of the crystallization front and its propagation in the three-dimensional plasma-dust system with a velocity on the order of the average interparticle distance per second were discovered.

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.

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.

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 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.

We study low frequency dust density waves excited in the microparticle streams of the Plasmakristall-4 chamber under microgravity conditions on board the International Space Station. Dispersion relations and instability conditions are analyzed for the dust density modes propagating along the axial electric field and in the reversed direction, revealing some important implications for the dust diagnostics. The results clearly point out to the Havnes parameter as a key factor for the excitation of the dust density modes.

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.

The effect of cloud density on the grain charging is of great importance in complex plasma physics. The quasi-neutrality condition brings about changing of ratio of a spatially averaged ion density to electron density in the dust cloud. Strong interaction between ions and highly charged grain complicates the analysis of the effect of ion density increasing on ion flux on the grain. The theoretical approach to ion flux correction on the grain space charge by the use of effective ion density instead of spatially averaged density is discussed. The experimental measurements of charges of solitary grains and the grains in the cloud under similar plasma parameters are used to prove the proposed approach.

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.

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.

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...

Complex (dusty) plasmas are composed of weakly ionized gas and charged micro-particles and represent the plasma state of soft matter. Due to the "heavy" component, microparticles, and the low density of the surrounding medium, the rarefied gas and plasma, it is necessary to perform experiments under microgravity conditions to cover a broad range of experimental parameters which are not available on ground. The investigations have been performed onboard the International Space Station with the help of the PK-3 Plus laboratory. This laboratory was mainly built to investigate the crystalline state of complex plasma, the so-called plasma crystal, its phase transitions and processes in multi-particle mixtures. Due to the manipulation of the interaction potential between the microparticles it is possible to initiate a phase transition from isotropic plasma into electrorheological plasma. The crystal–liquid phase transition was obtained in large three-dimensional isotropic dusty plasma system. First observations of a transition of the dusty plasma system state due to variations of the plasma component density are presented.

Influence of the elongated dust cloud on the intensities of different neon spectral lines in visible and near ir spectral ranges in the uniform positive column has been experimentally investigated using the Russian-European space apparatus "Plasma Kristall-4" (SA PK-4) on board of the International Space Station (ISS). The investigation was performed in the low pressure (0.5 mbar) direct current (dc, 1 mA) gas discharge in neon. Microgravity allowed us to perform experiments with a large dust cloud in the steady-state regime. To avoid the dust cloud drift in the dc electric field a switching dc polarity discharge mode has been applied. During the experiment a dust cloud of 9 mm in diameter in the discharge tube of 30 mm in diameter with the length of about 100 mm has been observed in the steady-state regime. In this regard, the intensities of neon spectral lines corresponding to 3p → 3s electronic transitions have increased by a factor of 1.4 times, while the intensities of neon spectral lines corresponding to 3d → 3p electronic transitions have increased by a factor of 1.6 times. The observed phenomenon is explained on the basis of the Schottky approach by a self-consistent rising dc electric field in the dusty plasma cloud resulting in an increase of the electron temperature.

PK-4 is an experiment designed to investigate complex plasmas in a combined dc/RF discharge under microgravity conditions on board of the International Space Station. The dc discharge is produced in a glass tube with a length of 35 cm and a diameter of 3 cm. In addition, an RF discharge can be applied by external RF coils. The setup is especially suited for studying the liquid phase of the complex plasmas, e.g., flow phenomena such as turbulence or nozzles, and forces acting on the microparticles. Experiments in the laboratory and in parabolic flights have been used to determine the charge of the microparticles as well as the ion drag force acting on them

PK-4 is an experiment designed to investigate complex plasmas (low-temperature plasmas containing microparticles, e.g. dust grains) in a combined dc/rf discharge under microgravity conditions on board of the International Space Station. Within the 35th and 36th ESA parabolic flight campaigns first experiments under microgravity conditions in a specially designed experiment set-up have been performed. The particle flow inside the tube, the appearance of dust waves, and lane formation in interpenetrating particle clouds have been observed.

The PK-4 experiment is a continuation of the successful dusty plasma experiments PK-1, PK-2 and PK-3 conducted on board of the orbital space stations Mir and International Space Station. For all these experiments it is important to avoid the strong influence of gravity, exerting an external stress on the system. Whereas PK-3 and PK-3 Plus experiments are using a planar rf capacitive discharge, PK-4 studies complex plasmas in a long cylindrical chamber with a combined dc/rf discharge. Such a configuration of the chamber will provide a particular advantage for investigation of different dynamical phenomena in complex plasmas such as sheared laminar flow of a highly nonideal dusty liquid and its transition to the turbulent regime, nozzle flow, boundary layers and instabilities, shock waves (solitons) formation and propagation, dust particle lane formation, and space dust grain separation according to their size.

Experiments with flow of microparticles were conducted in a DC discharge. A sharp threshold in the neutral gas pressure for the onset of an unstable low-frequency dust wave mode was observed. Highly space- and time-resolved measurements of the microparticle flow combined with probe measurements of the plasma parameters have allowed detailed comparison with a theoretical model. The model demonstrates good qualitative and quantitative agreement with the experimental results providing accurate estimates of the particle charge.

The PK-4 experiment is a continuation of the successful dusty plasma experiments PK-1, PK-2 and PK-3 conducted on board of the orbital space stations Mir and ISS (International Space Station). The aim of the PK-4 experiment is an investigation of physical processes in complex (dusty) plasmas under microgravity conditions in a combined dc/rf discharge plasma.

A spontaneous excitation of a grain density wave in a dusty cloud of monodisperse particles suspended at the diffuse edge of an rf inductive gas discharge has been discovered. The main physical parameters of this wave (phase velocity, wavelength, and growth rate) and of the background plasma (distributions of the electron density, electron temperature, and space potential) were measured. A theoretical model of the observable phenomenon based on the theory of dust acoustic waves in a collisional dusty plasma correlates well with the experimental data in a broad range of experimental conditions. The influence of a varying dust grain charge on the development of the observed dusty plasma instability has been analyzed. It is shown that the necessary condition for the instability excitation is the availability of a permanent electrical field (E0 ⩾3 V/cm) in the dusty cloud region.

Spontaneous excitation of a dust-particle density wave is observed in a dust cloud levitating in the region of the diffused edge of an rf inductive low-pressure gas-discharge plasma. The main physical parameters of this wave and of the background plasma are measured. The analytic model proposed for the observed phenomenon is based on the theory of dust sound and successfully correlates with experimental data in a wide range of experimental conditions. The effect of variable charge of dust particles on the evolution of the observed dust-plasma instability is studied analytically. It is shown that the necessary condition for the development of the dust-acoustic instability is the presence of a dc electric field in the dust cloud region.

Complex (dusty) plasmas are composed of weakly ionized gas and charged microparticles and represent the plasma state of soft matter. Due to the "heavy" component, microparticles, and the low density of the surrounding medium, the rarefied gas and plasma, it is necessary to perform experiments under microgravity conditions to cover a broad range of experimental parameters which are not available on ground. The investigations have been performed onboard the International Space Station (ISS) with the help of the PK-3 Plus laboratory. This laboratory was mainly built to investigate the crystalline state of complex plasma, the so-called plasma crystal, its phase transitions and processes in multi-particle mixtures. The crystal–liquid phase transition was obtained in large three-dimensional isotropic dusty plasma system. Observations of a transition of the dusty plasma system state due to the particle charge reduction and due to variations of the plasma component density are presented.

The absorption spectrum of gaseous 2,4,6-trinitrotoluene (TNT) was recorded by conventional absorption spectroscopy (AS) as well as cavity ringdown spectroscopy (CRDS) methods in the spectral regions 195–300 and 225–235 nm, respectively. These spectra were normalized by using the saturated TNT vapor-number density for the measured cell temperature to obtain the absorption cross section of TNT. No spectral features were found in the spectra; this result is consistent with a repulsive electronic excited state of TNT. The temperature dependence of the absorption coefficient of saturated TNT vapor was measured within the temperature range 5–110 °C. The limit of detection of TNT vapor by CRDS is less than 1 ppb. Real-time CRDS measurements of the TNT vapor density at 21 and 37 °C are presented. The TNT evaporation rates were found to be 7 × 108 and 4 × 1010 molecules/cm2 × s at 21 and 37 °C, respectively.

The results of an experimental study and numerical simulation of the relaxation of the composition of the flow of high-temperature combustion products in cylindrical channels with cooled and uncooled walls are presented. Primary attention is given to the effect of parameters such as the channel radius, the oxidizer excess coefficient in the initial mixture, and the wall temperature on the relaxation of carbon monoxide CO and on the level of its concentration at the channel outlet.

Results are presented from the experimental studies and numerical simulations of the behavior of dust grains in the plasma of an inductive RF discharge. The experiments were carried out with neon at a pressure of 25–500 Pa and with 1.87-µm melamine formaldehyde grains. The discharge was excited by a ring inductor supplied from a generator operating at a 100-MHz frequency. The effective dust-grain interaction potential used in numerical simulations involved the spatial dependence of the grain charge on the plasma floating potential, grain-interaction anisotropy resulting from the focusing of the drift ion current by the negatively charged grains, and specific features of the shielding of the dust grains by the plasma electrons and ions recombining both in the plasma bulk and on the grain surface. The results of Monte Carlo simulations show that the dust grains form specific filament structures observed experimentally in the plasma of an inductive electrodeless discharge.

The results of the experimental investigation of the behaviour of micron-sized dust particles in plasma of an inductive RF discharge are presented. The discharge was initiated in different inert gases at a frequency of 100 MHz. The formation of ordered filamentary structures from monodisperse polymer particles is demonstrated and the conditions of their existence are discussed.

The results of experimental research of behaviour of microparticles in plasma of inductive radio-frequency discharge are presented. The experiments were performed in inert gases with monodisperse and polydisperse particles. The discharge was initiated by radio-frequency power supply at 27 and 100 MHz. There are presented different kinds of dusty particle structures and conditions of their formation.

Results are reported from the experimental investigation and numerical simulation of the laminar boundary layer formed on the flat plate in flowing combustion products of propane in air. During the experiments such parameters as the temperature of the plate surface and the composition of the oncoming flow (due to a change of the fuel to air equivalence ratio φ) have been varied. In addition, when performing numerical simulations, the catalytic properties of the surface have been changed. The comparison of the measured profiles of velocity, temperature, OH and CO concentrations with the results of the calculation based on the chemically reacting boundary layer model has revealed that the model adequately described both distributions of parameters in the boundary layer and their dependence on the boundary conditions and parameters of the free stream flow.

The experimental investigations devoted to the Na-D spectral line broadening by molecular gases are reviewed. The theoretical interpretation of the data within the framework of the classical theory of atomic spectral line broadening is given. The effect of adiabatic and diabatic collisions on the shape of both impact and quasi-static spectral-line wings is discussed. The different forms of spectral-line shape representation and the connections between them are considered. The experimental cross-sections of the Lorentzian broadening of the Na-D lines by molecules are presented and compared with the theoretical ones. The problem of restoration of the adiabatic interaction potentials from quasi-static wing profiles is noted. The C6, C12 parameters of the effective atom-molecular potentials of the Lennard-Jones 12-6 form, restored on quasi-static wing profiles, are presented.

The new experimental data on the reduced absorption coefficient K(Δσ) in far wings of the Na-D doublet broadened by O2 and N2 molecules are reported. These profiles were measured in the blue wing of the Na-D2 line from about 2.5–1000 cm-1 and in the red wing of the Na-D1 line from about 2.5–2500 cm-1 with respect to the corresponding line center. The measurements were performed by the absorption–emission method in the uniform, premixed, laminar, shielded C2H2/O2 and C2H2/O2/N2 flames with sodium seed at T1=2105 K and T2=2135 K, respectively, and a pressure of 1 atm. The new wing profiles K j ( Δ σ) (j= O 2 , N 2 ) are compared with the K j ( Δ σ) (j= CO 2 , H 2 O ) measured by the authors earlier. It is shown, that the K O 2 ( Δ σ), K N 2 ( Δ σ), K CO 2 ( Δ σ) and K H 2 O ( Δ σ) profiles differ from each other by one or two orders of magnitude in the far quasi–static region (Δσ>100 cm-1), while this difference is less than the factor 2 in the impact region (Δσ j= N 2 , O 2 , CO 2 , H 2 O ) was proposed.

The reduced absorption coefficient K(Δσ) was measured in the red Na-D1 line wing from about 3 to 4000 cm−1 and in the blue Na-D2 line wing from about 3 to 1500 cm−1 with respect to the corresponding line center. These wings were measured in uniform, premixed, laminar, shielded, stoichiometric H 2 O 2 flames with H2O as diluent gas and sodium seed at a pressure of 1 atm for three temperatures—1770, 2180, and 2560 K—in order to determine the influence of temperature on K(Δσ). The absorption coefficient k(Δσ) was obtained from the emission spectrum of Na-D1,2 lines and corrected for the spectral overlap of Na-D1 and Na-D2 lines using an iterative method. The number density of sodium was measured by the full absorption method, the criteria of applicability of which to our conditions are discussed in detail. As it is found, the value of X of the measured and corrected for spectral overlap K(Δσ) ~ (Δσ)−X power dependence at the (mainly) impact range (Δσ ⩽ 20 cm−1) depends on the flame temperature and tends to the Lorentzian two as the temperature is increased. The measured absolute K(Δσ) values as well as their temperature dependence in the very far blue wing (Δσ ⩾ 1000 cm−1) of the Na-D2 line approximately correspond to those under a broadening by nitrogen molecules and indicate the repulsive effective Na(3S)-H2O interaction. On the contrary, the measured K(Δσ) values in the very far red wing of the Na-D1 line proved to be one or two orders of magnitude greater than under a broadening by nitrogen molecules. The temperature dependence of the absolute value of K(Δσ) at the Δσ = 4000 cm−1 indicates on the attractive effective Na(3S)-H2O interaction with the magnitude of 0.4 eV.

A new kind of dusty plasma instability was observed in the joint Russian-European "Plasma Kristall-4" space experiment on board of the International Space Station. An elongated cylindrical dust particle cloud of 0.9 cm diameter with a length of 20 cm was formed in the uniform positive column of a dc discharge operating in a polarity switching mode (dc/ps-mode). The discharge was operated in a glass tube of 3 cm inner diameter with a total length of 85 cm filled by argon at a pressure of 0.5 mbar. The dc/ps discharge was operated at 1 mA with a polarity switching frequency of 500 Hz. During the experiment, all the dust particles vibrated synchronized in the same phase in the direction perpendicular to the tube axis with a frequency of 24 Hz and peak-to-peak amplitude of 0.2 mm. The vibration was attended by discharge glow fluctuation. The nature of the cloud vibration is discussed.

Numerical and experimental investigations of the DC discharge in Neon in the tube with radius unevenly changed from 13.8 to 30 mm are presented. A formation of a dusty structure in standing stratum has been observed and simulated.

New results from the recent experiments using the “Plasma Kristall‐4”, “PK‐4”, onboard of the parabolic flight plane A‐300 Zero‐G are presented. These are: a) structural and dynamics properties of dusty plasma clouds containing elongated dust particles—microrods; b) formation of a boundary‐free dust cluster due to attractive forces caused by ion fluxes in a bulk plasma region; c) shock wave in dusty plasma driven by the electrical manipulative electrode in polarity switching direct current discharge. Physical models of the observed phenomena are presented and discussed. Universal possibilities of the “PK‐4” setup are demonstrated.

The photo-fragmentation cross-section of 2,4,6-trinitrotoluene (TNT) vapor at room temperature was determined at different ultraviolet wavelengths (254, 300, 340, and 400 nm) by measuring the concentration of NO molecule with cavity ring down spectroscopy and correcting for the photo-fragmentation cross-section of NO2. Nitric oxide (NO) molecules are produced by the TNT photo-fragmentation processes via an intermediate production of NO2. Our results reveal that the photo-fragmentation cross-section of TNT changes appreciably with change in wavelength with xenon arc lamp illumination, increasing with decreasing excitation wavelength. The maximum value of cross-section was observed at the shortest photo-fragmentation wavelength studied (254 nm), which is closest to the wavelength of an absorption peak of TNT near 220 nm.

Complex plasmas consist of micron-sized particles inside a low temperature plasma usually generated by a rf or dc discharge in a gas at a pressure around 1 mbar. The particles collect negative charges of several thousand electrons and interact with themselves, the plasma, and (confining) electric fields. By adjusting the plasma parameters complex plasmas can be put in nearly any state from crystalline ('plasma crystal') to super-critical. This can be visualized by cameras and allows to study the behavior of matter on the kinetic level of individual particles. To get rid of the disturbing influence of gravity complex plasma experiments were performed onboard the space station ISS and on parabolic flights. The PK-4 setup uses a dc discharge in a glass tube to create complex plasmas to study fluids on the particle level. Experiments on parabolic flights exhibit effects of self- organization in complex plasma flows, such as 'lane formation'. Particles in interpenetrating clouds form lanes to minimize the energy to pass through each other. These effects are also known from granular media and even moving crowds. The behavior of complex plasma flows in PK-4 is compared to these cases.

Spectral measurements of Doppler profiles of the hydrogen Hα spectral line in a glow direct current discharge in a mixture of 1% hydrogen in argon at pressures of 60, 120 and 180 Pa were performed. Solid and mesh cathodes were used. It is brought out that the line shapes are Gaussian with the width corresponding to the effective temperature of the excited atoms of 35-60 eV for emission from the cathode region. The line width does not change with the pressure and the distance from cathode in the region of negative glowing and increases slightly with the distance from the mesh cathode outside of the discharge gap.