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.