10,438 research outputs found
Control considerations for high frequency, resonant, power processing equipment used in large systems
Addressed is a class of resonant power processing equipment designed to be used in an integrated high frequency (20 KHz domain), utility power system for large, multi-user spacecraft and other aerospace vehicles. It describes a hardware approach, which has been the basis for parametric and physical data used to justify the selection of high frequency ac as the PMAD baseline for the space station. This paper is part of a larger effort undertaken by NASA and General Dynamics to be sure that all potential space station contractors and other aerospace power system designers understand and can comfortably use this technology, which is now widely used in the commercial sector. In this paper, we will examine control requirements, stability, and operational modes; and their hardware impacts from an integrated system point of view. The current space station PMAD system will provide the overall requirements model to develop an understanding of the performance of this type of system with regard to: (1) regulation; (2) power bus stability and voltage control; (3) source impedance; (4) transient response; (5) power factor effects, and (6) limits and overloads
Finite-size Scaling and Universality above the Upper Critical Dimensionality
According to renormalization theory, Ising systems above their upper critical
dimensionality d_u = 4 have classical critical behavior and the ratio of
magnetization moments Q = ^2 / has the universal value 0.456947...
However, Monte Carlo simulations of d = 5 Ising models have been reported which
yield strikingly different results, suggesting that the renormalization
scenario is incorrect. We investigate this issue by simulation of a more
general model in which d_u < 4, and a careful analysis of the corrections to
scaling. Our results are in a perfect agreement with the renormalization theory
and provide an explanation of the discrepancy mentioned.Comment: 5 pages RevTeX, 1 PostScript figure. Accepted for publication in
Physical Review Letter
Quantum States of Neutrons in Magnetic Thin Films
We have studied experimentally and theoretically the interaction of polarized
neutrons with magnetic thin films and magnetic multilayers. In particular, we
have analyzed the behavior of the critical edges for total external reflection
in both cases. For a single film we have observed experimentally and
theoretically a simple behavior: the critical edges remain fixed and the
intensity varies according to the angle between the polarization axis and the
magnetization vector inside the film. For the multilayer case we find that the
critical edges for spin up and spin down polarized neutrons move towards each
other as a function of the angle between the magnetization vectors in adjacent
ferromagnetic films. Although the results for multilayers and single thick
layers appear to be different, in fact the same spinor method explains both
results. An interpretation of the critical edges behavior for the multilyers as
a superposition of ferromagnetic and antifferomagnetic states is given.Comment: 6 pages, 5 figure
New skeletal tuberculosis cases in past populations from Western Hungary (Transdanubia)
The distribution, antiquity and epidemiology of tuberculosis (TB) have previously been studied in osteoarchaeological material in the eastern part of Hungary, mainly on the Great Plain. The purpose of this study is to map the occurrence of skeletal TB in different centuries in the western part of Hungary, Transdanubia, and to present new cases we have found. Palaeopathological analysis was carried out using macroscopic observation supported by radiographic and molecular methods. A large human osteoarchaeological sample (n = 5684) from Transdanubian archaeological sites ranging from the 2nd to the 18th centuries served as a source of material. Spinal TB was observed in seven individuals (in three specimens with Pott's disease two of which also had cold abscess) and hip TB was assumed in one case. The results of DNA for Mycobacterium tuberculosis were positive in seven of the eight cases identified by paleopathology, and negative in the assumed case of hip TB. However, the molecular results are consistent with highly fragmented DNA, which limited further analysis. Based on the present study and previously published cases, osteotuberculosis was found in Transdanubia mainly during the 9th–13th centuries. However, there are no signs of TB in many other 9th–13th century sites, even in those that lie geographically close to those where osteotuberculous cases were found. This may be due to a true absence of TB caused by the different living conditions, way of life, or origin of these populations. An alternative explanation is that TB was present in some individuals with no typical paleopathology, but that death occurred before skeletal morphological features could develop
Zero-variance principle for Monte Carlo algorithms
We present a general approach to greatly increase at little cost the
efficiency of Monte Carlo algorithms. To each observable to be computed we
associate a renormalized observable (improved estimator) having the same
average but a different variance. By writing down the zero-variance condition a
fundamental equation determining the optimal choice for the renormalized
observable is derived (zero-variance principle for each observable separately).
We show, with several examples including classical and quantum Monte Carlo
calculations, that the method can be very powerful.Comment: 9 pages, Latex, to appear in Phys. Rev. Let
Advancing the Evidence Base of Sport for Development: A New Open-Access, Peer-Reviewed Journal
We are pleased to release the first edition of the Journal of Sport for Development (JSFD) and we would like to take this opportunity to briefly describe its origins and objectives. In doing so, we endeavour to clarify for researchers, implementers, funders and policy-makers how we believe JSFD fits into the expanding sport for development (SFD) landscape. It is widely accepted that the United Nations International Year of Sport and Physical Education (IYSPE) in 2005 was an advocacy success and sparked a mass expansion in the SFD sector.1 This built on several previous international resolutions that recognised recreational play as a human right and emphasised the social potential of sport.2-9 Over the last decade, SFD has enjoyed widespread and international growth, in terms of resources, constituents, and public awareness.10 During this period several entities have attempted to define and demarcate the SFD sector. We believe that establishing a common definition is a critical step towards unifying a diverse range of stakeholders, many of which separately articulate the role of sport for social change and peace. However, we prefer to view these areas as integral parts of the sector and have adapted a previously described broad and inclusive definition for SFD
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