Dynamic balance of forces in dusty (complex) plasmas

Zafiu, Ciprian

Starting in 1994 with the first obtained plasma crystal, the study of dusty plasmas developed explosively since then as an autonomous research field. The dynamic equilibrium of forces acting on micro-spheres in the plasma is a fundamental problem for understanding the dynamics of complex plasmas. The most relevant forces are gravitation force, neutral gas friction, electric field force, ion drag and thermophoresis. The last two are introduced into the plasma physics only in connection to dusty plasma. Even though they are usually very small compared to the particle weight, they become very important when the gravitation is reduced, as in the case of microgravity experiments, or by falling particles in our second experiment. In this thesis two experiments on the forces acting on dust particles are presented under two completely different conditions: in the plasma sheath, and in the plasma bulk. These two situations differ by the fact that the sheath is a non-equilibrium environment governed by directional ion flows, where the particles are always trapped in 2D structures, while the plasma bulk is an equilibrium environment, where the particle trapping is possible under microgravity conditions, when 3D particle systems can be studied. In the first experiment, nonlinear oscillations of individual micro-spheres have been excited in the plasma sheath by low frequency modulation of the RF power applied to the electrode. The potential well of the particles trapped within the sheath is determined exclusively by the balance of the gravitation force and the electric field force. Other forces like thermophoresis or ion drag are about one order of magnitude smaller than the gravitation under these conditions and act in the same vertical direction as the weight force. From the characteristics of the nonlinear resonances, the contributions of electric field and the particle charging to the observed nonlinearity have been derived. It was found that the nonlinearity can be fully attributed to the position dependent charge of the particles. This oscillation method is a reliable technique for determining the forces acting on particles trapped in the non-equilibrium sheath. In the second experiment, the behaviour of free falling particles through a long plasma column is studied. This is a novel experimental approach to the study of the forces acting on the charged particles. The gravitation force is balanced in the vertical direction by the neutral gas friction force. Smaller forces like thermophoresis or ion drag act in the radial direction and this allows to decouple the dominant vertical forces from the more subtle ones, which act horizontally. The study of the radial forces is carried out for various plasma conditions, by analysing the trajectories of the falling particles. From the experiments, a good agreement with theoretical models of the thermophoretic and ion drag force is obtained. This approach provides accurate qualitative and quantitative results on the acting forces and allows direct comparisons with dusty plasma experiments under microgravity conditions e.g. in questions of the mechanism of the "void" phenomenon. Concluding, this thesis offers reliable methods of analysis for investigating both trapped and untrapped particles for an improved understanding of dynamic processes in complex plasmas.



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Zafiu, Ciprian: Dynamic balance of forces in dusty (complex) plasmas. 2002.


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