5,715 research outputs found
Stability of Magneto-optical Traps with Large Field Gradients: Limits on the Tight Confinement of Single Atoms
We report measurements of the stability of magneto-optical traps (MOTs) for neutral atoms in the limit of tight confinement of a single atom. For quadrupole magnetic field gradients at the trap center greater than ∼1 kG/cm, we find that stochastic diffusion of atoms out of the trapping volume becomes the dominant particle loss mechanism, ultimately limiting the MOT size to greater than ∼5 μm. We measured and modeled the diffusive loss rate as a function of laser power, detuning, and field gradient for trapped cesium atoms. In addition, for as few as two atoms, the collisional loss rates become very high for tightly confined traps, allowing the direct observation of isolated two-body atomic collisions in a MOT
30 Vragen en antwoorden over fosfaat in relatie tot landbouw en milieu
Deze uitgave geeft een antwoord op veel gestelde vragen over fosfaat. Het betreft zowel algemene als landbouwkundige en milieukundige vragen. Deze publicatie is opgesteld voor beleidsmedewerkers van de rijksoverheid en regionale overheden (provincies, waterschappen), maatschappelijke organisaties en voor landbouwers en hun organisaties. In feite voor allen die met fosfaatbeleid te maken hebben. Van nature zit er weinig fosfor in de bodem, zo’n 0,01 tot 0,1%. Fosfor wordt aangevoerd via veevoer en kunstmest. In Nederland wordt meer fosfaat aangevoerd dan afgevoerd. Er is dan ook sprake van een overschot aan fosfaat. Het overgrote deel van het overschot komt op landbouwgronden terecht. De fosfaatophoping in de bodem varieerde in de afgelopen decennia tussen de 80 en 40 miljoen kilo per jaar. Door uit- en afspoeling van fosfaten uit de bodem ontstaan negatieve effecten voor waterecosystemen, zoals een sterke groei van fytoplankton ("algenbloei”). Als gevolg hiervan verdwijnen waterplanten door lichtgebrek en sterven vissen door zuurstofgebrek. Blauwalgen halen zelfs regelmatig de media vanwege de giftige stoffen die zij afscheiden. Het terugdringen van het fosfaatoverschot is dan ook een van de hoofddoelen van het Nederlandse mestbeleid
Equipotential Surfaces and Lagrangian points in Non-synchronous, Eccentric Binary and Planetary Systems
We investigate the existence and properties of equipotential surfaces and
Lagrangian points in non-synchronous, eccentric binary star and planetary
systems under the assumption of quasi-static equilibrium. We adopt a binary
potential that accounts for non-synchronous rotation and eccentric orbits, and
calculate the positions of the Lagrangian points as functions of the mass
ratio, the degree of asynchronism, the orbital eccentricity, and the position
of the stars or planets in their relative orbit. We find that the geometry of
the equipotential surfaces may facilitate non-conservative mass transfer in
non-synchronous, eccentric binary star and planetary systems, especially if the
component stars or planets are rotating super-synchronously at the periastron
of their relative orbit. We also calculate the volume-equivalent radius of the
Roche lobe as a function of the four parameters mentioned above. Contrary to
common practice, we find that replacing the radius of a circular orbit in the
fitting formula of Eggleton (1983) with the instantaneous distance between the
components of eccentric binary or planetary systems does not always lead to a
good approximation to the volume-equivalent radius of the Roche-lobe. We
therefore provide generalized analytic fitting formulae for the
volume-equivalent Roche lobe radius appropriate for non-synchronous, eccentric
binary star and planetary systems. These formulae are accurate to better than
1% throughout the relevant 2-dimensional parameter space that covers a dynamic
range of 16 and 6 orders of magnitude in the two dimensions.Comment: 12 pages, 10 figures, 2 Tables, Accepted by the Astrophysical Journa
Interacting Binaries with Eccentric Orbits. Secular Orbital Evolution Due To Conservative Mass Transfer
We investigate the secular evolution of the orbital semi-major axis and
eccentricity due to mass transfer in eccentric binaries, assuming conservation
of total system mass and orbital angular momentum. Assuming a delta function
mass transfer rate centered at periastron, we find rates of secular change of
the orbital semi-major axis and eccentricity which are linearly proportional to
the magnitude of the mass transfer rate at periastron. The rates can be
positive as well as negative, so that the semi-major axis and eccentricity can
increase as well as decrease in time. Adopting a delta-function mass-transfer
rate of 10^{-9} M_\sun {\rm yr}^{-1} at periastron yields orbital evolution
timescales ranging from a few Myr to a Hubble time or more, depending on the
binary mass ratio and orbital eccentricity. Comparison with orbital evolution
timescales due to dissipative tides furthermore shows that tides cannot, in all
cases, circularize the orbit rapidly enough to justify the often adopted
assumption of instantaneous circularization at the onset of mass transfer. The
formalism presented can be incorporated in binary evolution and population
synthesis codes to create a self-consistent treatment of mass transfer in
eccentric binaries.Comment: 16 pages, 8 figures, Accepted by The Astrophysical Journa
Weather radar for urban hydrological applications: lessons learnt and research needs identified from 4 pilot catchments in North-West Europe
International audienceThis study investigates the impact of rainfall estimates of different spatial resolutions on the hydraulic outputs of the models of four of the EU RainGain project’s pilot locations (the Cranbrook catchment (UK), the Herent catchment (Belgium), the Morée-Sausset catchment (France) and the Kralingen District (The Netherlands)). Two storm events, one convective and one stratiform, measured by a polarimetric X-band radar located in Cabauw (The Netherlands) were selected for analysis. The original radar estimates, at 100 m and 1 min resolutions, were aggregated to a spatial resolution of 1000 m. These estimates were then applied to the high-resolution semi-distributed hydraulic models of the four urban catchments, all of which have similar size (between 5 and 8 km2), but different morphological, hydrological and hydraulic characteristics. When doing so, methodologies for standardising rainfall inputs and making results comparable were implemented. The response of the different catchments to rainfall inputs of varying spatial resolution is analysed in the light of model configuration, catchment and storm characteristics. Rather surprisingly, the results show that for the two events under consideration the spatial resolution (i.e. 100 m vs 1000 m) of rainfall inputs does not have a significant influence on the outputs of urban drainage models. The present study will soon be extended to more storms as well as model structures and resolutions, with the final aim of identifying critical spatial-temporal resolutions for urban catchment modelling in relation to catchment and storm event characteristics
Rigorous results on the local equilibrium kinetics of a protein folding model
A local equilibrium approach for the kinetics of a simplified protein folding
model, whose equilibrium thermodynamics is exactly solvable, was developed in
[M. Zamparo and A. Pelizzola, Phys. Rev. Lett. 97, 068106 (2006)]. Important
properties of this approach are (i) the free energy decreases with time, (ii)
the exact equilibrium is recovered in the infinite time limit, (iii) the
equilibration rate is an upper bound of the exact one and (iv) computational
complexity is polynomial in the number of variables. Moreover, (v) this method
is equivalent to another approximate approach to the kinetics: the path
probability method. In this paper we give detailed rigorous proofs for the
above results.Comment: 25 pages, RevTeX 4, to be published in JSTA
Generation of a flat-top laser beam for gravitational wave detectors by means of a nonspherical Fabry-Perot resonator
We have tested a new kind of Fabry-Perot long-baseline optical resonator proposed to reduce the thermal noise sensitivity of gravitational wave interferometric detectors--the "mesa beam" cavity--whose flat top beam shape is achieved by means of an aspherical end mirror. We present the fundamental mode intensity pattern for this cavity and its distortion due to surface imperfections and tilt misalignments, and contrast the higher order mode patterns to the Gauss-Laguerre modes of a spherical mirror cavity. We discuss the effects of mirror tilts on cavity alignment and locking and present measurements of the mesa beam tilt sensitivity
Tank test of vessel entry and exit for third set of Panama locks
The Panama Canal Authority (ACP) plans to extend the capacity of and allow larger ships to the Canal by building a so-called third lane, consisting of a new set of locks parallel to the existing ones. The design ship is a so-called Post-Panamax 12000 TEU container carrier. In order to optimise the approach to and the manoeuvres within the locks, in 2007-2008 experimental research has been conducted at Flanders Hydraulics Research, Antwerp, Belgium on a 1/80 scale model of the approach lane, the lock and the design vessel. With the ship model moving along a guiding rail, lateral forces acting on the ship and movements of the vessel can be recorded during a selection of entrance and exit manoeuvres. During a number of tests, the effect of density exchange current was examined. The present article offers an overview of the test-setup with a discussion of a few interesting results of the test runs
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