Imaging in turbid media with intensity-modulated lasers

We describe the formation of a narrow beam for intensity-modulated electromagnetic radiation propagating through highly scattering materials. We propose to use this beam to reconstruct images, similar to X-ray back-projection techniques. For sufficiently high modulation frequency, the photon density wave is primarily carried by photons that suffer small or no large-angle scattering, which gives rise to the beam’s narrow divergence. The beam-narrowing concept is supported by large-scale numerical simulations to examine the quality of the imaging.     

Review of time lag permeation technique as a method for characterisation of porous media and membranes

The time lag permeation technique has proven to bean effective method for characterisation. Because of the simple nature of the permeation experiment, transport parameters can be directly obtained from experimental data hence avoiding the intensive mathematical treatment required by other techniques. The method has historically been applied to diffusion and adsorption in porous membranes and diffusion in polymer membranes. Since its origins in 1920, interest in the time lag method has expanded because of its value in characterising simple permeation processes and also complex systems of diffusion with simultaneous adsorption and surface diffusion. This review focuses on presenting the asymptotic solution of the mass balance diffusion equations and includes applications of time lag analysis, in order to give a critical and broad perspective of this method as a tool for characterisation. It includes much of the previously published literature in order to show that for most cases the asymptotic solution of the transport equations is simple, and for more complex cases that an analytical solution is possible hence avoiding cumbersome numerical techniques.     

Closest packing in dense oxides: The structure of a polymorph of Co3(AsO4)2

Crystals of Co₃(AsO₄)₂ were grown from the melt of a mixture of Co₂As₂O₇ and As₂O₅. The crystals are isostructural with Mg₃(AsO₄)₂ and are tetragonal with a = 6.858(2), c = 18.872(5) Å, Z = 6, and space group $\text{I}\text{4}\text{2d}$. A total of 1048 independent reflections were measured by diffractometer and used in the full-matrix refinement to a final R value of 0.069. The structure contains two distinct AsO₄ groups. Two of the cobalt ions are octahedrally coordinated and a third occupies a $\text{4}$ site with four short and four long CoO distances. The crystal structure of Co₃ (AsO₄)₂ is not based on the continuous three-dimensional closest packing of oxygen atoms. Nevertheless the number of oxygen atoms per cubic centimeter is 5.4 × 10²², which falls in the range of values for hexagonal and cubic closest packed structures. A better measure of the degree to which closest packing is achieved by a structure is suggested. It is based on an analysis of the polyhedra of oxygen atoms which surround each of the oxygen atoms in a structure and their relation to the polyhedra in ideally closest packed structures. In order to facilitate the analysis, polytopes of 11- and 12-vertex polyhedra were studied. A new decahexahedral 11-vertex polyhedron was found.     

Intramolecular energy migration and transfer in macromolecules. A fluorescence depolarization approach

Intramolecular energy migration and trapping by acceptor species which are an integral part of a macromolecule have been studied in three polymer systems using the techniques of fluorescence depolarization. The effect of energy migration produces depolarization of both donor and acceptor emissions. Copolymers of styrene with 1-vinylaphthalene or 2-phenyl-5-(p-vinyl)phenyloxazole have been studied. In addition, energy transfer to a terminal anthracene species has been studied in a polyvinyltoluene sample. When energy transfer occurs with unit efficiency, the depolarization of acceptor emission reflects the path length available to the migrating exciton. In cases of lesser transfer efficiency, the acceptor emission exhibits depolarization characteristics which reflect the distribution of migration lengths from the site of energy absorption.     

Separation of Isotopes in the Thermal Diffusion Column

Abstract

Gas liquid and solid mixtures undergo a slight demixing in the presence of a temperature difference. This phenomenon, known as thermal diffusion, is illustrated in Figure 1 for the isotopes of neon in the gas phase and for isotopically substituted carbon disulfide in the liquid phase. The separation effect, which is quite small even for relatively large temperature differences, was of no practical significance prior to the invention by Clusius and Dickel of the thermogravitational thermal diffusion column in 1938¹. In the thermal diffusion column (Figure 2) the fluid is confined between closely spaced vertical walls maintained at different temperatures. A convection current is set up with the fluid rising along the hot wall and descending along the cold wall. Thermal diffusion takes place In the horizontal direction. The combined effects of vertical countercurrent thermogravitational circulation and horizontal thermal diffusion lead to large separations in the vertical direction. The equivalent of several hundred separation stages can be obtained in apparatus no more than a few meters in length.     

Equilibrium and kinetics of water adsorption in carbon molecular sieve: theory and experiment

Measurements of water adsorption equilibrium and kinetics in Takeda carbon molecular sieve (CMS) were undertaken in an effort to characterize fundamental mechanisms of adsorption and transport. Adsorption equilibrium revealed a type III isotherm that was characterized by cooperative multimolecular sorption theory. Water adsorption was found to be reversible and did not display hysteresis upon desorption over the conditions studied. Adsorption kinetics measurements revealed that a Fickian diffusion mechanism governed the uptake of water and that the rate of adsorption decreased with increasing relative pressure. Previous investigations have attributed the observed decreasing trend in the rate of adsorption to blocking of micropores. Here, it is proposed that the decrease is attributed to the thermodynamic correction to Fick's law which is formulated on the basis of the chemical potential as the driving force for transport. The thermodynamically corrected formulation accounted for observations of transport of water and other molecules in CMS.     

Yb:YAG and Nd:YAG edge-pumped slab lasers

Experimental demonstrations of two edge-pumped zigzag slab lasers are presented. The Nd:YAG slab laser generated 127 W of multimode output power with 300W of pump power. Preliminary results with a Yb:YAG slab produced 46 W of output power with 315W of pump power. The edge-pumped slab design permits symmetric conduction cooling and efficient pump absorption and accepts large-numerical-aperture pump sources.     

Plasma Photocathodes

Plasma wakefield accelerators offer accelerating and focusing electric fields three to four orders of magnitude larger than state-of-the-art radiofrequency cavity-based accelerators. Plasma photocathodes can release ultracold electron populations within such plasma waves and thus open a path toward tunable production of well-defined, compact electron beams with normalized emittance and brightness many orders of magnitude better than state-of-the-art. Such beams will have far-reaching impact for applications such as light sources, but also open up new vistas on high energy and high field physics. This paper reviews the innovation of plasma photocathodes, and reports on the experimental progress, challenges, and future prospects of the approach. Details of the proof-of-concept demonstration of a plasma photocathode in 90° geometry at SLAC FACET within the E-210: Trojan Horse program are described. Using this experience, alongside theoretical and simulation-supported advances, an outlook is given on future realizations of plasma photocathodes such as the upcoming E-310: Trojan Horse-II program at FACET-II with prospects toward excellent witness beam parameter quality, tunability, and stability. Future installations of plasma photocathodes also at compact, hybrid plasma wakefield accelerators, will then boost capacities and open up novel capabilities for experiments at the forefront of interaction of high brightness electron and photon beams.