669 research outputs found
Compact graphene mode-locked wavelength-tunable erbium-doped fiber lasers: from all anomalous dispersion towards all normal dispersion
Soliton operation and soliton wavelength tuning of erbium-doped fiber lasers
mode locked with atomic layer graphene was experimentally investigated under
various cavity dispersion conditions. It was shown that not only wide range
soliton wavelength tuning but also soltion pulse width variation could be
obtained in the fiber lasers. Our results show that the graphene mode locked
erbium-doped fiber lasers provide a compact, user friendly and low cost
wavelength tunable ultrahsort pulse source
Very High Resolution Solar X-ray Imaging Using Diffractive Optics
This paper describes the development of X-ray diffractive optics for imaging
solar flares with better than 0.1 arcsec angular resolution. X-ray images with
this resolution of the \geq10 MK plasma in solar active regions and solar
flares would allow the cross-sectional area of magnetic loops to be resolved
and the coronal flare energy release region itself to be probed. The objective
of this work is to obtain X-ray images in the iron-line complex at 6.7 keV
observed during solar flares with an angular resolution as fine as 0.1 arcsec -
over an order of magnitude finer than is now possible. This line emission is
from highly ionized iron atoms, primarily Fe xxv, in the hottest flare plasma
at temperatures in excess of \approx10 MK. It provides information on the flare
morphology, the iron abundance, and the distribution of the hot plasma.
Studying how this plasma is heated to such high temperatures in such short
times during solar flares is of critical importance in understanding these
powerful transient events, one of the major objectives of solar physics. We
describe the design, fabrication, and testing of phase zone plate X-ray lenses
with focal lengths of \approx100 m at these energies that would be capable of
achieving these objectives. We show how such lenses could be included on a
two-spacecraft formation-flying mission with the lenses on the spacecraft
closest to the Sun and an X-ray imaging array on the second spacecraft in the
focal plane \approx100 m away. High resolution X-ray images could be obtained
when the two spacecraft are aligned with the region of interest on the Sun.
Requirements and constraints for the control of the two spacecraft are
discussed together with the overall feasibility of such a formation-flying
mission
Bubbling AdS and droplet descriptions of BPS geometries in IIB supergravity
This paper focuses on supergravity duals of BPS states in N=4 super
Yang-Mills. In order to describe these duals, we begin with a sequence of
breathing mode reductions of IIB supergravity: first on S^3, then S^3 x S^1,
and finally on S^3 x S^1 x CP^1. We then follow with a complete supersymmetry
analysis, yielding 1/8, 1/4 and 1/2 BPS configurations, respectively (where in
the last step we take the Hopf fibration of S^3). The 1/8 BPS geometries, which
have an S^3 isometry and are time-fibered over a six-dimensional base, are
determined by solving a non-linear equation for the Kahler metric on the base.
Similarly, the 1/4 BPS configurations have an S^3 x S^1 isometry and a
four-dimensional base, whose Kahler metric obeys another non-linear,
Monge-Ampere type equation.
Despite the non-linearity of the problem, we develop a universal bubbling AdS
description of these geometries by focusing on the boundary conditions which
ensure their regularity. In the 1/8 BPS case, we find that the S^3 cycle
shrinks to zero size on a five-dimensional locus inside the six-dimensional
base. Enforcing regularity of the full solution requires that the interior of a
smooth, generally disconnected five-dimensional surface be removed from the
base. The AdS_5 x S^5 ground state corresponds to excising the interior of an
S^5, while the 1/8 BPS excitations correspond to deformations (including
topology change) of the S^5 and/or the excision of additional droplets from the
base. In the case of 1/4 BPS configurations, by enforcing regularity
conditions, we identify three-dimensional surfaces inside the four-dimensional
base which separate the regions where the S^3 shrinks to zero size from those
where the S^1 shrinks.Comment: 94 pages, 6 figures, latex, typos corrected, references added, one
new Appendi
Composing JSON-based Web APIs
International audienceThe development of Web APIs has become a discipline that companies have to master to succeed in the Web. The so-called API economy is pushing companies to provide access to their data by means of Web APIs, thus requiring web developers to study and integrate such APIs into their applications. The exchange of data with these APIs is usually performed by using JSON, a schemaless data format easy for computers to parse and use. While JSON data is easy to read, its structure is implicit, thus entailing serious problems when integrating APIs coming from di erent vendors. Web developers have therefore to understand the domain behind each API and study how they can be composed. We tackle this issue by presenting an approach able to both discover the domain of JSON-based Web APIs, and identify composition links among them. Our approach allows developers to easily visualize what is behind APIs and how they can be composed to be used in their applications
Towards the noise reduction of piezoelectrical-driven synthetic jet actuators
This paper details an experimental investigation aimed at reducing the noise output of piezoelectrical-driven synthetic jet actuators without compromising peak jet velocity. Specifically, the study considers double-chamber ('back-to-back') actuators for anti-phase noise suppression and corrugated-lobed orifices as a method to enhance turbulent mixing of the jets to suppress jet noise. The study involved the design, manufacture and bench test of interchangeable actuator hardware. Hot-wire anemometry and microphone recordings were employed to acquire velocity and noise measurements respectively for each chamber configuration and orifice plate across a range of excitation frequencies and for a fixed input voltage. The data analysis indicated a 32% noise reduction (20 dBA) from operating a singlechamber, circular orifice SJA to a double-chamber, corrugated-lobed orifice SJA at the Helmholtz resonant frequency. Results also showed there was a small reduction in peak jet velocity of 7% (~3 m/s) between these two cases based on orifices of the same discharge area. Finally, the electrical-to-fluidic power conversion efficiency of the double-chamber actuator was found to be 15% across all orifice designs at the resonant frequency; approximately double the efficiency of a single-chamber actuator. This work has thus demonstrated feasible gains in noise reduction and power efficiency through synthetic jet actuator design
New supersymmetric solutions of N=2, D=5 gauged supergravity with hyperscalars
We construct new supersymmetric solutions, including AdS bubbles, in an N=2
truncation of five-dimensional N=8 gauged supergravity. This particular
truncation is given by N=2 gauged supergravity coupled to two vector multiples
and three incomplete hypermultiplets, and was originally investigated in the
context of obtaining regular AdS bubble geometries with multiple active
R-charges. We focus on cohomogeneity-one solutions corresponding to objects
with two equal angular momenta and up to three independent R-charges.
Curiously, we find a new set of zero and negative mass solitons asymptotic to
AdS_5/Z_k, for k \ge 3, which are everywhere regular without closed timelike
curves.Comment: Latex 3 times, 42 page
Comparative genome analysis of multiple vancomycin-resistant Enterococcus faecium isolated from two fatal cases
Enterococcus faecium is both a commensal of the human intestinal tract and an opportunistic pathogen. The increasing incidence of enterococcal infections is mainly due to the ability of this organism to develop resistance to multiple antibiotics, including vancomycin. The aim of this study was to perform comparative genome analyses on four vancomycin-resistant Enterococcus faecium (VREfm) strains isolated from two fatal cases in a tertiary hospital in Malaysia. Two sequence types, ST80 and ST203, were identified which belong to the clinically important clonal complex (CC) 17. This is the first report on the emergence of ST80 strains in Malaysia. Three of the studied strains (VREr5, VREr6, VREr7) were each isolated from different body sites of a single patient (patient Y) and had different PFGE patterns. While VREr6 and VREr7 were phenotypically and genotypically similar, the initial isolate, VREr5, was found to be more similar to
VRE2 isolated from another patient (patient X), in terms of the genome contents, sequence types and phylogenomic relationship. Both the clinical records and genome sequence data suggested that patient Y was infected by multiple strains from different clones and the strain that infected patient Y could have derived from the same clone from patient X. These multidrug resistant strains harbored a number of virulence genes such as the epa locus and pilus-associated genes which could enhance their persistence. Apart from that, a homolog of E. faecalis bee locus was identified in VREr5 which might be involved in biofilm formation. Overall, our comparative genomic analyses had provided insight into the genetic relatedness, as well as the virulence potential, of the four clinical strains
An evaluation of possible mechanisms for anomalous resistivity in the solar corona
A wide variety of transient events in the solar corona seem to require
explanations that invoke fast reconnection. Theoretical models explaining fast
reconnection often rely on enhanced resistivity. We start with data derived
from observed reconnection rates in solar flares and seek to reconcile them
with the chaos-induced resistivity model of Numata & Yoshida (2002) and with
resistivity arising out of the kinetic Alfv\'en wave (KAW) instability. We find
that the resistivities arising from either of these mechanisms, when localized
over lengthscales of the order of an ion skin depth, are capable of explaining
the observationally mandated Lundquist numbers.Comment: Accepted, Solar Physic
Energy-Dependent Timing of Thermal Emission in Solar Flares
We report solar flare plasma to be multi-thermal in nature based on the
theoretical model and study of the energy-dependent timing of thermal emission
in ten M-class flares. We employ high-resolution X-ray spectra observed by the
Si detector of the "Solar X-ray Spectrometer" (SOXS). The SOXS onboard the
Indian GSAT-2 spacecraft was launched by the GSLV-D2 rocket on 8 May 2003.
Firstly we model the spectral evolution of the X-ray line and continuum
emission flux F(\epsilon) from the flare by integrating a series of isothermal
plasma flux. We find that multi-temperature integrated flux F(\epsilon) is a
power-law function of \epsilon with a spectral index (\gamma) \approx -4.65.
Next, based on spectral-temporal evolution of the flares we find that the
emission in the energy range E= 4 - 15 keV is dominated by temperatures of T=
12 - 50 MK, while the multi-thermal power-law DEM index (\gamma) varies in the
range of -4.4 and -5.7. The temporal evolution of the X-ray flux F(\epsilon,t)
assuming a multi-temperature plasma governed by thermal conduction cooling
reveals that the temperature-dependent cooling time varies between 296 and 4640
s and the electron density (n_e) varies in the range of n_e= (1.77-29.3)*10^10
cm-3. Employing temporal evolution technique in the current study as an
alternative method for separating thermal from non-thermal components in the
energy spectra, we measure the break-energy point ranging between 14 and
21\pm1.0 keV.Comment: Solar Physics, in pres
Magnetic field diagnostics and spatio-temporal variability of the solar transition region
Magnetic field diagnostics of the transition region from the chromosphere to
the corona faces us with the problem that one has to apply extreme UV
spectro-polarimetry. While for coronal diagnostic techniques already exist
through infrared coronagraphy above the limb and radio observations on the
disk, for the transition region one has to investigate extreme UV observations.
However, so far the success of such observations has been limited, but there
are various projects to get spectro-polarimetric data in the extreme UV in the
near future. Therefore it is timely to study the polarimetric signals we can
expect for such observations through realistic forward modeling.
We employ a 3D MHD forward model of the solar corona and synthesize the
Stokes I and Stokes V profiles of C IV 1548 A. A signal well above 0.001 in
Stokes V can be expected, even when integrating for several minutes in order to
reach the required signal-to-noise ratio, despite the fact that the intensity
in the model is rapidly changing (just as in observations). Often this
variability of the intensity is used as an argument against transition region
magnetic diagnostics which requires exposure times of minutes. However, the
magnetic field is evolving much slower than the intensity, and thus when
integrating in time the degree of (circular) polarization remains rather
constant. Our study shows the feasibility to measure the transition region
magnetic field, if a polarimetric accuracy on the order of 0.001 can be
reached, which we can expect from planned instrumentation.Comment: Accepted for publication in Solar Physics (4.Mar.2013), 19 pages, 9
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