Finite clusters with a small number of charged particles immersed in a complex-plasma environment have been investigated experimentally. Finite clusters in complex plasmas are shown to be a unique system for the excitation and observation of normal modes in bounded charged-particle systems. In systems of 3, 4, and 7 particles, normal-mode oscillations have been excited and from the frequencies of the different modes the key parameters, particle charge, and the screening strength have been derived. This method is proposed to be applied in future microgravity experiments.

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.

Self-excited dust-density waves are experimentally studied in a dusty plasma under microgravity. Two types of waves are observed: a mode inside the dust volume propagating in the direction of the ion flow and another mode propagating obliquely at the boundary between the dusty plasma and the space charge sheath. The dominance of oblique modes can be described in the frame of a fluid model. It is shown that the results fom the fluid model agree remarkably well with a kinetic electrostatic model of Rosenberg [J. Vac. Sci. Technol. A 14, 631 (1996)]. In the experiment, the instability is quenched by increasing the gas pressure or decreasing the dust density. The critical pressure and dust density are well described by the models.

The dynamical stability of finite two-dimensional clusters of charged micrometer sized particles is studied experimentally in a dusty plasma, where the particles experience a screened interaction. For clusters containing 19 and 20 particles the intershell rotation barrier height is measured quantitatively by applying a well defined torque on the clusters' outer shell by the radiation pressure of two opposing laser beams. Moreover, a complex behavior is found for the 19-particle cluster where structural transitions occur together with the intershell rotation showing that the presence of shielding can basically change the least stable modes in finite clusters. The measured crystal properties have been compared with Monte Carlo simulations. Quantitative agreement is found when nonideal effects are taken into account. PRB 61 KLINDWORTH, MELZER, PIEL, AND SCHWEIGERT

Dust-free regions around a Langmuir probe are studied in a complex plasma under microgravity. The dust particles settle in the presheath of the probe, where an equilibrium of the electric field force and the ion-drag force is established. The size and shape of the dust cloud are discussed with simple models. A more sophisticated presheath model is solved numerically to analyze the acting forces and the equilibrium position of the dust. The formation of distinct particle layers in the dust shell can be explained by the force gradients of the effective potential well.