[show abstract][hide abstract] ABSTRACT: Experiments in a dusty plasma under the microgravity conditions of parabolic flights are presented. Under microgravity, extended dust structures and a central dust-free region (“void”) are formed. Here, the forces and the force balance at the void boundary are studied by means of laser manipulation of the dust particles: A focused laser beam is moved in a controlled way to drive particles in the extended dust cloud and at the void boundary. From the observed particle motion, the forces on the particles in the dust cloud and at the void boundary are derived. Together with Langmuir probe measurements, a quantitative description of the force balance has been achieved. Special attention has been paid to the ion drag force, which is crucial in understanding the void formation. The results are compared to prevalent ion drag models.
Physics of Plasmas 12/2007; 14(12):123707-123707-10. · 2.38 Impact Factor
[show abstract][hide abstract] ABSTRACT: Experiments on the laser manipulation of dust particles in a 3-D dust cloud under microgravity conditions are presented. The experiments have been performed on two parabolic flight campaigns in 2005 and 2006 to study the formation of the dust-free region ("void") in the center of the cloud. The manipulation laser is used to drive particles from the void edge into the void. From the particle trajectories, the forces on the particles are derived. The measured forces are compared to models of the ion drag force
IEEE Transactions on Plasma Science 05/2007; · 0.87 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: The use of lasers to actively manipulate charged microparticles in a plasma has been a valuable tool in the dusty plasma community. However, almost all of these studies have been performed in the strongly-coupled regime in rf discharge dusty plasmas. This presentation reports on the application of laser manipulation techniques to a dc discharge dusty plasma. In these studies, a Nd:YAG laser with a maximum output power of 600 mW is used as the manipulation laser. The manipulation laser uses a voltage-controlled, two-mirror system that allows the laser to be scanned throughout the volume of the dusty plasma. This presentation will highlight three main results. First, it will demonstrate the active manipulation of microparticles in the plasma. Second, it will use the manipulation of the particles at the particle cloud plasma interface to characterize the potential structure at that boundary. Third, it will illustrate the effect of scanning the laser through the particle cloud and observing driven flows.
[show abstract][hide abstract] ABSTRACT: Experiments on the heating and melting of two-dimensional finite dust crystals are performed using random laser excitation of the dust particles by a rapidly moving laser beam. The achievable dust temperatures scale with the square of the laser power. The heating process is described for different dust clusters under various plasma and cluster conditions. A single-particle model is developed to explain the observed behavior of the cluster under the random laser excitation. Good quantitative agreement is found when the radiation pressure is made responsible for the particle excitation by the laser. The dynamical properties of the system during heating are analyzed and the dominant modes are identified. From this, it is demonstrated that the heating process is of a nearly equilibrium nature in contrast to previous melting experiments. Finally, the melting of the dust cluster by laser heating is studied. From these experiments, a precise determination of the critical coupling parameter for the solid-fluid transition was possible. It is measured as Gamma = 270-480 for an N = 18 cluster.
[show abstract][hide abstract] ABSTRACT: The heating and melting of two-dimensional dust clusters with one additional particle in the lower layer has been investigated experimentally in a gas discharge. The full dynamical properties of the system during the entire phase transition were determined in terms of the spectral power densities of the crystal modes. A two-step melting transition is identified when the gas pressure in the discharge is reduced: first, a sudden increase of the dust temperature takes place due to an instability of the lower-layer particle resulting in a hot crystalline state of the cluster, and second, the actual transition into a fluid state is observed at a decisively lower gas pressure.
[show abstract][hide abstract] ABSTRACT: The melting of 2D dust clusters caused by one additional particle in the lower layer has experimentally been observed to undergo a two-step transition, which divides the phase of the cluster into three stages. The first transition is a jump of the dust kinetic energy due to the onset of an instability of the lower-layer particle, shifting the cluster from an ordinary to a hot crystalline state. The second transition is the actual phase transition into a liquid state, which occurs at a decisively lower gas pressure. The detailed dynamical properties of the system during the transition were determined in terms of the normal mode analysis.