V. N. Naumkin

Fluid-solid phase transitions in three-dimensional complex plasmas under microgravity conditions

Phase behavior of large three-dimensional (3D) complex plasma systems under microgravity conditions onboard the International Space Station is investigated. The neutral gas pressure is used as a control parameter to trigger phase changes. Detailed analysis of structural properties and evaluation of three different melting-freezing indicators reveal that complex plasmas can exhibit melting by increasing the gas pressure. Theoretical estimates of complex plasma parameters allow us to identify main factors responsible for the observed behavior. The location of phase states of the investigated systems on a relevant equilibrium phase diagram is estimated. Important differences between the melting process of 3D complex plasmas under microgravity conditions and that of flat 2D complex plasma crystals in ground based experiments are discussed.

Direct measurement of the speed of sound in a complex plasma under microgravity conditions

We present a direct measurement of the speed of sound in a three-dimensional complex plasma —a room-temperature plasma that contains micrometer-sized particles as fourth component. In order to obtain an undisturbed system, the setup was placed under microgravity conditions on board the International Space Station. The speed of sound was measured with the help of Mach cones excited by a supersonic probe particle moving through the extended particle cloud at Mach numbers M3. We use the Mach cone relation to infer the particle charge and compare with that predicted by standard theories. In addition, we compare our results with a numerical simulation. In both experiment and simulation, we observe a double Mach cone structure.

The corona discharge in nuclear excited plasma as a way of obtaining ordered dust particle structures

A model of a corona discharge in a nuclear excited dust plasma at the pressures of 1–100 atm is proposed. The distributions of the electric field and current-voltage characteristics of a corona discharge in a nuclear-excited dust plasma are found for a particular cylindrical geometry case at different methods of corona-producing electrode positioning. The conditions for the existence of a stationary corona discharge are obtained. A mathematical model describing the behavior of dust particles in a nuclear plasma that allows taking into account the key physical processes occurring in a nuclear-excited dust plasma is considered. The analyzed plasma properties are as follows: (1) shielding of the Coulomb forces of the interaction between dust particles, (2) the energy exchange and the stochastic character of the dust particle interaction with a buffer gas and ambient plasma, and (3) strong spatial inhomogeneity of the nuclear-excited plasma. The use of a corona discharge in a nuclear-excited plasma will make it possible to ensure stability of plasma-dust structures and more efficient conversion of the nuclear energy into laser radiation.

Measurement of the speed of sound by observation of the Mach cones in a complex plasma under microgravity conditions

We report the first observation of the Mach cones excited by a larger microparticle (projectile) moving through a cloud of smaller microparticles (dust) in a complex plasma with neon as a buffer gas under microgravity conditions. A collective motion of the dust particles occurs as propagation of the contact discontinuity. The corresponding speed of sound was measured by a special method of the Mach cone visualization. The measurement results are incompatible with the theory of ion acoustic waves. The estimate for the pressure in a strongly coupled Coulomb system and a scaling law for the complex plasma make it possible to derive an evaluation for the speed of sound, which is in a reasonable agreement with the experiments in complex plasmas.

Density distribution of a dust cloud in three-dimensional complex plasmas

We propose a method of determination of the dust particle spatial distribution in dust clouds that form in three-dimensional (3D) complex plasmas under microgravity conditions. The method utilizes the data obtained during the 3D scanning of a cloud, and it provides reasonably good accuracy. Based on this method, we investigate the particle density in a dust cloud realized in gas discharge plasma in the PK-3 Plus setup onboard the International Space Station. We find that the treated dust clouds are both anisotropic and inhomogeneous. One can isolate two regimes in which a stationary dust cloud can be observed. At low pressures, the particle density decreases monotonically with the increase of the distance from the discharge center; at higher pressures, the density distribution has a shallow minimum. Regardless of the regime, we detect a cusp of the distribution at the void boundary and a slowly varying density at larger distances (in the foot region). A theoretical interpretation of the obtained results is developed that leads to reasonable estimates of the densities for both the cusp and the foot. The modified ionization equation of state, which allows for violation of the local quasineutrality in the cusp region, predicts the spatial distributions of ion and electron densities to be measured in future experiments.

EkoPlasma - Experiments with Grid Electrodes in Microgravity

First results of complex plasma experiments in microgravity with grid electrodes in the Ekoplasma facility are presented. These electrodes separate the region of plasma production from the working volume where the microparticles are injected. Grid electrodes allow a control of plasma parameters, and provide a nearly force-free environment for the microparticles. The experiments show that the grid electrode method can be applied to complex plasmas and that this technology yields a promising new experimental method for future research topics.

Dust coupling parameter of radio-frequency-discharge complex plasma under microgravity conditions

Oscillation of particles in a dust crystal formed in a low-pressure radio-frequency gas discharge under microgravity conditions is studied. Analysis of experimental data obtained in our previous study shows that the oscillations are highly isotropic and nearly homogeneous in the bulk of a dust crystal; oscillations of the neighboring particles are significantly correlated. We demonstrate that the standard deviation of the particle radius vector along with the local particle number density fully define the coupling parameter of the particle subsystem. The latter proves to be of the order of 100, which is two orders of magnitude lower than the coupling parameter estimated for the Brownian diffusion of particles with the gas temperature. This means significant kinetic overheating of particles under stationary conditions. A theoretical interpretation of the large amplitude of oscillation implies the increase of particle charge fluctuations in the dust crystal. The theoretical estimates are based on the ionization equation of state for the complex plasma and the equation for the plasma perturbation evolution. They are shown to match the results of experimental data processing. Estimated order of magnitude of the coupling parameter accounts for the existence of the solid-liquid phase transition observed for similar systems in experiments.

Study of the projectile motion in a dust crystal under microgravity conditions

Using the PK-3 Plus laboratory onboard the International Space Station motion of a large projectile in the bulk of a dust crystal formed by negatively charged small particles was investigated. It is demonstrated that a subsonic projectile moves almost freely inside the dust crystal. A hydrodynamic theory of projectile nonviscous motion confirms the experimental data.

Freezing and melting of 3D complex plasma structures driven by neutral gas pressure manipulation in PK‐3 Plus experiment

We discuss experimental results from the PK‐3 Plus facility on solid‐fluid phase transitions in complex plasmas.

Corona discharge in a nuclear excited dusty plasma

A nuclear excited dusty plasma can be used effec� tively in systems of direct nuclear energy conversion, for example, into optical radiation energy. However, to compensate for gravity and to confine charged dust particles, a potential trap must be formed in a nuclear excited plasma. High pressures (1–100 atm) are needed to effectively use a nuclear excited plasma. Here, we investigate the possibility of using a corona electric discharge to provide the existence of dust structures before the action of hard radiation on a plasma–dust medium. The problem is considered in terms of continuum equations. We consider the case where a nuclear excited dusty plasma is in a cylinder of length L and radius R (L/R 1) with a wire of radius r 0 stretched through the center. The distributions of the number densities of electrons (ne), singly charged atomic (ni) and molecular (nmi) ions, excited atoms (n*), and metastable atoms (nam) and molecules (nMm) depend on the specific kinetics of elementary processes and the local electric field Er(r). These stationary distribu� tions can be found by simultaneously solving the par� ticle number balance equations,