Alexander Piel

Experimental determination of the charge on dust particles forming Coulomb lattices

Abstract Large dust grains ( r p =10 μ m) are found to form hexagonal Coulomb lattices with an interparticle distance of R p =880 μ m in the sheath of an argon rf discharge. The charge on the particle is measured from its resonant response to an external periodic force. The charge is found to be -200000 e .

Measurement of the electric charge on particulates forming Coulomb crystals in the sheath of a radiofrequency plasma

Monodisperse electrically charged particulates (9.4 mu m diameter) are trapped in the sheath of an argon RF parallel plate discharge. The particulates form Coulomb crystals consisting of horizontal hexagonal layers. A method for the determination of the charge on the particles is demonstrated that uses the response to a modulation of the electrode bias. Charges about three to five thousand elementary units are measured. The commonly applied orbital motion limited (OML) charging model leads to charges higher than the measured values. The strong coupling system is compared to the one component plasma (OCP model) and to a model with Yukawa potentials.

Dynamical processes in complex plasmas

In this review, a systematic overview of dynamical processes in complex (dusty) plasmas is given. Complex plasmas consist of electrons, ions, neutrals, and microparticles of nanometre to micrometre size, which are responsible for the unusual properties of this kind of plasmas, such as the formation of liquid or solid phases at strong electrostatic coupling. The examples represent the progress in this field within the last five years and mostly such cases were chosen, in which experimental results could be compared with theory or where the phenomenon has a diagnostic application. The presentation begins with single particle effects, where levitation, confinement in plasma traps, charging, and oscillations are involved. It is shown that vertical oscillations can be used for particle charge measurements. Nonlinear and parametric effects as well as self-excited oscillations are discussed. Then the interaction force between the particulates is explored in few-particle systems. Scattering experiments show that the interaction in the levitation plane can be described by shielded Yukawatype potentials, while the vertical interaction is governed by additional attractive wake fields. The latter are shown to be asymmetric and lead to the formation of vertical strings. The normal modes of one-dimensional or two-dimensional particle arrangements are useful tools for diagnostics. Many-particle effects are discussed in terms of low-frequency electrostatic waves. The dust-acoustic wave, the dust ion-acoustic wave, and two types of dust lattice waves (shear and compressional) are discussed with respect to diagnostic applications, among them Mach cones in plasma crystals. Laser methods are now established for the excitation of such modes. Two novel types of instabilities, the wakefield instability and the void-forming instability, as well as first results on dust ion-acoustic shocks are presented.

Laser-excited dust lattice waves in plasma crystals

Abstract Laser-excited waves in a two-dimensional dust plasma crystal have been observed experimentally in a parallel plate rf discharge. The measured dispersion relation is compared with theoretical models. Agreement is found with dust lattice waves, whereas deviations from dust acoustic waves exist. From the dispersion relation of a dust lattice wave the screening of the particles in the rf sheath is determined.

Dust confinement and dust-acoustic waves in weakly magnetized anodic plasmas

Experiments on dust-acoustic waves (DAW) in a magnetized anodic plasma are presented for the regime of low collisionality. The dust trapping and the self-excited and synchronized DAW dynamics are studied. Based on Langmuir and emissive probe measurements the dust confinement is found to be well described with respect to size, stability, and position of the dust cloud by an effective potential well formed by ion drag and Coulomb forces. Moreover, the measurements indicate the necessity for a kinetic model for the wave dispersion. By means of singular value decomposition the local wavelengths and growth rates of the waves are measured systematically. It is found that the measured mean wave number is well described by kinetic theory, while the theoretical growth rates overestimate the experiments. A novel observation for the DAW is a systematic variation of the wavelength inside the dust cloud.

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.

Measurement of the ion drag force on falling dust particles and its relation to the void formation in complex (dusty) plasmas

Experiments on the quantitative determination of the weaker forces (ion drag, thermophoresis, and electric field force) on free-falling dust particles in a rf discharge tube are presented. The strongest force, gravity, is balanced by gas friction and the weaker forces are investigated in the radial (horizontal) plane. Under most discharge conditions, the particles are found to be expelled from the central plasma region. A transition to a situation where the falling particles are focused into the plasma center is observed at low gas pressures using small particles. These investigations allow a quantitative understanding of the mechanism of unwanted dust-free areas (so-called voids) in dusty plasmas under microgravity. Good quantitative agreement with standard models of the ion drag is found.

Effect of neutral gas motion on the rotation of dust clusters in an axial magnetic field

Experiments are carried out to investigate the rotation of dust clusters in a radio-frequency plasma sheath with a vertical magnetic field. Our observations are in disagreement with the standard model, in which it was assumed that the neutral gas is at rest and that a steady rotation is attained when the ion-drag force is balanced by neutral friction. Here, we re-examine this basic assumption by carefully designed experiments. Our results suggest that the neutral gas is set into rotation by E×B induced ion flow through ion-neutral collisions and that the dust particles are advected by this flow. A hydrodynamic model is proposed to describe the rotation of the neutral gas and it can explain our observations.

Nonlinear dynamical behavior of thermionic low pressure discharges. II. Experimental

Strongly nonlinear relaxation oscillations of discharge current and plasma potential are investigated in a magnetized thermionic plasma discharge. The quasi‐one‐dimensional electron motion allows a direct comparison with one‐dimensional models and computer simulations. Two different stable discharge modes can be established, the low‐current space charge limited and the high‐current temperature limited mode. Time resolved probe measurements of the plasma potential distribution demonstrate that the current oscillations result from a strongly nonlinear instability of the potential structure in the weak current discharge mode. This confirms the model based on particle‐in‐cell simulations [F. Greiner et al., Phys. Plasmas 2, 1810 (1995)]. The oscillation process consists of three distinct phases. The sequence of events and the observed parameter dependencies of the oscillation frequency is in accordance with the model. The periodically driven system shows the characteristic behavior of nonlinear oscillators: q...