Dynamics of a Spherically Confined Yukawa Plasma: Shell Formation and Collective Excitations

The dynamics of spherical dust crystals created in a gaseous plasma (so-called "Yukawa Balls") is investigated by means of first-principle simulations and an analytic "cold-fluid" theory. The analysis focuses on the dynamic formation of the characteristic shell structure and the collective excitations in the strongly coupled liquid state. It is based on a model of charged particles with screened Coulomb (Yukawa) interaction confined in an isotropic harmonic trap. The emergence of the shell structure in spherically confined plasmas is analyzed by time-resolved Langevin dynamics and thermodynamic Monte Carlo simulations. The former are used to cool a Yukawa Ball from a weakly coupled initial state to a strongly coupled final state. In this scenario, shell formation generally begins at the outer plasma edge, but the screening parameter strongly determines the time for the formation of the shells inside the cluster. Monte Carlo simulations are then used to study other trapping potentials. It is found that the process can be initiated inside the plasma if the confinement is modified such that the particles cannot enter the inner region of the trap. The second main topic concerns the collective modes of Yukawa Balls in the strongly coupled liquid phase. They are studied analytically by solving the linearized fluid equations for a cold plasma. In this "cold-fluid" theory, the mode spectrum of Yukawa Balls is fully characterized by three mode numbers and a single parameter describing the effective range of the interaction potential. Since the cold-fluid theory is based on the mean-field approximation and does not account for thermal effects, its results are compared with mode spectra from molecular dynamics (MD) simulations. The comparison shows that the cold-fluid theory accurately describes the low-order modes in systems with weak to moderate screening. Larger deviations, however, are observed for the high-order modes and in the strong screening regime. The MD simulations of strongly coupled Yukawa Balls further contain additional low-frequency excitations that are not predicted by the cold-fluid theory. Both the cold-fluid theory and the MD simulations yield additional insights into the properties of the "breathing mode" in a confined Yukawa plasma and thereby complement existing results in the literature

On the Wake Structure in Streaming Complex Plasmas

The theoretical description of complex (dusty) plasmas requires multiscale concepts that adequately incorporate the correlated interplay of streaming electrons and ions, neutrals, and dust grains. Knowing the effective dust-dust interaction, the multiscale problem can be effectively reduced to a one-component plasma model of the dust subsystem. The goal of the present publication is a systematic evaluation of the electrostatic potential distribution around a dust grain in the presence of a streaming plasma environment by means of two complementary approaches: (i) a high precision computation of the dynamically screened Coulomb potential from the dynamic dielectric function, and (ii) full 3D particle-in-cell simulations, which self-consistently include dynamical grain charging and non-linear effects. The applicability of these two approaches is addressed

Sadržaj

The self-diffusion phenomenon in a two-dimensional dusty plasma at extremely strong (effective) magnetic fields is studied experimentally and by means of molecular dynamics simulations. In the experiment the high magnetic field is introduced by rotating the particle cloud and observing the particle trajectories in a co-rotating frame, which allows reaching effective magnetic fields up to 3000 Tesla. The experimental results confirm the predictions of the simulations: (i) super-diffusive behavior is found at intermediate time-scales and (ii) the dependence of the self-diffusion coefficient on the magnetic field is well reproduced.Comment: accepted by Physical Review

Thermodynamic and transport coefficients from the dynamic structure factor of Yukawa liquids

The ion-ion dynamic structure factor (DSF) of warm dense matter and dense plasmas contains information on collective ionic modes and various thermodynamic and transport coefficients, which are important for modeling the interiors of giant planets or the dense plasmas occurring in inertial confinement fusion. Here, it is demonstrated, using the Yukawa liquid as a reduced model, that the complete hydrodynamic information encoded in the DSF can be extracted with an accuracy comparable to that of dedicated methods. This is achieved by applying a generalized hydrodynamic and a viscoelastic model and extrapolating the results at finite wave numbers into the hydrodynamic limit. Very good agreement with previous data is obtained for the sound speed and the viscosity. The thermal diffusivities deduced from different methods exhibit somewhat larger deviations

Dynamic structure factor of the magnetized one-component plasma: Crossover from weak to strong coupling

Plasmas in strong magnetic fields have been mainly studied in two distinct limiting cases—that of weak and strong nonideality with very different physical properties. While the former is well described by the familiar theory of Braginskii, the latter regime is closer to the behavior of a Coulomb liquid. Here we study in detail the transition between both regimes. We focus on the evolution of the dynamic structure factor of the magnetized one-component plasma from weak to strong coupling, which is studied with first-principle molecular dynamics simulations. The simulations show the vanishing of Bernstein modes and the emergence of higher harmonics of the upper hybrid mode across the magnetic field, a redistribution of spectral power between the two main collective modes under oblique angles, and a suppression of plasmon damping along the magnetic field. Comparison with results from various models, including the random phase approximation, a Mermin-type dielectric function, and the quasilocalized charge approximation show that none of the theories is capable of reproducing the crossover that occurs when the coupling parameter is on the order of unity. The findings are relevant to the scattering spectra, stopping power, and transport coefficients of correlated magnetized plasmas

Magnetic field effects and waves in complex plasmas

Magnetic fields can modify the physical properties of a complex plasma in various different ways. Weak magnetic fields in the mT range affect only the electrons while strong fields in the Tesla regime also magnetize the ions. In a rotating dusty plasma, the Coriolis force substitutes the Lorentz force and can be used to create an effective magnetization for the strongly coupled dust particles while leaving electrons and ions unaffected. Here, we present a summary of our recent experimental and theoretical work on magnetized complex plasmas. We discuss the dynamics of dust particles in magnetized discharges, the wave spectra of strongly coupled plasmas, and the excitations in confined plasmas