2,758 research outputs found
Analysis of Nd3+:glass, solar-pumped, high-powr laser systems
The operating characteristics of Nd(3+):glass lasers energized by a solar concentrator were analyzed for the hosts YAG, silicate glass, and phosphate glass. The modeling is based on the slab zigzag laser geometry and assumes that chemical hardening methods for glass are successful in increasing glass hardness by a factor of 4. On this basis, it was found that a realistic 1-MW solar-pumped laser might be constructed from phosphate glass 4 sq m in area and 2 mm thick. If YAG were the host medium, a 1-MW solar-pumped laser need only be 0.5 sq m in area and 0.5 cm thick, which is already possible. In addition, Nd(3+) doped glass fibers were found to be excellent solar-pumped laser candidates. The small diameter of fibers eliminates thermal stress problems, and if their diameter is kept small (10 microns), they propagate a Gaussian single mode which can be expanded and transmitted long distances in space. Fiber lasers could then be used for communications in space or could be bundled and the individual beams summed or phase-matched for high-power operation
String amplitudes in the Hpp-wave limit of AdS3xS3
We compute string amplitudes on pp-waves supported by NS-NS 3-form fluxes and
arising in the Penrose limit of AdS3xS3xM. We clarify the role of the
non-chiral accidental SU(2) symmetry of the background. We comment on the
extension of our results to the superstring and propose a holographic formula
in the BMN limit of the AdS3/CFT2 correspondence valid for any correlator.Comment: Latex,no figures, 47 p
The Economic Value of Basin Protection to Improve the Quality and Reliability of Potable Water Supply: Some Evidence from Ecuador
This study estimates the willingness to pay (WTP) of Loja’s households to protect two micro-basins that supply over 40 percent of potable water to the city. Results indicate that households have an average WTP of $5.80 per month, which corresponds to a 25 percent increase in the self-reported monthly water bill, to preserve the basins.Basin protection, contingent valuation, Loja, Ecuador, Environmental Economics and Policy, Land Economics/Use,
Hamiltonian and physical Hilbert space in polymer quantum mechanics
In this paper, a version of polymer quantum mechanics, which is inspired by
loop quantum gravity, is considered and shown to be equivalent, in a precise
sense, to the standard, experimentally tested, Schroedinger quantum mechanics.
The kinematical cornerstone of our framework is the so called polymer
representation of the Heisenberg-Weyl (H-W) algebra, which is the starting
point of the construction. The dynamics is constructed as a continuum limit of
effective theories characterized by a scale, and requires a renormalization of
the inner product. The result is a physical Hilbert space in which the
continuum Hamiltonian can be represented and that is unitarily equivalent to
the Schroedinger representation of quantum mechanics. As a concrete
implementation of our formalism, the simple harmonic oscillator is fully
developed.Comment: 19 pages, 2 figures. Comments and references added. Version to be
published in CQ
New design model for brittle failure in the parallel-to-grain direction of timber connections with large diameter fasteners
Timber connections with steel dowel-type fasteners may fail in ductile or brittle ways. The design of this joints was traditionally focused on ductile failure, assuming that brittle failure could be avoided by means of
geometrical limitations. However, recent studies demonstrated that these assumptions do not always guarantee ductile failure. A new design model for timber connections with small diameter dowel-type fasteners is provided in this paper. Its improved accuracy is demonstrated by analysing an extensive database of experimental tests and comparing its prediction accuracy to that of existing models
Geochemistry and Geochronology of the Guajira Eclogites, northern Colombia : evidence of a metamorphosed primitive Cretaceous Caribbean Island-arc
The chemical composition of eclogites, found as boulders in a Tertiary conglomerate from the Guajira Peninsula, Colombia suggests that these rocks are mainly metamorphosed basaltic andesites. They are depleted in LILE elements compared to MORB, have a negative Nb-anomaly and flat to enriched REE patterns, suggesting that their protoliths evolved in a subduction related tectonic setting. They show island-arc affinities and are similar to primitive islandarc rocks described in the Caribbean. The geochemical characteristics are comparable to low-grade greenschists from the nearby Etpana Terrane, which are interpreted as part of a Cretaceous intra-oceanic arc. These data support evidence that the eclogites and the Etpana terrane rocks formed from the same volcano-sedimentary sequence. Part of this sequence was accreted onto the margin and another was incorporated into the subduction channel and metamorphosed at eclogite facies conditions. 40Ar-39Ar ages of 79.2±1.1Ma and 82.2±2.5Ma determined on white micas, separated from two eclogite samples, are interpreted to be related to the cooling of the main metamorphic event. The formation of a common volcano-sedimentary protolith and subsequent metamorphism of these units record the ongoing Late Cretaceous continental subduction of the South American margin within the Caribbean intra-oceanic arc subduction zone. This gave way to an arc-continent collision between the Caribbean and the South American plates, where this sequence was exhumed after the Campanian
Voltage rectification by a SQUID ratchet
We argue that the phase across an asymmetric dc SQUID threaded by a magnetic
flux can experience an effective ratchet (periodic and asymmetric) potential.
Under an external ac current, a rocking ratchet mechanism operates whereby one
sign of the time derivative of the phase is favored. We show that there exists
a range of parameters in which a fixed sign (and, in a narrower range, even a
fixed value) of the average voltage across the ring occurs, regardless of the
sign of the external current dc component.Comment: 4 pages, 4 EPS figures, uses psfig.sty. Revised version, to appear in
Physical Review Letters (26 August 1996
Epidemiology of salmonellosis in sow units of Catalonia (Spain)
A bacteriological study aimed to the detection of sows excreting Salmonella was carried out in 74 sow units. Individual faecal samples were collected and a detailed questionnaire was filled in each farm and the data gathered were used in a bivariate logistic regression analysis to determine risk factors for Salmonella positivity. In 18 farms (24.32 %) Salmonella carriers were detected. Of the total 1480 samples analysed, 50 were positive (3.38 %), accounting for 11 different serotypes. Within positive farms, 13.8 % of sows were positive. Epidemiologic analysis of the results showed three main risk factors: to have open-flushed drainage of slurry, rodent control, and the number of sows in the unit. Our results showed that, in infected farms, the proportion of sows actively excreting Salmonella was considerably high (3.38 %). This fact, and the risk factors detected should be taken into account when designing effective plans for the control of salmonellosis in sows
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