Jan Schablinski

On the influence of wakefields on three-dimensional particle arrangements

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.

Laser heating of finite two-dimensional dust clusters: A. Experiments

Laser manipulation allows to control the kinetic particle temperature in dusty plasmas. Different methods of laser heating for plasma crystals are benchmarked experimentally. The methods are analyzed with respect to homogeneity and isotropy in a spatial, temporal, and statistical sense. It is shown that it is possible to achieve particle dynamics very close to thermal equilibrium and that laser heating methods allow for a detailed study of phase transitions in finite size systems.

Charging and coupling of a vertically aligned particle pair in the plasma sheath

The phenomenon of particle chain formation is studied in a two-particle system. A wake of positive ions leads to an alignment of the negatively charged particles parallel to the ion flow. The dynamic response of this dust system to a small external perturbation is evaluated. It is shown that the eigenfrequency of the downstream particle is reduced compared to an isolated particle. This effect can be identified as a decharging of the particle by the focused ion flow in the wake of the upstream particle. Furthermore, a strong asymmetry of the particle interaction parallel to the ion flow is found. This asymmetry may not be attributed entirely to the interaction forces mediated by the ion wake.

Network analysis of three-dimensional complex plasma clusters in a rotating electric field

Network analysis was used to study the structure and time evolution of driven three-dimensional complex plasma clusters. The clusters were created by suspending micron-size particles in a glass box placed on top of the rf electrode in a capacitively coupled discharge. The particles were highly charged and manipulated by an external electric field that had a constant magnitude and uniformly rotated in the horizontal plane. Depending on the frequency of the applied electric field, the clusters rotated in the direction of the electric field or remained stationary. The positions of all particles were measured using stereoscopic digital in-line holography. The network analysis revealed the interplay between two competing symmetries in the cluster. The rotating cluster was shown to be more cylindrical than the nonrotating cluster. The emergence of vertical strings of particles was also confirmed.

Laser heating of finite two-dimensional dust clusters: B. Simulations

Laser heating of monolayer dusty plasmas is investigated theoretically by Langevin dynamics simulations. The laser radiation pressure is used to externally control the dust temperature without changing the plasma properties. We show that the laser scanning pattern has a major influence on both the velocity distribution function and the stationary structure of the cluster. Furthermore, the heating effect is found to be enhanced when the laser spots move with slightly higher frequencies than the trap frequency. The simulations confirm that a proper thermodynamic excitation of the dust particles is possible.

Controlling strongly correlated dust clusters with lasers

Lasers have been used extensively to manipulate matter in a controlled way ? from single atoms and molecules up to macroscopic materials. They are particularly valuable for the analysis and control of mesoscopic systems such as few-particle clusters. Here we report on recent work on finite size complex (dusty) plasma systems. These are unusual types of clusters with a very strong inter-particle interaction so that, at room temperature, they are practically in their ground state. Lasers are employed as a tool to achieve excited states and phase transitions.The most attractive feature of dusty plasmas is that they allow for a precise diagnostic with single-particle resolution. From such measurements, the structural properties of finite two-dimensional (2D) clusters and three-dimensional (3D) spherical crystals in nearly harmonic traps?so-called Yukawa balls?have been explored in great detail. Their structural features?the shell compositions and the order within the shells?have been investigated and good agreement to theoretical predictions was found. Open questions on the agenda are the excitation behaviour, the structural changes and phase transitions that occur at elevated temperature.Here we report on recent experimental results where laser heating methods were further improved and applied to finite 2D and 3D dust clusters. Comparing to simulations, we demonstrate that laser heating indeed allows to increase the temperature in a controlled manner. For the analysis of thermodynamic properties and phase transitions in these finite systems, we present theoretical and experimental results on the basis of the instantaneous normal modes, pair distribution function and the recently introduced centre-two-particle correlation function.

Effect of rotating electric field on 3D complex (dusty) plasma

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.

An optical tweezer for complex plasmas

This paper describes the experimental realization of an optical trap for microparticles levitating in the plasma sheath. Single particles can be trapped in a laser beam comparable to optical tweezers known from colloidal suspensions. The trapping mechanism is discussed and two applications of the system are shown.

Selective mode excitation in finite size plasma crystals by diffusely reflected laser light

The possibility to use diffuse reflections of a laser beam to exert a force on levitating dust particles is studied experimentally. Measurements and theoretical predictions are found to be in good agreement. Further, the method is applied to test the selective excitation of breathing-like modes in finite dust clusters.

Sheared and unsheared rotation of driven dust clusters

Finite size plasma crystals confined in an anisotropic potential well were studied under a rotating and radially unsheared drive in experiment and simulation at moderate rotational frequencies. A radially sheared rotation of these strongly coupled systems is observed for most cluster configurations with a low symmetry. The results show that a differential rotation can be effected by a non-sheared driving force.