Structure and Dynamics of Dust Clusters: Rotation and Magnetic Fields

Carstensen, Jan

Ensembles of charged, micrometer-sized dust particles conned in the sheath of a rf-discharge are typically in a strongly-coupled state and form crystal-like structures (dust clusters). This thesis focuses on the eects of magnetic elds and rotation on the structure and dynamics of dust clusters. Both topics, magnetization and rotation, are linked, since dust structures in the presence of magnetic elds are usually in a rotating state. Furthermore, the Larmor theorem states that the Lorentz force and the Coriolis force are mathematically equivalent and aect the dynamics of dust clusters in a similar way. In order to systematically study the eects of cluster rotation, a dust centrifuge is introduced as a new technique to set dust clusters into rotation. This allows to clearly separate the eects of rotational motion from the in uence of magnetic elds. In a frame of a rotating cluster two pseudo-forces appear: the centrifugal force, which eectively weakens the radial connement of the dust clusters and oers the possibility to probe the dust-dust interaction. Moreover, the Coriolis force, which modies the dynamics of a dust cluster in a similar way as it is expected from a giant magnetic eld. Here, a proof-of-principle is given that the Coriolis force can be used to effctively magnetize a small dust cluster and to study the simultaneous eects of (pseudo) magnetization and strong coupling in a dusty plasma device. The second focus of this thesis lies on the eects of magnetic elds on the dynamics of dust clusters. The mechanism of cluster rotation in the presence of an axial agnetic eld is discussed. It is found that the cluster rotation is accompanied by a co-rotation of the neutral gas column, which can result in an unexpected sense of rotation and magnitude of the rotation frequencies in comparison to an established model of cluster rotation, which is based on a balance of ion and neutral drag force. Furthermore, the dust-dust interaction force is of special interest, which is substantially modied by the ow of ions in the plasma sheath and the formation of ion wakes. Here, the phase-resolved resonance method is introduced to measure the nonreciprocity of the interaction force and the partial decharging of particles in the wake of another with high precision. It turns out that both eects are not independent of each other and that the decharging eect enhances the nonreciprocity of the particle interaction due to a coupling with the external connement. In a further step, the in uence of a strong magnetic eld on the ion-wake-mediated particle interaction force is studied. It is shown for the rst time that a magnetic eld parallel to the ion ow severely reduces the interaction forces, which is in qualitative agreement with linear response calculations predicting a reduction of the wake charge. At last, an alternative approach to reach the realms of fully magnetized dusty plasmas is discussed. Going down in particle size from micrometer-sized dust particles to nanoparticles might allow to produce a dusty plasma, where even the motion of the dust is directly aected by a strong magnetic eld. Here, two important steps towards this aim are made. First, a modied discharge geometry is proposed, which allows to conne dust clouds in a plasma column in the presence of strong magnetic elds. Second, an "Imaging Mie Ellipsometer" is introduced, which provides important parameters, such as the dust size and the dust density, with high spatiotemporal resolution.

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Carstensen, Jan: Structure and Dynamics of Dust Clusters: Rotation and Magnetic Fields. 2013.

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