Multipole expansion in plasmas: Effective interaction potentials between compound particles

Non-equilibrium molecular dynamics study of heat transfer parameters in two-dimensional Yukawa systems under uniform magnetic field

The present study explores the effect of a magnetic field on the thermal conductivity of two-dimensional (2D) Yukawa systems in a wide range of system parameters using the non-equilibrium molecular dynamic method (NEMD). We consider an external magnetic field with Ω = ω c / ω p ≤ 1 (with Ω being the ratio of the cyclotron frequency to plasma frequency) and the coupling parameter values in the range 1 ≤ Γ ≤ 100 (with Γ being the ratio of the Coulomb interaction energy at mean inter-particle distance to the thermal energy of particles). The results show that an external uniform magnetic field results in the reduction of the thermal conductivity at the considered values of the coupling parameter Γ . Additionally, we found that the effect of the magnetic field on thermal conduction is weaker at larger values of the system coupling parameter. To ensure that calculated results for the thermal conductivity are accurate and reliable, we performed a detailed investigation of the convergence of the results with respect to simulation parameters in NEMD with a strong external magnetic field. We believe that the presented results will serve as useful benchmark data for the theoretical models of (2D) Yukawa systems.

Shear viscosity in two-dimensional dipole systems

The results of modeling shear flows in classical two-dimensional (2D) dipole systems are presented. We used the method of nonequilibrium molecular dynamics to calculate the viscosity at various shear rates. The coefficients of shear viscosity are given in the limit of low shear rates for various regimes of interparticle correlation from a weakly correlated gaseous state to a strongly nonideal liquid state near the crystallization point. The calculations were carried out for bare (unscreened) dipole systems, as well as for dipole systems in a polarizable medium that provide screening of the dipole-dipole interaction. The effect of shear thinning in 2D dipole systems is reported for low values of the coupling parameter. In addition, it is shown that dipole systems can become both less and more viscous due to the presence of a screening medium, depending on the degree of interparticle correlation. The optimal simulation parameters are discussed within the framework of the method of nonequilibrium molecular dynamics for determining the shear viscosity of two-dimensional dipole systems. Moreover, we present a simple fitting curve which provides a universal scaling law for both bare dipole-dipole interaction and screened dipole-dipole interaction.

Melting, freezing, and dynamics of two-dimensional dipole systems in screening bulk media

This paper reports on the molecular dynamics simulations of classical two-dimensional (2D) electric dipole systems. The properties of 2D systems with bare (nonscreened) and screened dipole-dipole interactions have been investigated. Based on the polygon construction method, we present simulation results on the phase transition, and we locate the melting and freezing points of 2D dipole systems in terms of a polygon disorder parameter, with the polygon disorder parameter being the sum of nontriangular polygon order parameters. It was found that the phase transition of the system occurs when the polygon disorder parameter has a value 0.165. This result was cross-checked by using both local and overall orientational order parameters. We also identified that the value of the average local orientational order parameter at the phase transition point is 0.67. These results are valid for the ordinary (bare) dipole-dipole interaction as well as the screened dipole-dipole interaction, and they are expected to be general for other 2D systems with repulsive pair interaction. We observed that both melting and freezing points shift to lower values of temperature due to screening. In the liquid state, the radial distribution function and polygon construction method show the loss of order in a structure as screening becomes more severe. Furthermore, the impact of screening on the system's collective excitation spectra and diffusive characteristics at liquid and solid states has been studied. Results show the decrease in the values of both longitudinal and transverse sound speeds and the emergence of anomalous superdiffusive motion in the liquid state due to screening.

Plasma-grain interaction mediated by streaming non-Maxwellian ions

A comprehensive parametric study of plasma-grain interaction for non-Maxwellian streaming ions in steady-state employing particle-in-cell simulations is delineated. Instead of considering the intergrain interaction potential to be the linear sum of isolated grain potentials, we incorporate the numerical advancement developed fully for grain shielding by including nonlinear contributions from the plasma and shadowing effect. The forces acting on grains versus intergrain distance, streaming velocity of the ions, and impact of trapped ions density (number) are characterized for non-Maxwellian ions in the presence of charge-exchange collisions. It is found that the nonlinear plasma response considerably modifies the plasma-grain interaction. Unlike the stationary plasma case, for two identical grains separated by a distance in the presence of streaming ions, the electrostatic force is neither repulsive for all grain separations nor equivalent to the force due to one isolated grain. Inadequacy of the linear response formalism in dealing with the systems having very large grain charges is also discussed. The smallest intergrain separation for which the role of the shadow effect can be ignored is reported.