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Abstract
Experimental studies on dusty plasmas containing systems of (super-)paramagnetic dust particles are presented. In our experiments, external (homogeneous as well as inhomogeneous) magnetic fields in the mT range are applied to study the effect on single particles or few-particle systems that are trapped inside the sheath region. The behavior of the paramagnetic dust particles is considerably different than that of dielectric plastic particles, which are widely used in dusty plasmas. It is revealed that especially non-magnetic contributions play an important role in the interaction between superparamagnetic particles.
This thesis discusses three publications in the field of dusty plasmas.
In the first section, measurements of the ir absorption of silica nanoparticles confined in an argon radiofrequency plasma discharge using a Fourier transform infrared spectrometer have been performed. By varying the gas pressure of the discharge and duty cycle of the applied radiofrequency voltage, a shift of the absorption peak of silica is observed. This shift is attributed to charge-dependent absorption features of silica. The charge-dependent shift has been calculated for silica particles, and from comparisons with the experiment the particle charge has been retrieved using the infrared phonon resonance shift method. With the two different approaches of changing the gas pressure and altering the duty cycle, one is able to deduce a relative change of the particle charge with pressure variations and an absolute estimate of the charge with the duty cycle.
In the second part, infrared (IR) absorption spectra of melamine-formaldehyde (MF) microparticles confined in an rf plasma are studied at different plasma conditions. Several absorption peaks have been analysed in dependence of plasma power and their temporal evolution. For comparison, the IR absorption spectra of heated MF microparticles without plasma exposition are used to determine the general influence of the temperature on the IR spectra. Measuring the temperature of the particles inside the plasma shows that the temperature is not the only process changing the particles' IR spectra. Chemical changes of the MF particles with increasing plasma power influence the absorption peak structure.
Finally, experiments on dust clusters trapped in the sheath of a radio frequency discharge have been performed for different magnetic field strengths ranging from a few milliteslas to 5.8 T. The dynamics of the dust clusters are analyzed in terms of their normal modes. From that, various dust properties such as the kinetic temperature, the dust charge, and the screening length are derived. It is found that the kinetic temperature of the cluster rises with the magnetic field, whereas the dust charge nearly remains constant. The screening length increases slightly at intermediate magnetic field strengths. Generally, the dust properties seem to correlate with magnetization parameters of the plasma electrons and ions, however only to a small degree.