The charging of a dust grain in supersonic plasma flows in the wake of another grain is studied by numerical simulations. While entering the Mach cone originating from the upstream grain, the grain is discharged by scattered ions. Electrostatic forces acting on the grain in the wake will move it to the stable position in the wake at a distance close to the electron Debye length from the upstream grain. The onset for discharging can be used to estimate the ion flow speed in the system. The simulations are carried out with the DiP3D code, a three-dimensional particle-in-cell code where both electrons and ions are represented as numerical particles [W. J. Miloch et al., Nonlinear Processes Geophys. 14, 575 (2007); New J. Phys. 11, 043005 (2009)].

The role of wakefields on the three-dimensional (3D) particle arrangement in finite dust clouds at low neutral gas pressures is investigated experimentally. Using a novel stereoscopic digital in-line holography approach for the instantaneous measurement of the 3D particle positions, the structural properties of finite dust clouds are studied. The competition of a nested shell structure (known from Yukawa balls) and vertical particle chains (known for particle clouds confined in the plasma sheath) results in a markedly different particle arrangement. With experiments studying the dynamical response in a two-particle system for identical plasma and confinement conditions, we show that the presence of an ion focus in the wake of the particle is responsible for the observed structural differences.

A digital in-line holography (DIH) approach for the instantaneous measurement of the three-dimensional positions of micrometer sized particles in a radio frequency gas discharge is presented. The applicability of this method to a strongly coupled dusty plasma is demonstrated. The performance of DIH is benchmarked and special attention is paid to its spatial and temporal resolution.

To study the structural and dynamical properties of finite 3D dust clouds (Yukawa balls) new diagnostic tools have been developed. This contribution describes the progress towards 3D diagnostics for measuring the particle positions. It is shown that these diagnostics are capable of investigating the structural and dynamical properties of Yukawa balls and gaining insight into their basic construction principles.

The effect of rotating electric field on 3D particle clusters suspended in rf plasma was studied experimentally. Spheroidal clusters were suspended inside a glass box mounted on the lower horizontal rf electrode, with gravity partially balanced by thermophoretic force. Clusters rotated in the horizontal plane, in response to rotating electric field that was created inside the box using conducting coating on its inner surfaces (“rotating wall” technique). Cluster rotation was always in the direction of applied field and had a shear in the vertical direction. The angular speed of rotation was 104–107 times lower than applied frequency. The experiment is compared to a recent theory.