S. Käding

Three-dimensional stereoscopy of Yukawa (Coulomb) balls in dusty plasmas

A stereoscopic approach with standard video cameras for positioning and tracking of micrometer sized polymer particles in a radio-frequency gas discharge is presented. The stereoscopy is applied to simultaneously determine the positions of all particles in a three-dimensional strongly coupled spherical dusty plasma (Yukawa ball). The accuracy of the stereoscopic method is discussed. The shell structure and the occupation number of various Yukawa balls are determined and compared to recent simulations and models.

Experiments on metastable states of three-dimensional trapped particle clusters

Small three-dimensional charged-dust clusters, so-called Yukawa balls, are analyzed with regard to their construction principle. For that purpose, in an experimental approach, different (metastable) configurations of clusters with fixed particle number (N<100) have been generated under identical plasma and trapping conditions. Metastable states are frequently observed. In combination with molecular dynamics simulations, it is shown that particle interaction with screening strongly affects the appearance probabilities of metastable configurations. Small clusters show different average density distributions with screened interaction compared to pure Coulomb, although having the same ground state configurations.

Probability of metastable configurations in spherical three-dimensional Yukawa crystals.

Recently the occurrence probabilities of ground and metastable states of three-dimensional Yukawa clusters with 27 and 31 particles have been analyzed in dusty plasma experiments [D. Block, Phys. Plasmas 15, 040701 (2008)]. There it was found that, in many cases, the ground state appeared substantially less frequently than excited states. Here we analyze this question theoretically by means of molecular dynamics (MD) and Monte Carlo simulations and an analytical method based on the canonical partition function. We confirm that metastable states can occur with a significantly higher probability than the ground state. The results strongly depend on the screening parameter of the Yukawa interaction and the damping coefficient used in the MD simulations. The analytical method allows one to gain insight into the mechanisms being responsible for the occurrence probabilities of metastable states in strongly correlated finite systems.

Shell transitions between metastable states of Yukawa balls

Spherical dust clusters composed of several concentric shells are experimentally investigated with particular interest on transitions between different configurations and transitions of particles between different shells. Transitions between different ground and metastable configurations are frequently observed. The experimental analysis allows us to derive the energy differences of different configurations from particles traveling between shells. The observed transitions and transition probabilities are compared to molecular dynamics simulations.

Complex plasmas: forces and dynamical behaviour

Some peculiar features of complex (dusty) plasmas are reviewed and discussed. In the plasma sheath, the supersonic ion flow causes charge accumulation in the ion wake, which leads to attractive forces between like-charged dust particles. Ion drag is an important force under microgravity conditions, where it produces dust-free regions (voids). It is shown that the same mechanism determines dust confinement in anodic plasmas and leads to torus-shaped dust clouds in magnetized plasmas. Spherical plasma crystals are an interesting object of study which have led to an understanding of the influence of shielding on the structure of trapped particle clouds.

Stereoscopic Investigations of 3D Coulomb Balls

In dusty plamas particles are arranged due to the influence of external forces and the Coulomb interaction. Rescently Arp et al. were able to generate 3D spherical dust clouds, so‐called Coulomb balls. Here, we present measurements that reveal the full 3D particle trajectories from stereoscopic imaging.

Strongly coupled dusty plasmas: finite and extended systems

Systems of confined microspheres in a gaseous plasma environment are studied. These systems of trapped “dust” particles can form extended structures where the size of the system is much larger than interparticle distance. With specially tailored confinement potentials also finite arrangements of microspheres can be formed. In these systems the structure and dynamics dramatically depend on the exact particle number. Here, three-dimensional finite systems of microspheres, so-called Yukawa balls, are investigated and their structural properties are discussed with respect to the realized configurations of ground and metastable states.

Structure And Dynamics Of Finite Dust Clouds

Two novel three‐dimensional (3D) diagnostics, stereoscopic imaging and digital holography, enable us to provide a critical comparison of experimental results with simulations and theory and thus to gain a detailed insight into the structural and dynamical properties of strongly coupled dust clouds. Special attention is paid to the influence of screening and the role of metastable states in dust clouds containing just a very few particles.