S. A. Khrapak

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.

Hybrid approach to the ion drag force

A detailed calculation of the ion drag force acting on a single grain in a collisionless Maxwellian plasma with an arbitrary velocity of the ion flow is carried out. The traditional binary collision approach to the problem is combined with the linear kinetic formalism. It is shown that for a pointlike particle the binary collision approach yields correct results provided that the effective plasma screening length is chosen appropriately. The correct choice follows from the self-consistent kinetic theory. On the other hand, the binary collision approach accounts consistently for the effects of finite grain size and grain charging. Taking these effects into account an expression for the ion drag force is obtained. Calculations are performed for a typical (exemplary) set of complex plasma parameters. The relevance for recent complex plasma experiments is briefly discussed.

Compressional waves in complex (dusty) plasmas under microgravity conditions

Complex plasmas consist of electrons, ions and charged microparticles, with typical charge-to-mass ratios 1:10−5:10−13. The interest in these systems has grown explosively, because they can be investigated at the kinetic level (the microparticles). However, on Earth the supporting forces (against gravity) are of the same order as the electrostatic interparticle forces—and hence only strongly compressed systems can be investigated. Under microgravity conditions these “body forces” are a factor 102 smaller which allows the experimental investigation of weakly compressed three-dimensional complex plasmas. One way to study these systems is by the controlled excitation of low-frequency compressional waves. The first such experiments, conducted with the PKE-Nefedov laboratory on the International Space Station is reported. The waves were excited by modulating the voltage on the rf electrodes. By varying the modulation frequency the dispersion relation was measured. The results are compared with existing theoret...

Charging properties of a dust grain in collisional plasmas

Charging related properties of a small spherical grain immersed in a collisional plasma are investigated. Asymptotic expressions for charging fluxes, grain surface potential, long range electrostatic potential, and the properties of grain charge fluctuations due to the discrete nature of the charging process are obtained. These analytical results are in reasonable agreement with the available results of numerical modeling.

Scattering in the attractive Yukawa potential: application to the ion-drag force in complex plasmas

Scattering in the attractive screened Coulomb (Yukawa) potential is investigated. The momentum-transfer cross section is numerically calculated and analytical approximations are presented. The results are applied to estimate the ion-drag force acting on an isolated micron-sized grain in low-pressure bulk plasmas.

Freezing and melting of 3D complex plasma structures under microgravity conditions driven by neutral gas pressure manipulation.

Freezing and melting of large three-dimensional complex plasmas under microgravity conditions is investigated. The neutral gas pressure is used as a control parameter to trigger the phase changes: Complex plasma freezes (melts) by decreasing (increasing) the pressure. The evolution of complex plasma structural properties upon pressure variation is studied. Theoretical estimates allow us to identify the main factors responsible for the observed behavior.

Structural properties of dense hard sphere packings

We numerically study structural properties of mechanically stable packings of hard spheres (HS), in a wide range of packing fractions 0.53 ≤ ϕ ≤ 0.72. Detailed structural information is obtained from the analysis of orientational order parameters, which clearly reveals a disorder–order phase transition at the random close packing (RCP) density, ϕc ≃ 0.64. Above ϕc, the crystalline nuclei form 3D-like clusters, which upon further desification transform into alternating planar-like layers. We also find that particles with icosahedral symmetry survive only in a narrow density range in the vicinity of the RCP transition.

An interpolation formula for the ion flux to a small particle in collisional plasmas

Ion collisionality is known to be a major factor which determines the magnitude of the surface (floating) potential of an individual particle immersed in a plasma. In this paper a simple interpolation formula for the ion flux collected by such a particle in the entire range of ion collisionality is proposed. The dependency of the floating potential on ion collisionality calculated using this formula as well as using other analytic approximations developed recently are compared. The reliability of different approaches is discussed.

Model of grain charging in collisional plasmas accounting for collisionless layer

Grain charging in collision dominated plasmas is investigated. The transition from a thin collisionless region around the grain, i e a, to a thick one, i e a, is studied under the assumptions i e D and a D, where i e is the ion electron mean free path, a is the grain radius, and D is the plasma screening length. It is also assumed that no ionization and recombination occur in the vicinity of the grain. With these assumptions, the analytical model of grain charging is constructed, the expressions for the ion and electron fluxes to the grain surface are derived, and the grain charge is obtained from their balance. The analytical results are then compared with the available experimental results. The behavior of ion and electron number densities in the vicinity of the grain is briefly discussed.

Drag force on an absorbing body in highly collisional plasmas

The force acting on a small absorbing body embedded in a highly collisional plasma with drifting ions is calculated using the linear response formalism. It is shown that the absorption introduces physical effects leading to a drastic reduction of the force. The importance of this result is discussed, mostly in the context of complex (dusty) plasma research, but it can be relevant to many other situations, ranging from astrophysics, thunderclouds, dust in fusion devices, colloidal suspensions, biological systems, etc.