50WM1139 Nonlinear Waves in Dusty Plasmas under Microgravity


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Project objectives

We investigate complex plasmas under microgravity to avoid sedimentation of the particles, which occurs in laboratory experiments, and to study the weaker forces that act on the particles. The most prominent feature of a complex plasma in weightlessness is the formation of central particle-free regions in the plasma ("void"), which are created by the drag force from the radial outflow of ions. A sophisticated Langmuir-probe diagnostic system is used to measure profiles of plasma density, plasma potential and electron temperature. With these data the force equilibria are studied that lead to void formation. The Langmuir probe system is a predevelopment for the planned IMPACT facility aboard the International Space Station.

Project leader

Prof. Dr. A. Piel
Dr. K. Menzel


Approach

The distribution of the dust particles is recorded with video cameras in a vertical 2D section that is illuminated by a thin sheet of laser light. Two standard CCD cameras with 25 frames/s cover a small and a large field of view. In addition, a fast CMOS camera with 100 frames/s is used to observe dust acoustic waves or fast particle motion after laser manipulation. The laser manipulation is a joint effort with Prof. Dr. A. Melzer, University of Greifswald.



Project status

In this funding period we have participated in the DLR parabolic flight campaigns 5, 6, 7 and 8. PFC7 was performed in September 2005 from Bordeaux-Merignac airport, PFC8 from Köln-Bonn airport. On these campaigns we have combined the new IMPF-K plasma chamber with the laser manipulation laser-facility and high-speed camera developed at Greifswald University. The IMPF-K chamber is equipped with segmented electrodes that can be operated independently and with retractable glass tubes for dust confinement.


Voids and dust-density waves

Major topics of our microgravity studies are related to the "void" phenomenon â the appearance of a dust-free area in the center of a weightless complex plas-ma. Of particular interest are the driving mechanisms of the void and the intensive dust density waves (see movie) which are excited at its edges. Here, video observations, profiles of basic plasma parameters measured with the probe and laser measurements of forces acting on the particles have to be combined to derive a consistent idea of the underlying physics.

Cuvette experiments and laser manipulation

Using the glas tubes (cuvettes) in combination with corresponding RF-power and DC-bias on the sections of the electrodes, the ion-drag force which is mainly responsible for the "void" phenomenon can be reversed and almost homogeneous dust clouds are created in the cuvettes. Perturbations of single particles with the manipulation laser allow the exploration of the exceptional particle confinement potential.



Probe measurements in complex plasmas under microgravity

Video observations cannot provide all information about the physical systems which we study. Special attention has to be paid to the properties of the plasma, in which the particles are embedded, and the interaction of dust and plasma. A dense dust cloud has direct impact on the plasma density and potential as we have shown with Langmuir probe measurements.

Tracer particle experiments


With only a few particles injected into the plasma, which are assumed to have no influence on the plasma, it is possible to reveal the equilibrium surfaces on which the the opposing dominant forces on the particles, the ion-drag force and the electric field force , compensate each other. In this way, we have shown that the "void" is already predetermined in a dust-free plasma and we could visualize the sheath edge around objects inserted in the plasma.





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