107,705 research outputs found
Thermal And Mechanical Analysis of High-power Light-emitting Diodes with Ceramic Packages
In this paper we present the thermal and mechanical analysis of high-power
light-emitting diodes (LEDs) with ceramic packages. Transient thermal
measurements and thermo-mechanical simulation were performed to study the
thermal and mechanical characteristics of ceramic packages. Thermal resistance
from the junction to the ambient was decreased from 76.1 oC/W to 45.3 oC/W by
replacing plastic mould to ceramic mould for LED packages. Higher level of
thermo-mechanical stresses in the chip were found for LEDs with ceramic
packages despite of less mismatching coefficients of thermal expansion
comparing with plastic packages. The results suggest that the thermal
performance of LEDs can be improved by using ceramic packages, but the mounting
process of the high power LEDs with ceramic packages is critically important
and should be in charge of delaminating interface layers in the packages.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Phenological response of vegetation to upstream river flow in the Heihe Rive basin by time series analysis of MODIS data
Liquid and solid precipitation is abundant in the high elevation, upper reach of the Heihe River basin in northwestern China. The development of modern irrigation schemes in the middle reach of the basin is taking up an increasing share of fresh water resources, endangering the oasis and traditional irrigation systems in the lower reach. In this study, the response of vegetation in the Ejina Oasis in the lower reach of the Heihe River to the water yield of the upper catchment was analyzed by time series analysis of monthly observations of precipitation in the upper and lower catchment, river streamflow downstream of the modern irrigation schemes and satellite observations of vegetation index. Firstly, remotely sensed NDVI data acquired by Terra-MODIS are used to monitor the vegetation dynamic for a seven years period between 2000 and 2006. Due to cloud-contamination, atmospheric influence and different solar and viewing angles, however, the quality and consistence of time series of remotely sensed NDVI data are degraded. A Fourier Transform method – the Harmonic Analysis of Time Series (HANTS) algorithm – is used to reconstruct cloud- and noise-free NDVI time series data from the Terra-MODIS NDVI dataset. Modification is made on HANTS by adding additional parameters to deal with large data gaps in yearly time series in combination with a Temporal-Similarity-Statistics (TSS) method developed in this study to seek for initial values for the large gap periods. Secondly, the same Fourier Transform method is used to model time series of the vegetation phenology. The reconstructed cloud-free NDVI time series data are used to study the relationship between the water availability (i.e. the local precipitation and upstream water yield) and the evolution of vegetation conditions in Ejina Oasis from 2000 to 2006. Anomalies in precipitation, streamflow, and vegetation index are detected by comparing each year with the average year. The results showed that: the previous year total runoff had a significant relationship with the vegetation growth in Ejina Oasis and that anomalies in the spring monthly runoff of the Heihe River influenced the phenology of vegetation in the entire oasis. Warmer climate expressed by the degree-days showed positive influence on the vegetation phenology in particular during drier years. The time of maximum green-up is uniform throughout the oasis during wetter years, but showed a clear S-N gradient (downstream) during drier years
Downlink Steered Space-Time Spreading Assisted Generalised Multicarrier DS-CDMA Using Sphere-Packing-Aided Multilevel Coding
This paper presents a novel generalised Multi-Carrier Direct Sequence Code Division Multiple Access (MC DS-CDMA) system invoking smart antennas for improving the achievable performance in the downlink, as well as employing multi-dimensional Sphere Packing (SP) modulation for increasing the achievable diversity product. In this contribution, the MC DS-CDMA transmitter considered employs multiple Antenna Arrays (AA) and each of the AAs consists of several antenna elements. Furthermore, the proposed system employs both time- and frequency- (TF) domain spreading for extending the achievable capacity, when combined with a novel user-grouping technique for reducing the effects of Multiuser Interference (MUI). Moreover, in order to further enhance the system’s performance, we invoke a MultiLevel Coding (MLC) scheme, whose component codes are determined using the so-called equivalent capacity based constituent-code rate-calculation procedure invoking a 4-dimensional bit-to-SP-symbol mapping scheme. Our results demonstrate an approximately 3.8 dB Eb/N0 gain over an identical throughput scheme dispensing with SP modulation at a BER of 10?5
Faintest Galaxy Morphologies from WFPC2 Imaging of the Hawaii Survey Fields
We present very deep WFPC2 images in the F814W filter of two Hawaii
Survey fields, SSA13 and SSA22. Using these data with previous ground-based
imaging and spectroscopy, we compare the colors, star-forming properties and
morphologies of the faintest galaxies with a reference sample of bright nearby
galaxies and analyze the changes in field galaxy morphology with magnitude. Our
principal result is the identification of a new morphological class of
``chain'' galaxies at the faintest magnitudes. Based on limited spectroscopy,
we tentatively conclude that these are linearly organized giant star-forming
regions at and, if this is correct, that these are large galaxies
in the process of formation.Comment: 18 pages + 1 table of text as 1 LaTeX file (uses aastex style macros:
aaspp.sty, flushrt.sty) plus 1 uuencoded compressed tar file of 12 PostScript
figures (Figs. 3-9, 16-17, and 21-23). The remaining gray-scale plots are
available by anonymous ftp at
ftp://hubble.ifa.hawaii.edu/pub/preprints/plates To appear in the October
1995 Astronomical Journa
CMB Lensing Reconstruction on the Full Sky
Gravitational lensing of the microwave background by the intervening dark
matter mainly arises from large-angle fluctuations in the projected
gravitational potential and hence offers a unique opportunity to study the
physics of the dark sector at large scales. Studies with surveys that cover
greater than a percent of the sky will require techniques that incorporate the
curvature of the sky. We lay the groundwork for these studies by deriving the
full sky minimum variance quadratic estimators of the lensing potential from
the CMB temperature and polarization fields. We also present a general
technique for constructing these estimators, with harmonic space convolutions
replaced by real space products, that is appropriate for both the full sky
limit and the flat sky approximation. This also extends previous treatments to
include estimators involving the temperature-polarization cross-correlation and
should be useful for next generation experiments in which most of the
additional information from polarization comes from this channel due to
sensitivity limitations.Comment: Accepted for publication in Phys. Rev. D; typos correcte
Gluon GPDs and Exclusive Photoproduction of a Quarkonium in Forward Region
Forward photoproduction of can be used to extract Generalized Parton
Distributions(GPD's) of gluons. We analyze the process at twist-3 level and
study relevant classifications of twist-3 gluon GPD's. At leading power or
twist-2 level the produced is transversely polarized. We find that at
twist-3 the produced is longitudinally polarized. Our study shows that
in high energy limit the twist-3 amplitude is only suppressed by the inverse
power of the heavy quark mass relatively to the twist-2 amplitude. This
indicates that the power correction to the cross-section of unpolarized
can have a sizeable effect. We have also derived the amplitude of the
production of at twist-3, but the result contains end-point
singularities. The production of other quarkonia has been briefly discussed.Comment: Discussions of results are adde
An Extremely Luminous Galaxy at z=5.74
We report the discovery of an extremely luminous galaxy lying at a redshift
of z=5.74, SSA22-HCM1. The object was found in narrowband imaging of the SSA22
field using a 105 Angstrom bandpass filter centered at 8185 Angstroms during
the course of the Hawaii narrowband survey using LRIS on the 10 m Keck II
Telescope, and was identified by the equivalent width of the emission
W_lambda(observed)=175 Angstroms, flux = 1.7 x 10^{-17} erg cm^{-2} s^{-1}).
Comparison with broadband colors shows the presence of an extremely strong
break (> 4.2 at the 2 sigma level) between the Z band above the line, where the
AB magnitude is 25.5, and the R band below, where the object is no longer
visible at a 2 sigma upper limit of 27.1 (AB mags). These properties are only
consistent with this object's being a high-z Ly alpha emitter. A 10,800 s
spectrum obtained with LRIS yields a redshift of 5.74. The object is similar in
its continuum shape, line properties, and observed equivalent width to the
z=5.60 galaxy, HDF 4-473.0, as recently described by Weymann et al. (1998), but
is 2-3 times more luminous in the line and in the red continuum. For H_0 = 65
km s^{-1} Mpc^{-1} and q_0 = (0.02, 0.5) we would require star formation rates
of around (40, 7) solar masses per year to produce the UV continuum in the
absence of extinction.Comment: 5 pages, 4 figures, Latex with emulateapj style file; to appear in
the Astrophysical Journal (Letters
Fluctuations of the vacuum energy density of quantum fields in curved spacetime via generalized zeta functions
For quantum fields on a curved spacetime with an Euclidean section, we derive
a general expression for the stress energy tensor two-point function in terms
of the effective action. The renormalized two-point function is given in terms
of the second variation of the Mellin transform of the trace of the heat kernel
for the quantum fields. For systems for which a spectral decomposition of the
wave opearator is possible, we give an exact expression for this two-point
function. Explicit examples of the variance to the mean ratio of the vacuum energy density of a
massless scalar field are computed for the spatial topologies of and , with results of , and
respectively. The large variance signifies the importance
of quantum fluctuations and has important implications for the validity of
semiclassical gravity theories at sub-Planckian scales. The method presented
here can facilitate the calculation of stress-energy fluctuations for quantum
fields useful for the analysis of fluctuation effects and critical phenomena in
problems ranging from atom optics and mesoscopic physics to early universe and
black hole physics.Comment: Uses revte
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