Michael Himpel

Three-dimensional single particle tracking in dense dust clouds by stereoscopy of fluorescent particles

In dense dust clouds of a dusty plasma single particle trajectories are impossible to follow due to occlusion of particles and ambiguities in particle correspondences. By stereoscopic imaging of fluorescent tracer particles, we were able to reconstruct 3D single particle trajectories within dense dust clouds. Several measurements are shown that justify to regard the tracer particles as suitable representatives for the whole dust system. A first analysis of dust density waves in dense clouds already shows that these waves exhibit three-dimensional dynamics at larger wave amplitudes that cannot be resolved by 2D imaging techniques: a broad velocity distribution perpendicular to the oscillation plane due to dust-dust collisions is seen, while the velocity distribution in the oscillation direction is bimodal and shifted due to the bulk wave propagation.

Three-view stereoscopy in dusty plasmas under microgravity: A calibration and reconstruction approach

A three-camera stereoscopy setup is presented that allows to reconstruct the trajectories of particles in dusty plasmas under microgravity. The calibration procedure for the three-camera setup takes the special circumstances into account that occur in close-range imaging of small particles. Additionally, a reconstruction algorithm is presented that is based on the epipolar geometry and delivers the essential particle correspondences. Further improvements are achieved by analyzing the dynamic particle behavior. Two applications of our calibration and reconstruction procedure are presented: A two-dimensional dust structure in the laboratory with a large percentage of hidden particles, and particles inside the void of a dust cloud under microgravity.

Spatially resolved three-dimensional particle dynamics in the void of dusty plasmas under microgravity using stereoscopy

Three-dimensional (3D) dynamical properties of fast particles being injected into the void region of a dusty plasma under microgravity conditions have been measured. For that purpose, a stereoscopic camera setup of three cameras has been developed that is able to track and reconstruct the 3D trajectories of individual dust particles. From more than 500 particle trajectories, the force field inside the void region and its influence on particle movement are derived and analyzed in 3D. It is shown that the force field is dominated by forces pointing radially out of the void and that this radial character is reflected in the velocity distributions of particles leaving the void. Furthermore, the structure of the force field is used for measuring the neutral gas friction for the particles inside the void.

Computer tomography of large dust clouds in complex plasmas

The dust density is a central parameter of a dusty plasma. Here, a tomography setup for the determination of the three-dimensionally resolved density distribution of spatially extended dust clouds is presented. The dust clouds consist of micron-sized particles confined in a radio frequency argon plasma, where they fill almost the entire discharge volume. First, a line-of-sight integrated dust density is obtained from extinction measurements, where the incident light from an LED panel is scattered and absorbed by the dust. Performing these extinction measurements from many different angles allows the reconstruction of the 3D dust density distribution, analogous to a computer tomography in medical applications.

Stereoscopy of dust density waves under microgravity: Velocity distributions and phase-resolved single-particle analysis

Experiments on dust-density waves have been performed in dusty plasmas under the microgravity conditions of parabolic flights. Three-dimensional measurements of a dust density wave on a single particle level are presented. The dust particles have been tracked for many oscillation periods. A Hilbert analysis is applied to obtain trajectory parameters such as oscillation amplitude and three-dimensional velocity amplitude. While the transverse motion is found to be thermal, the velocity distribution in wave propagation direction can be explained by harmonic oscillations with added Gaussian (thermal) noise. Additionally, it is shown that the wave properties can be reconstructed by means of a pseudo-stroboscopic approach. Finally, the energy dissipation mechanism from the kinetic oscillation energy to thermal motion is discussed and presented using phase-resolved analysis.

Stereoscopic imaging of dusty plasmas

The fundamentals of stereoscopy and their application to dusty plasmas are described. It is shown that stereoscopic methods allow us to measure the three-dimensional particle positions and trajectories with high spatial and temporal resolution. The underlying technical implications are presented and requirements and limitations are discussed. The stereoscopic method is demonstrated for dust particles in dust-density waves under microgravity conditions.

Phase Separation of Binary Charged Particle Systems with Small Size Disparities using a Dusty Plasma.

The phase separation in binary mixtures of charged particles has been investigated in a dusty plasma under microgravity on parabolic flights. A method based on the use of fluorescent dust particles was developed that allows us to distinguish between particles of slightly different size. A clear trend towards phase separation even for smallest size (charge) disparities is observed. The diffusion flux is directly measured from the experiment and uphill diffusion coefficients have been determined.

Oscillation Amplitudes in 3-D Dust Density Waves in Dusty Plasmas Under Microgravity Conditions

Large dust clouds in dusty plasmas exhibiting self-excited dust density waves (DDWs) have been investigated in a microgravity environment. With the help of the tracer particle technique [1], 3-D trajectories of single dust particles within the dust cloud have been measured using a stereoscopic camera setup. With the availability of the full phase-space information, 3-D wave properties can be accessed. In this paper, the spatial variation of the oscillation amplitude of single particles participating in a DDW is presented. We find that the amplitude increases in the direction of wave propagation and is nearly homogenous in the plane perpendicular to that direction.

Three-Dimensional Force Field Measurements in the Void of Dusty Plasmas Under Microgravity Conditions

In dusty plasmas, when the gravitational force on the particles is compensated, usually an extended dust cloud with a central dust-free void emerges. Fast particles are injected into the void and followed along their trajectories in three dimensions. They act as probes to measure the forces forming the void. An image illustrating this three-dimensional resolved force field inside the void is presented.

Analysis of 3D vortex motion in a dusty plasma

Dust clusters of about 50–1000 particles have been confined near the sheath region of a gaseous radio-frequency plasma discharge. These compact clusters exhibit a vortex motion which has been reconstructed in full three dimensions from stereoscopy. Smaller clusters are found to show a competition between solid-like cluster structure and vortex motion, whereas larger clusters feature very pronounced vortices. From the three-dimensional analysis, the dust flow field has been found to be nearly incompressible. The vortices in all observed clusters are essentially poloidal. The dependence of the vorticity on the cluster size is discussed. Finally, the vortex motion has been quantitatively attributed to radial gradients of the ion drag force.