904 research outputs found

    Suppressing low-order eigenmodes with local control for deformable mirrors

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    To improve the mechanical characteristics of actively controlled continuous faceplate deformable mirrors in adaptive optics, a strategy for reducing crosstalk between adjacent actuators and for suppressing low-order eigenmodes is proposed. The strategy can be seen as extending Saint-Venant’s principle beyond the static case, for small local families of actuators. An analytic model is presented, from which we show the feasibility of the local control. Also, we demonstrate how eigenmodes and eigenfrequencies are affected by mirror parameters, such as thickness, diameter, Young’s modulus, Poisson’s ratio, and density. This analysis is used to evaluate the design strategy for a large deformable mirror, and how many actuators are needed within a family

    Die Proteine

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    Layoutgetreues Digitalisat der Ausg.: Marburg : Elwert, 1900 tandort: Fachgebiet fĂŒr Geschichte der Medizin (192) Signatur: 793/IV Provenienz: Behring, Emil vo

    Longitude : a privacy-preserving location sharing protocol for mobile applications

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    Location sharing services are becoming increasingly popular. Although many location sharing services allow users to set up privacy policies to control who can access their location, the use made by service providers remains a source of concern. Ideally, location sharing providers and middleware should not be able to access users’ location data without their consent. In this paper, we propose a new location sharing protocol called Longitude that eases privacy concerns by making it possible to share a user’s location data blindly and allowing the user to control who can access her location, when and to what degree of precision. The underlying cryptographic algorithms are designed for GPS-enabled mobile phones. We describe and evaluate our implementation for the Nexus One Android mobile phone

    Intraplate strike-slip faulting in East Antarctica: new geophysical views from the Rennick Graben and Wilkes Subglacial Basin

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    Intraplate strike-slip faulting can occur in association with different geodynamic settings, ranging from subduction-related to collision and extension. Geological and geophysical research in Northern Victoria Land (NVL) in East Antarctica, has led to the interpretation that major fault systems that were active during the early Paleozoic Ross Orogen were reactivated much later as right-lateral intraplate strike-slip fault systems from ca 48 Ma, and that these faults may have accomodated differential shear along evolving oceanic transform faults located between southeastern Australia and Tasmania. One of the main structures in NVL that has been inferred to relate to this unusual geodynamic process is the Rennick Graben (RG), but its age, extent and kinematics have remained both poorly constrained and controversial. Even less well-understood are the potential tectonic linkages between the RG and the deep sub-basins that lie within the much broader Wilkes Subglacial Basin (WSB), in the hinterland of the Transantarctic Mountains. Here, we present new interpretations of enhanced potential field images derived from aeromagnetic and airborne and land-gravity observations to help constrain the extent and architecture of the RG and the sub-basins within the WSB. We show that the RG is a composite pull-part basin that extends from the Oates Coast towards the margin of the Ross Sea Rift, part of the West Antarctic Rift System. We suggest that the more cratonic WSB region was also affected by extensional and transtensional processes, the latter potentially linked to an evolving and distributed left-lateral Paleogene(?) strike-slip plate boundary between East Antarctica and Australia

    Mass fractionation of noble gases in synthetic methane hydrate : implications for naturally occurring gas hydrate dissociation

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    This paper is not subject to U.S. copyright. The definitive version was published in Chemical Geology 339 (2013): 242-250, doi:10.1016/j.chemgeo.2012.09.033.As a consequence of contemporary or longer term (since 15 ka) climate warming, gas hydrates in some settings may presently be dissociating and releasing methane and other gases to the ocean–atmosphere system. A key challenge in assessing the impact of dissociating gas hydrates on global atmospheric methane is the lack of a technique able to distinguish between methane recently released from gas hydrates and methane emitted from leaky thermogenic reservoirs, shallow sediments (some newly thawed), coal beds, and other sources. Carbon and deuterium stable isotopic fractionation during methane formation provides a first-order constraint on the processes (microbial or thermogenic) of methane generation. However, because gas hydrate formation and dissociation do not cause significant isotopic fractionation, a stable isotope-based hydrate-source determination is not possible. Here, we investigate patterns of mass-dependent noble gas fractionation within the gas hydrate lattice to fingerprint methane released from gas hydrates. Starting with synthetic gas hydrate formed under laboratory conditions, we document complex noble gas fractionation patterns in the gases liberated during dissociation and explore the effects of aging and storage (e.g., in liquid nitrogen), as well as sampling and preservation procedures. The laboratory results confirm a unique noble gas fractionation pattern for gas hydrates, one that shows promise in evaluating modern natural gas seeps for a signature associated with gas hydrate dissociation.Partial support for this research was provided by Interagency Agreements DE-FE0002911 and DE-NT0006147 between the U.S. Geological Survey Gas Hydrates Project and the U.S. Department of Energy's Methane Hydrates Research and Development Program

    Corrigendum to “Insights into methane dynamics from analysis of authigenic carbonates and chemosynthetic mussels at newly-discovered Atlantic Margin seeps” [Earth Planet. Sci. Lett. 449 (2016) 332–344]

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    This paper is not subject to U.S. copyright. The definitive version was published in Earth and Planetary Science Letters 475 (2017): 268, doi:10.1016/j.epsl.2017.07.037

    Parametrically-stimulated recovery of a microwave signal using standing spin-wave modes of a magnetic film

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    The phenomenon of storage and parametrically-stimulated recovery of a microwave signal in a ferrite film has been studied both experimentally and theoretically. The microwave signal is stored in the form of standing spin-wave modes existing in the film due to its finite thickness. Signal recovery is performed by means of frequency-selective amplification of one of these standing modes by double- requency parametric pumping process. The time of recovery, as well as the duration and magnitude of the recovered signal, depend on the timing and amplitudes of both the input and pumping pulses. A mean-field theory of the recovery process based on the competitive interaction of the signal-induced standing spin-wave mode and thermal magnons with the parametric pumping field is developed and compared to the experimental data
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