Oliver Arp

Structural Properties of Screened Coulomb Balls

Small three-dimensional strongly coupled charged particles in a spherical confinement potential arrange themselves in a nested shell structure. By means of experiments, computer simulations, and theoretical analysis, the sensitivity of their structural properties to the type of interparticle forces is explored. While the normalized shell radii are found to be independent of shielding, the shell occupation numbers are sensitive to screening and are quantitatively explained by an isotropic Yukawa model.

Confinement of Coulomb balls

A model for the confinement of the recently discovered Coulomb balls is proposed. These spherical three-dimensional plasma crystals are trapped inside a rf discharge under gravity conditions and show an unusual structural order in complex plasmas. Measurements of the thermophoretic force acting on the trapped dust particles and simulations of the plasma properties of the discharge are presented. The proposed model of confinement considers thermophoretic, ion-drag, and electric field forces, and shows excellent agreement with the observations. The findings suggest that self-confinement does not significantly contribute to the structural properties of Coulomb balls.

Classical and quantum Coulomb crystals

Strong correlation effects in classical and quantum plasmas are discussed. In particular, Coulomb (Wigner) crystallization phenomena are reviewed focusing on one-component non-neutral plasmas in traps and on macroscopic two-component neutral plasmas. The conditions for crystal formation in terms of critical values of the coupling parameters and the distance fluctuations and the phase diagram of Coulomb crystals are discussed.

Langmuir probe diagnostics in the IMPF device and comparison with simulations and tracer particle experiments

The plasma parameters in the IMPF (International Microgravity Plasma Facility) device for studying complex plasmas are determined with a 2D-scanning Langmuir probe system, which is suitable for operation on the International Space Station. The probe characteristics in low density, low pressure radio frequency (rf) discharges are evaluated in the framework of the radial-motion theory with corrections for collisions. A critical comparison between experimental values for plasma potential and ion density with SIGLO simulations (Kinema Software) is made, which yields good overall agreement. In particular, the shaping of the plasma profile by different rf power in the plasma centre and in an outer ring is well described. Small amounts of dust particles are used as a novel method to mark the boundary, where ion-drag force and electric field force are balanced. Again close agreement with simulation is found and demonstrates the applicability of the tracer technique as a quantitative diagnostic means.

3D Coulomb balls: experiment and simulation

Spherically symmetric three-dimensional charged particle clusters are analyzed experimentally and theoretically. Based on accurate molecular dynamics simulations ground state configurations and energies with clusters for N ≤ 160 are presented which correct previous results of Hasse and Avilov [Phys. Rev. A 44, 4506 (1991)]. A complete table is given in the appendix. Further, the lowest metastable states are analyzed.

On the origin of dust vortices in complex plasmas under microgravity conditions

Under microgravity conditions, microparticles in a radio-frequency plasma form an extended dust cloud. In such clouds, self-excited large-scale vortices are observed. New experimental observations are reported, which exhibit a simple double vortex structure or a more complex quadrupole-like topology. Modeling the fields of the main acting forces, namely, the electric field force and the ion drag force, and calculating the curl of these forces reveal their non-conservative character and the possible driving mechanism of the vortices. It is shown that the curl of the ion drag force and of the electric field force has opposite sign and the combination could thus lead to the complex structures, also found in the observations.

Force measurements in dusty plasmas under microgravity by means of laser manipulation

Experiments in a dusty plasma under the microgravity conditions of parabolic flights are presented. Under microgravity, extended dust structures and a central dust-free region (“void”) are formed. Here, the forces and the force balance at the void boundary are studied by means of laser manipulation of the dust particles: A focused laser beam is moved in a controlled way to drive particles in the extended dust cloud and at the void boundary. From the observed particle motion, the forces on the particles in the dust cloud and at the void boundary are derived. Together with Langmuir probe measurements, a quantitative description of the force balance has been achieved. Special attention has been paid to the ion drag force, which is crucial in understanding the void formation. The results are compared to prevalent ion drag models.

Spatially extended void-free dusty plasmas in a laboratory radio-frequency discharge

Laboratory experiments with thermophoretic levitation of dust are described that aim at the closure of a central dust-free void region. A careful study of the void structure as a function of the discharge and levitation parameters leads to the discovery of an extended parameter region where stable void-free equilibria are found. The void closure is effected by a novel mechanism that involves a self-organized change in the discharge topology, in which the dust cloud becomes surrounded by a toroidal region of plasma production. In this geometry ions are found to stream radially inwards instead of outwards as in clouds with a central void. This change in ion flow is proved by a reversal of the propagation direction of dust-density waves.

Finite dust clusters in dusty plasmas

We review recent experiments on the formation of finite systems of charged microspheres in dusty plasmas. There, finite arrangements of these dust clusters can be studied in different geometries ranging from 1D to 3D. The structure and the mode dynamics in these systems will be discussed.

The Structure of Self-Excited Dust-Density Waves Under Microgravity

Dusty plasmas under microgravity conditions are a great opportunity to observe dynamical processes in strongly coupled systems. For example, in such systems, self-excited dust-density waves can occur at low gas pressures in extended regions of the discharge. Recently, we have performed a series of measurements in a parallel-plate RF reactor during parabolic flights. It reveals that the waves can appear in two completely different states. One of them yields a high spatial and temporal coherence of the density fluctuations. This feature allows us to utilize scanning video microscopy to obtain information on the structure of the 3-D wave field. Under different experimental conditions, we also found that a wave field with multiple different wavelengths can arise in the dust volume. This results in defects in the wave pattern due to merging wavefronts. We determine their temporal evolution, which can be derived accurately from the phase information.