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Antonova, Tetyana (2007): Interaction of particles with complex electrostatic structures and 3D clusters. Dissertation, LMU München: Faculty of Physics
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Abstract

Particles of micrometer size externally introduced in plasmas usually find their positions of levitation in the plasma sheath, where the gravity force is compensated by the strong electric field. Here due to electrostatic interaction they form different structures, which are interesting objects for the investigation of strongly coupled systems and critical phenomena. Because of the low damping (e.g. in comparison to colloidal suspension) it is possible to measure the dynamics up to the relevant highest frequency (e.g. Einstein frequency) at the most elementary level of single particle motion. The task of this work was to analyze the three dimensional structure, dynamical processes and the limit of the cooperative behavior in small plasma crystals. In addition to the study of the systems formed, the immersed particles themselves may be used for diagnostics of the plasma environment: estimation of parameters or monitoring of the processes inside plasma. The laboratory experiments are performed in two radio-frequency (RF) plasma reactors with parallel plate electrodes, where the lower electrode is a so-called "adaptive electrode". This electrode is segmented into 57 small "pixels" independently driven in DC (direct current) and/or RF voltage. When RF voltage is applied to one of these pixels, a bright localized glow, "secondary plasma ball", appears above. Three dimensional dust crystals with less than 100 particles are formed inside this "plasma ball" - the ideal conditions for the investigation of the transition from cluster systems to collective systems. The investigation of the particle interactions in crystals is performed with an optical diagnostic, which allows determination of all three particle coordinates simultaneously with time resolution of 0.04 sec. The experimental results are: 1. The binary interaction among particles in addition to the repelling Coulomb force exhibits also an attractive part, which is experimentally determined for the first time. 2. Analysis of the dynamical evolution shows the tendency of the systems to approach the state with minimum energy by rearranging particles inside. 3. The measured 63 particles' crystal vibrations are in close agreement with vibrations of a drop with surface tension. This indicates that even a 63 particle crystal already exhibits properties normally associated with the cooperative regime. The possibility to use levitated particles as a new powerful diagnostic of the plasma sheath region is proposed. The existence of different equilibrium positions of microparticles suspended in an Oxygen discharge provides evidence of a structured electronegative plasma sheath, a feature so far only mathematically and numerically investigated.