Electrical performance characteristics of high power converters for space power applications

The first goal of this project was to investigate various converters that would be suitable for processing electric power derived from a nuclear reactor. The implementation is indicated of a 20 kHz system that includes a source converter, a ballast converter, and a fixed frequency converter for generating the 20 kHz output. This system can be converted to dc simply by removing the fixed frequency converter. This present study emphasized the design and testing of the source and ballast converters. A push-pull current-fed (PPCF) design was selected for the source converter, and a 2.7 kW version of this was implemented using three 900 watt modules in parallel. The characteristic equation for two converters in parallel was derived, but this analysis did not yield any experimental methods for measuring relative stability. The three source modules were first tested individually and then in parallel as a 2.7 kW system. All tests proved to be satisfactory; the system was stable; efficiency and regulation were acceptable; and the system was fault tolerant. The design of a ballast-load converter, which was operated as a shunt regulator, was investigated. The proposed power circuit is suitable for use with BJTs because proportional base drive is easily implemented. A control circuit which minimizes switching frequency ripple and automatically bypasses a faulty shunt section was developed. A nonlinear state-space-averaged model of the shunt regulator was developed and shown to produce an accurate incremental (small-signal) dynamic model, even though the usual state-space-averaging assumptions were not met. The nonlinear model was also shown to be useful for large-signal dynamic simulation using PSpice

A comparative study of electric power distribution systems for spacecraft

The electric power distribution systems for spacecraft are compared concentrating on two interrelated issues: the choice between dc and high frequency ac, and the converter/inverter topology to be used at the power source. The relative merits of dc and ac distribution are discussed. Specific converter and inverter topologies are identified and analyzed in detail for the purpose of detailed comparison. Finally, specific topologies are recommended for use in dc and ac systems

Robust Optimal Risk Sharing and Risk Premia in Expanding Pools

We consider the problem of optimal risk sharing in a pool of cooperative agents. We analyze the asymptotic behavior of the certainty equivalents and risk premia associated with the Pareto optimal risk sharing contract as the pool expands. We first study this problem under expected utility preferences with an objectively or subjectively given probabilistic model. Next, we develop a robust approach by explicitly taking uncertainty about the probabilistic model (ambiguity) into account. The resulting robust certainty equivalents and risk premia compound risk and ambiguity aversion. We provide explicit results on their limits and rates of convergence, induced by Pareto optimal risk sharing in expanding pools

Does cognitive ability influence responses to the Warwick-Edinburgh Mental Well-Being Scale?

It has been suggested that how individuals respond to self-report items relies on cognitive processing. We hypothesized that an individual's level of cognitive ability may influence these processes such that, if there is a hierarchy of items within a particular questionnaire, as demonstrated by Mokken scaling, the strength of that hierarchy will vary according to cognitive ability. Using data on 8,643 men and women from the National Child Development Survey (1958 birth cohort; Power, & Elliott, 2006), we investigated, using Mokken scaling, whether the 14 items that make up the Warwick-Edinburgh Mental Well-Being Scale (Tennant et al., 2007)-completed when the participants were 50 years of age-form a hierarchy and whether that hierarchy varied according to cognitive ability at age 11 years. Among the sample as a whole, we found a moderately strong unidimensional hierarchy of items (Loevinger's coefficient [H] = 0.48). We split participants into 3 groups according to cognitive ability and analyzed the Mokken scaling properties of each group. Only the medium and high cognitive ability groups had acceptable (?0.3) invariant item ordering (assessed using the HT statistic). This pattern was also found when the 3 cognitive ability groups were assessed within men and women separately. Greater attention should be paid to the content validity of questionnaires to ensure they are applicable across the spectrum of mental ability

Non-collinear interaction of photons with orbital angular momentum

We elucidate the consequences of a phase-matching theory that describes second-harmonic generation of two non-collinear incident light beams that carry orbital angular momentum (OAM). More specifically, the two incident beams generate a third that, depending on the incident OAM, may experience a significantly smaller conversion efficiency in comparison to that based on the conventional phase-matching theory. This is the case even for incident angles substantially less than those required for non-conservation of OAM in the nonlinear interaction. Experiments are performed under different conditions and are in excellent agreement with the theory. Our results have implications beyond the specific case studied here of second-harmonic generation, in particular for parametric down-conversion of photons.Comment: 6 pages, 4 figure

A multi-photon magneto-optical trap

We demonstrate a Magneto-Optical Trap (MOT) configuration which employs optical forces due to light scattering between electronically excited states of the atom. With the standard MOT laser beams propagating along the {\it x}- and {\it y}- directions, the laser beams along the {\it z}-direction are at a different wavelength that couples two sets of {\it excited} states. We demonstrate efficient cooling and trapping of cesium atoms in a vapor cell and sub-Doppler cooling on both the red and blue sides of the two-photon resonance. The technique demonstrated in this work may have applications in background-free detection of trapped atoms, and in assisting laser-cooling and trapping of certain atomic species that require cooling lasers at inconvenient wavelengths.Comment: 10 pages, 5 figure

Modelling aspects of oviduct fluid formation in vitro

© 2017 Society for Reproduction and Fertility. Oviduct fluid is the microenvironment that supports early reproductive processes including fertilisation, embryo cleavage and genome activation. However, the composition and regulation of this critical environment remain rather poorly defined. This study uses an in vitro preparation of the bovine oviduct epithelium to investigate the formation and composition of in vitro-derived oviduct fluid (ivDOF) within a controlled environment. We confirm the presence of oviduct-specific glycoprotein 1 in ivDOF and show that the amino acid and carbohydrate content resembles that of previously reported in vivo data. In parallel, using a different culture system, a panel of oviduct epithelial solute carrier genes and the corresponding flux of amino acids within ivDOF in response to steroid hormones were investigated. We next incorporated fibroblasts directly beneath the epithelium. This dual culture arrangement represents more faithfully the in vivo environment and impacts on ivDOF composition. Lastly, physiological and pathophysiological endocrine states were modelled and their impact on the in vitro oviduct preparation was evaluated. These experiments help clarify the dynamic function of the oviduct in vitro and suggest a number of future research avenues, such as investigating epithelial-fibroblast interactions, probing the molecular aetiologies of subfertility and optimising embryo culture media

Coherent control of light interaction with graphene

We report the experimental observation of all-optical modulation of light in a graphene film. The graphene film is scanned across a standing wave formed by two counter-propagating laser beams in a Sagnac interferometer. Through a coherent absorption process the on-axis transmission is modulated with close to 80% efficiency. Furthermore we observe modulation of the scattered energy by mapping the off-axis scattered optical signal: scattering is minimized at a node of the standing wave pattern and maximized at an antinode. The results highlight the possibility to switch and modulate any given optical interaction with deeply sub-wavelength films.Comment: 4 pages, 4 figure

An isogeometric finite element formulation for phase transitions on deforming surfaces

This paper presents a general theory and isogeometric finite element implementation for studying mass conserving phase transitions on deforming surfaces. The mathematical problem is governed by two coupled fourth-order nonlinear partial differential equations (PDEs) that live on an evolving two-dimensional manifold. For the phase transitions, the PDE is the Cahn-Hilliard equation for curved surfaces, which can be derived from surface mass balance in the framework of irreversible thermodynamics. For the surface deformation, the PDE is the (vector-valued) Kirchhoff-Love thin shell equation. Both PDEs can be efficiently discretized using $C^1$-continuous interpolations without derivative degrees-of-freedom (dofs). Structured NURBS and unstructured spline spaces with pointwise $C^1$-continuity are utilized for these interpolations. The resulting finite element formulation is discretized in time by the generalized-$\alpha$ scheme with adaptive time-stepping, and it is fully linearized within a monolithic Newton-Raphson approach. A curvilinear surface parameterization is used throughout the formulation to admit general surface shapes and deformations. The behavior of the coupled system is illustrated by several numerical examples exhibiting phase transitions on deforming spheres, tori and double-tori.Comment: fixed typos, extended literature review, added clarifying notes to the text, added supplementary movie file