Biomedical subjects

A Piel

Publications and source records attributed to A Piel.

18 recordsLinked to original sources

PubMed2006

Obliquely propagating dust-density plasma waves in the presence of an ion beam.

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 a new mode propagating obliquely at the boundary between the dusty plasma and the space-charge sheath. A model for dust-density waves propagating at an arbitrary angle with respect to the ion-flow direction is presented, which explains the preference for oblique or parallel modes as a function of ion velocity.

Journal Article↗

PubMed2006

Cutoff wave number for shear waves in a two-dimensional Yukawa system (dusty plasma).

The cutoff wave number for shear waves in a liquid-state strongly coupled plasma was measured experimentally. The phonon spectra of random particle motion were measured at various temperatures in a monolayer dusty plasma, where microspheres interact with a Yukawa potential. In the liquid state of this particle suspension, shear waves were detected only for wavelengths smaller than 20 to 40 Wigner-Seitz radii, depending on the Coulomb coupling parameter. The temperature of the suspension was controlled using a laser-heating method.

Journal Article↗

PubMed2006

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.

Journal Article↗

PubMed2004

Dust-free regions around Langmuir probes in complex plasmas under microgravity.

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.

Journal Article↗

PubMed2003

Compressional and shear wakes in a two-dimensional dusty plasma crystal.

Wakes composed of compressional and shear waves were studied experimentally in a two-dimensional screened-Coulomb crystal. Highly charged microspheres suspended in a plasma settled in a horizontal monolayer and arranged in a triangular lattice with a repulsive interparticle potential. Wakes were excited by a moving spot of Ar+ laser light. Depending on the laser spot speed, compressional waves formed a Mach cone and multiple lateral or transverse wakes, similar to ship wakes on the water surface, due to a combination of acoustic and dispersive properties. Shear waves, however, formed only a Mach cone, due to their nearly acoustic, i.e., dispersionless character. The experimental results show agreement with a recently developed theory and with molecular dynamics simulations, which assume a binary Yukawa interparticle potential. A generally useful method is presented for calculating the real part of the dispersion relation of the compressional waves based on the analysis of the spatial structure of a phonon wake. Fitting the resulting dispersion relation provides an independent measure of the interparticle potential, parametrized by the screening parameter kappa and particle charge Q.

Journal Article↗

PubMed2002

Experiments and molecular-dynamics simulation of elastic waves in a plasma crystal radiated from a small dipole source.

The radiation of elastic waves from a localized source is observed experimentally in a two-dimensional plasma crystal. An initial shear stress applied by a laser forms a small dipole source. The emerging complex wave pattern is shown to consist of outgoing compressional and shear wave pulses. Subsequent structures are identified as inward-going waves due to the finite size of the source region, which reappear on the opposite side. The compressional wave forms a trailing wave train due to strong dispersion, while the nondispersive shear wave evolves into a vortex-antivortex pair on either side. The experiments are compared with a molecular-dynamics simulation.

Journal Article↗

PubMed2002

Observation of shear-wave Mach cones in a 2D dusty-plasma crystal.

Mach cones composed of shear waves were observed experimentally in a two-dimensional screened-Coulomb crystal. Highly charged microspheres suspended in a plasma and interacting with a repulsive Yukawa potential arranged themselves in a triangular lattice with hexagonal symmetry. Mach cones were excited by applying a force from the radiation pressure of a moving laser beam. They had a single-cone structure, which is explained by the almost dispersionless character of shear waves. The cone's opening angle obeyed the Mach-cone-angle relation. Results are compared to a molecular-dynamics simulation.

Journal Article↗

PubMed2001

Normal modes of 2D finite clusters in complex plasmas.

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.

Journal Article↗

PubMed2001

Mode selective control of drift wave turbulence.

Experiments on spatiotemporal open-loop synchronization of drift wave turbulence in a magnetized cylindrical plasma are reported. The synchronization effect is modeled by a rotating current profile with prescribed mode structure. Numerical simulations of an extended Hasegawa-Wakatani model show good agreement with experimental results.

Journal Article↗

PubMed2001

Nonlinear resonances of particles in a dusty plasma sheath.

Vertical oscillations of microparticles trapped in the sheath of a capacitive rf discharge have been excited showing a strongly nonlinear resonance. The nonlinear oscillations are analyzed in terms of an anharmonic fourth-order potential energy curve. It is demonstrated that the observed nonlinearities can be related to a position dependent charge of the microspheres, whereas the electric field is found to be as nearly linearly increasing. The experimental results on the position dependent charging and electric field structure are compared to a numerical model.

Journal Article↗

PubMed2001

Synchronization of drift waves.

Based on a thorough experimental study of the equilibrium plasma state and the description of coherent drift modes in the framework of a linear, nonlocal model, the response of drift modes to different external driver signal is reported. Purely temporal perturbations as well as corotating and counter-rotating electric fields of various mode numbers are applied to the system. Drift mode synchronization and periodic pulling, typical for driven nonlinear oscillator systems, are investigated in detail. The full spatio-temporal behavior of periodic pulling is observed with multiprobe arrays.

Journal Article↗

PubMed2000

Melting of dust plasma crystals with defects

The role of defects in the melting transition of a bilayer dust plasma crystal with vertical alignment is studied using Langevin molecular dynamics simulations. Two types of defects are considered: (i) point defects and dislocations and (ii) additional particles which are placed below and above the bilayer crystal (i.e., the so-called strong defects). It is shown that the presence of point defects and a few dislocations does not change the two-step melting scenario previously found for nondefect bilayer crystals. This contrast with the influence of the strong defects which leads to a substantial increase of the kinetic energy of particles and to local heating of the bilayer dust crystal.

Journal Article↗

PubMed1996