Effective interaction between star polymers

The distance-resolved effective interaction between two star polymers in a good solvent is calculated by Molecular Dynamics computer simulations. The results are compared with a pair potential proposed recently by Likos et al. [Phys. Rev. Lett. 1998, 80, 4450] which is exponentially decaying for large distances and crosses over, at the corona diameter of the star, to an ultrasoft logarithmic repulsion for small distances. Excellent agreement is found in a broad range of star arm numbers.Comment: final version as published, 9 pages + 5 ps-figure

Soliton Staircases and Standing Strain Waves in Confined Colloidal Crystals

We show by computer simulation of a two-dimensional crystal confined by corrugated walls that confinement can be used to impose a controllable mesoscopic superstructure of predominantly mechanical elastic character. Due to an interplay of the particle density of the system and the width D of the confining channel, "soliton staircases" can be created along both parallel confining boundaries, that give rise to standing strain waves in the entire crystal. The periodicity of these waves is of the same order as D. This mechanism should be useful for structure formation in the self-assembly of various nanoscopic materials.Comment: 22 pages, 5 figure

Experimental validation of image contrast correlation between ultra-small-angle X-ray scattering and grating-based dark-field imaging using a laser-driven compact X-ray source: Experimentelle Verifizierung des Zusammenhangs zwischen Röntgen-Kleinwinkelstreuung und gitter-basierter Röntgen-Dunkelfeldbildgebung unter Verwendung eines laser-getriebenen Kompaktsynchrotrons

X-ray phase and dark-field contrast have recently been the source of much attention in the field of X-ray imaging, as they both contribute new imaging signals based on physical principles that differ from conventional X-ray imaging. With a so-called Talbot grating interferometer, both phase-contrast and dark-field images are obtained simultaneously with the conventional attenuation-based X-ray image, providing three complementary image modalities that are intrinsically registered. Whereas the physical contrast mechanisms behind attenuation and phase contrast are well understood, a formalism to describe the dark-field signal is still in progress. In this article, we report on correlative experimental results obtained with a grating interferometer and with small-angle X-ray scattering. Furthermore, we use a proposed model to quantitatively describe the results, which could be of great importance for future clinical and biomedical applications of grating-based X-ray imagin

SLAC/CERN high gradient tests on an X-band accelerating section

High frequency linear collider schemes envisage the use of rather high accelerating gradients: 50 to 100 MV/m for X-band and 80 MV/m for CLIC. Because these gradients are well above those commonly used in accelerators, high gradient studies of high frequency structures have been initiated and test facilities have been constructed at KEK [1], SLAC [2] and CERN [3]. The studies seek to demonstrate that the above mentioned gradients are both achievable and practical. There is no well-defined criterion for the maximum acceptable level of dark current but it must be low enough not to generate unacceptable transverse wakefields, disturb beam position monitor readings or cause RF power losses. Because there are of the order of 10,000 accelerating sections in a high frequency linear collider, the conditioning process should not be too long or difficult. The test facilities have been instrumented to allow investigation of field emission and RF breakdown mechanisms. With an understanding of these effects, the high gradient performance of accelerating sections may be improved through modifications in geometry, fabrication methods and surface finish. These high gradient test facilities also allow the ultimate performance of high frequency/short pulse length accelerating structures to be probed. This report describes the high gradient test at SLAC of an X-band accelerating section built at CERN using technology developed for CLIC

12/13/18/19: the making of Blind Spot

Global Challenges (FSW