1,905,261 research outputs found

    Status report of the three phase 25 kA, 1.5 kW thermally switched superconducting rectifier, transformer and switches

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    A 25 kA, 1.5 kW superconducting rectifier system has been developed. This rectifier system working like an a.c.-d.c, converter with a primary current of 35 A at 0.1Hz, will energize a 25 kA coil with an average power of 5.4 MJ/hr and a proposed energy efficiency of at least 96%. Such a highly efficient device might work instead of a 'normal' rectifier and a pair of 25 kA current leads with its energy loss of at least 2 W/kA. The 25 kA current step-up transformer has been tested succesfully concerning its maximum current (26.4 kA) and a.c. losses (2 W at 25 kA and 0.1Hz). A conductor for the 25 kA switches has been manufactured and processed into the switching system. Their construction is described

    Timing and causes of North African wet phases during the last glacial period and implications for modern human migration

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    We present the first speleothem-derived central North Africa rainfall record for the last glacial period. The record reveals three main wet periods at 65-61‚ÄČka, 52.5-50.5‚ÄČka and 37.5-33‚ÄČka that lead obliquity maxima and precession minima. We find additional minor wet episodes that are synchronous with Greenland interstadials. Our results demonstrate that sub-tropical hydrology is forced by both orbital cyclicity and North Atlantic moisture sources. The record shows that after the end of a Saharan wet phase around 70‚ÄČka ago, North Africa continued to intermittently receive substantially more rainfall than today, resulting in favourable environmental conditions for modern human expansion. The encounter and subsequent mixture of Neanderthals and modern humans ‚Äď which, on genetic evidence, is considered to have occurred between 60 and 50‚ÄČka ‚Äď occurred synchronously with the wet phase between 52.5 and 50.5‚ÄČka. Based on genetic evidence the dispersal of modern humans into Eurasia started less than 55‚ÄČka ago. This may have been initiated by dry conditions that prevailed in North Africa after 50.5‚ÄČka. The timing of a migration reversal of modern humans from Eurasia into North Africa is suggested to be coincident with the wet period between 37.5 and 33‚ÄČka

    Magnetization and Loss Measurements of YBCO CORC and Roebel Cables for Accelerators using ¬ĪT dipole Susceptometer

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    This CORC sample Magnetization reaches about 900 kA/m (cable volume) and 2200 kA/m (strand volume) near injection This Roebel cable reaches about 2000 kA/m (cable volume) and 2400 kA/m (strand volume) near injection These values agree well with individual tape values if (i) we normalize to strand volume, Correct CORC for twisting These values can be compared to NbTi Minj = 10 kA/m b3 = 3-6 units Nb3Sn Minj = 100 kA/m b3 = 30 units Here Minj is 100 X higher than NbTi and 10 X higher than NbTi Not insurmountable, but must be considered when thinking about particle beam steering magnet qualit

    Monophyly of brachiopods and phoronids: reconciliation of molecular evidence with Linnaean classification (the subphylum Phoroniformea nov.)

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    Molecular phylogenetic analyses of aligned 18S rDNA gene sequences from articulate and inarticulate brachiopods representing all major extant lineages, an enhanced set of phoronids and several unrelated protostome taxa, confirm previous indications that in such data, brachiopod and phoronids form a well-supported clade that (on previous evidence) is unambiguously affiliated with protostomes rather than deuterostomes. Within the brachiopod-phoronid clade, an association between phoronids and inarticulate brachiopods is moderately well supported, whilst a close relationship between phoronids and craniid inarticulates is weakly indicated. Brachiopod-phoronid monophyly is reconciled with the most recent Linnaean classification of brachiopods by abolition of the phylum Phoronida and rediagnosis of the phylum Brachiopoda to include tubiculous, shell-less forms. Recognition that brachiopods and phoronids are close genealogical allies of protostome phyla such as molluscs and annelids, but are much more distantly related to deuterostome phyla such as echinoderms and chordates, implies either (or both) that the morphology and ontogeny of blastopore, mesoderm and coelom formation have been widely misreported or misinterpreted, or that these characters have been subject to extensive homoplasy. This inference, if true, undermines virtually all morphology-based reconstructions of phylogeny made during the past century or more

    Evolution of postglacial vegetation in the Western Laptev Sea region (Siberian Arctic)

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    On the basis of a detailed study of the pollen-spore spectra and a detailed radiocarbon chronology of a sediment core obtained from the western outer Laptev Sea shelf, the long-term and high-resolution changes of vegetation in the northwestern Laptev Sea region were reconstructed for the last 12.0 cal. ka. Three major phases in the development of paleoenvironments and vegetation on the surrounding hinterland and the exposed Laptev Sea shelf were recognized. The period between 12.0 and 10.3 cal. ka BP was characterized by predominance of grass-sedge and moss tundra. Rapid expansion of herbaceous tundra with dwarf birch and alder started at about 10.3 and lasted until 8.0 cal. ka. Pollen spectra from this time interval evidence the warmest and most favorable climate conditions. After 8.0 cal. ka mosses and lichen vegetation with scare herbs typical for the modern arctic tundra dominated. German: Auf der Grundlage detaillierter Pollen- und Sporenspektren aus einem 14C-datierten Sedimentkern vom √§u√üeren Schelf des westlichen Laptewmeeres wurden die langfristigen und hochaufgel√∂sten Ver√§nderungen der Vegetation in den letzten 12 cal. ka in der nordwestlichen Laptewmeer-Region rekonstruiert. Es wurden drei Hauptphasen der Entwicklung von Umwelt und Vegetation im umgebenden Hinterland erkannt. Die Zeit zwischen 12,0 und 10,3 cal. ka BP war charakterisiert durch Riedgras- und Moos-Tundra. Rasche Ausdehnung der Kraut-Tundra mit Zwergbirke und Erle begann etwa um 10,3 cal. ka und dauerte bis 8,0 cal. ka. Pollenspektren aus diesem Zeitintervall beschreiben die w√§rmsten und besten Klimabedingungen. Nach 8,0 cal. ka dominierte die f√ľr die heutige arktische Tundra typische Moos- und Flechtenvegetation mit wenigen Kr√§utern

    On the exponential convergence of the Kaczmarz algorithm

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    The Kaczmarz algorithm (KA) is a popular method for solving a system of linear equations. In this note we derive a new exponential convergence result for the KA. The key allowing us to establish the new result is to rewrite the KA in such a way that its solution path can be interpreted as the output from a particular dynamical system. The asymptotic stability results of the corresponding dynamical system can then be leveraged to prove exponential convergence of the KA. The new bound is also compared to existing bounds

    Domestic chickens activate a piRNA defense against avian leukosis virus

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    PIWI-interacting RNAs (piRNAs) protect the germ line by targeting transposable elements (TEs) through the base-pair complementarity. We do not know how piRNAs co-evolve with TEs in chickens. Here we reported that all active TEs in the chicken germ line are targeted by piRNAs, and as TEs lose their activity, the corresponding piRNAs erode away. We observed de novo piRNA birth as host responds to a recent retroviral invasion. Avian leukosis virus (ALV) has endogenized prior to chicken domestication, remains infectious, and threatens poultry industry. Domestic fowl produce piRNAs targeting ALV from one ALV provirus that was known to render its host ALV resistant. This proviral locus does not produce piRNAs in undomesticated wild chickens. Our findings uncover rapid piRNA evolution reflecting contemporary TE activity, identify a new piRNA acquisition modality by activating a pre-existing genomic locus, and extend piRNA defense roles to include the period when endogenous retroviruses are still infectious. DOI: http://dx.doi.org/10.7554/eLife.24695.00

    Geometric phases for wave packets in a uniform magnetic field

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    A wave packet of a charged particle always make cyclic circular motion in a uniform magnetic field, just like a classical particle. The nonadiabatic geometric phase for an arbitrary wave packet can be expressed in terms of the mean value of a number operator. For a large class of wave packets, the geometric phase is proportional to the magnetic flux encircled by the orbit of the wave packet. For more general wave packets, however, the geometric phase contains an extra term.Comment: REVTeX4, 7 pages, no figur
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