Experimente zur Strukturbildung in Plasmen

Since the early days of plasma physics, it is known that plasma is more than a hotgaseous state of matter. The electric charge of the plasma constituents (usually electrons and ions) allows for collective behavior and the time scales of electron and iondynamics are well separated. Thus, plasmas exhibit a rich variety of phenomena whichare unknown to ordinary gases and contribute to the compelling properties of thisstate of matter. The wealth of phenomena, their relevance for astrophysical and technological applications, and their impact on other fields have driven plasma researchfor almost a century now and have provided much insight into the formation of staticstructures, oscillatory instabilities, waves, and turbulence

Synchronization of drift waves and its effect on fluctuation-induced transport

In this work the influence of external wave-like potential structures on the dynamics of drift waves in plasmas is investigated. In the center of interest is, besides a detailed, experimental investigation of the dynamics of the driven drift waves, the question, whether the anomalous fluctuation-induced transport related to drift waves can be influenced by a proper choice of the driver signal. A detailed analysis of the plasma equilibrium and a classification of the observed fluctuations as drift waves form the basis for investigations of the externally driven system. The main results of this work are: the characterization of the mode-selective interaction of drift wave and driver, the observation of temporal and spatial signatures of complete and incomplete synchronization and the identification of the influence of the azimuthal electric field of the exciter on the drift wave dynamics. These results permit an identification of the mechanism behind the observed transport enhancements and reductions. It turns out, that the radial mode structure of the driven drift modes determines the observed change of transport

Network analysis of 3D complex plasma clusters in a rotating electric field

Network analysis was used to study the structure and time evolution of driven three-dimensional complex plasma clusters. The clusters were created by suspending micron-size particles in a glass box placed on top of the rf electrode in a capacitively coupled discharge. The particles were highly charged and manipulated by an external electric field that had a constant magnitude and uniformly rotated in the horizontal plane. Depending on the frequency of the applied electric field, the clusters rotated in the direction of the electric field or remained stationary. The positions of all particles were measured using stereoscopic digital in-line holography. The network analysis revealed the interplay between two competing symmetries in the cluster. The rotating cluster was shown to be more cylindrical than the nonrotating cluster. The emergence of vertical strings of particles was also confirmed.Comment: 9 pages, 9 figures; corrected Fig.4 and typo

The effects of entrepreneurship education

Entrepreneurship education ranks high on policy agendas in Europe and the US, but little research is available to assess its impact. To help close this gap we investigate whether entrepreneurship education a?ects intentions to be entrepreneurial uniformly or whether it leads to greater sorting of students. The latter can reduce the average intention to be entrepreneurial and yet be socially beneficial. This paper provides a model of learning in which entrepreneurship education generates signals to students. Drawing on the signals, students evaluate their aptitude for entrepreneurial tasks. The model is tested using data from a compulsory entrepreneurship course. Using ex ante and ex post survey responses from students, we find that intentions to found decline somewhat although the course has significant positive e?ects on students’ self-assessed entrepreneurial skills. The empirical analysis supports the hypothesis that students receive informative signals and learn about their entrepreneurial aptitude. We outline implications for educators and public policy

Network analysis of 3D complex plasma clusters

Network analysis was used to study the structure and time evolution of driven three-dimensional complex plasma clusters. The clusters were created by suspending micron-size particles in a glass box placed on top of the rf electrode in a capacitively coupled discharge. The particles were highly charged and manipulated by an external electric �eld that had a constant magnitude and rotated uniformly in the horizontal plane. Depending on the frequency of the applied electric �eld, the clusters rotated in the direction of the electric �eld or remained stationary. The three-dimensional positions of all particles were measured using stereoscopic digital in-line holography. The network approach was used to elucidate the structural changes in the cluster consisting only of a very limited number of particles (64). The Analysis revealed an interplay between two competing symmetries in the cluster. Spherical and cylindrical ordering of the particles was examined by comparing network measures of the experimental data with null models. The null models were arti�cial data with a certain number of points in perfectly spherical order, and the rest in cylindrical order. The well established network measures local connectivity, clustering coe�cient and average path length were considered. Network analysis of the clusters showed that the rotating cluster was more cylindrical than the nonrotating cluster. These �ndings were in agreement with the estimate of the radial con�nement with the aid of a dynamical force balance. Neglecting friction and inertial forces due to the low particle velocities in the cluster, the pro�le of the electrical con�nement could be estimated by calculating the repulsing Yukawa-type interaction between the particles. The radial con�nement was shown to be stronger in the case of cluster rotation, increasing the cylindricity of the cluster. The emergence of vertical strings of particles was also con�rmed by using a network analysis. While the traditional method of a �xed threshold has limitations such as erroneously including passing by particles and a somewhat arbitrary threshold, community �nding algorithms yield a more elegant approach of �nding structures in complex systems. With the aid of multislice networks, it is possible to examine the whole time series at once and thus resolve the time evolution of the strings. As we demonstrated, network analysis is a powerful tool to analyze the structure of complex plasma clusters and may have numerous applications in other complex systems where the characertization of the spatial structure plays a vital role.

Dissociation of VE-PTP from VE-cadherin is required for leukocyte extravasation and for VEGF-induced vascular permeability in vivo

Artificially stabilizing contacts between VE-cadherin and VE-PTP reduce vascular permeability and leukocyte extravasation in vivo