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Schwabe, Mierk (2009): Dynamical effects in fluid complex plasmas. Dissertation, LMU München: Faculty of Physics
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Abstract

Complex plasmas consist of neutrals, ions, electrons and additional micrometer-sized grains. Fluxes of plasma particles onto the grain surface charge it, and the microparticles interact with each other. When the particles are illuminated, e.g., with a laser, recording the scattered light allows to perform fully resolved kinetic studies at all relevant frequencies, e.g., the Einstein frequency, Debye frequency, and plasma frequency. In this thesis, dynamical effects in fluid complex plasmas are investigated using the PK-3 Plus setup, consisting of a radio-frequency plasma chamber, on the International Space Station (ISS), during a parabolic flight, and on the ground. The objective is the study of dynamical phenomena on the particle level, e.g., to determine when "corpuscular" effects become important and when fluid dynamics provides an accurate description. "Periodgrams" are used to examine the global dynamic structures. Velocities and forces acting on individual particles were analyzed by studying the particle motion. In the ground laboratory, self-excited highly resolved wave structures were analyzed. Thermophoresis was used to vary the extent to which the particles were pushed into the sheath regions of the chamber. The self-excitation of the waves is due to the free energy in plasma ions, which stream fast relative to the microparticles, especially in the sheath region. This excitation mechanism also explains the observed pressure dependence of the excitation threshold. Using a simple model of the microparticle dynamics, the grain charge is estimated, which agrees well with that calculated using known theories. Self-excited waves were neither observed when the particles were completely suspended in the center of the chamber by the thermophoretic force nor during dedicated experiments during the microgravity phase of the parabolic flight. Microparticle waves were externally excited in a complex plasma under microgravity conditions on board the ISS by a variable, low frequency modulation voltage. In addition to the vertical slashing at the externally imposed frequency f_mod, waves propagated both in the vertical and horizontal direction. The horizontal oscillation did not depend on f_mod. When f_mod was resonant with the frequency of the horizontal oscillation, the excitation region of the vertically propagating waves started spreading obliquely towards the left part of the cloud. The dispersion relation of the oblique waves was quasi-sound like. Around f_mod = 9 Hz, the wave activity spread over the whole cloud. At higher frequencies, the oblique waves disappeared. The particle dynamics resembled those accompanying the self-excited waves observed in the ground setup. Under similar conditions as for the self-excited waves, a new phenomenon occurring near the surface of the particle cloud was investigated - the formation of microparticle bubbles, blobs, and surface cusps. The behavior of microparticles was similar to that observed in fluid drops and with sedimenting particles, including the breakup mechanism and vortex motion of particles inside the blobs. The forces acting on the microparticles are analyzed and the velocity scaling with pressure is shown to be compatible with a possible flow induced by thermal creep inside the plasma chamber. Various effects indicate the presence of surface tension, such as the formation of cusps with angles like in Taylor cones, the self-confinement of blobs inside the void, and the break-up of bubble lids. Qualitative agreement with the Rayleigh-Taylor instability is demonstrated.