94 research outputs found
Investigation into the effect of beam shape on melt pool characteristics using analytical modelling
An established analytical model is used to simulate an extended laser beam. Multiple Gaussian sources are superimposed to form a rectangular beam and results are compared with a single circular Gaussian source model as well as experimental results from a high power diode laser with a rectangular beam. Melt depth, and melt pool profile and progression have been predicted by modelling which are compared with experimental results from melting of Inconel 625. The model produced is shown to give a reasonable prediction of melt pool shape and can be usefully employed to help optimise overlap required for laser surface processing applications. The value of absorptivity used in the model can be used as a fitting parameter to optimise the match between experimental and predicted results
Multilayer broadband anti-reflective coatings for bulk heterojunction polymer solar cells
The photocurrent produced by solar cells is
limited by reflection losses for all types of
photovoltaic devices. The first reflection loss
occurs at the glass/air interface of the
photovoltaic device. A solar cell without a light
trapping mechanism in place loses around 4 %
of the solar energy at this interface. To
minimise the losses, a broadband multilayer
thin film anti-reflection (MAR) coating has
been designed and deposited onto the glass
surface of a solar cell. The coating consisted
of four dielectric layers of alternating thin films
of ZrO2 and SiO2. The layers were deposited
by using high rate pulsed DC magnetron
sputtering using time only for nanometre
thickness control. Spectrophotometer
measurements confirm that the transmission
increased over the spectrum utilized by the
bulk heterojunction (BHJ) solar cell (350nm-
700nm). The weighted average reflection
reduced from 4.22% to 0.99%. BHJ solar cells
with a PCDTBT:PCBM blend serving as the
active layer were prepared on a MAR coated
soda lime glass slides to verify the
effectiveness of the coating. The efficiency
increased by 0.18% at STC from 4.98% to
5.17% (a 3.7% relative increase). The gain
was achieved by increasing the photocurrent
from 11.96 mA/cm2
to 12.36 mA/cm2
Massive Electrodynamics and the Magnetic Monopoles
We investigate in detail the problem of constructing magnetic monopole
solutions within the finite-range electrodynamics (i.e., electrodynamics with
non-zero photon mass, which is the simplest extension of the standard theory;
it is fully compatible with the experiment). We first analyze the classical
electrodynamics with the additional terms describing the photon mass and the
magnetic charge; then we look for a solution analogous to the Dirac monopole
solution. Next, we plug the found solution into the Schr\"{o}dinger equation
describing the interaction between the the magnetic charge and the electron.
After that, we try to derive the Dirac quantization condition for our case.
Since gauge invariance is lost in massive electrodynamics, we use the method of
angular momentum algebra. Under rather general assumptions we prove the theorem
that the construction of such an algebra is not possible and therefore the
quantization condition cannot be derived. This points to the conclusion that
the Dirac monopole and the finite photon mass cannot coexist within one and the
same theory. Some physical consequences of this conclusion are considered. The
case of t'Hooft-Polyakov monopole is touched upon briefly.Comment: 24 pages, revtex, 1 figure appended as a PostScript fil
Electromagnetic probes
We introduce the seminal developments in the theory and experiments of
electromagnetic probes for the study of the dynamics of relativistic heavy ion
collisions and quark gluon plasma.Comment: 47 pages, 33 Figures; Lectures delivered by Dinesh K. Srivastava at
QGP Winter School (QGPWS08) at Jaipur, India, February 1-3, 200
The systematic study of the influence of neutron excess on the fusion cross sections using different proximity-type potentials
Using different types of proximity potentials, we have examined the trend of
variations of barrier characteristics (barrier height and its position) as well
as fusion cross sections for 50 isotopic systems including various collisions
of C, O, Mg, Si, S, Ca, Ar, Ti and Ni nuclei with condition
for compound systems. The results of our studies reveal that the relationships
between increase of barrier positions and decrease of barrier heights are both
linear with increase of ratio. Moreover, fusion cross sections also
enhance linearly with increase of this ratio.Comment: 28 pages, 7 figures, 5 Table
Hydrodynamics and Flow
In this lecture note, we present several topics on relativistic hydrodynamics
and its application to relativistic heavy ion collisions. In the first part we
give a brief introduction to relativistic hydrodynamics in the context of heavy
ion collisions. In the second part we present the formalism and some
fundamental aspects of relativistic ideal and viscous hydrodynamics. In the
third part, we start with some basic checks of the fundamental observables
followed by discussion of collective flow, in particular elliptic flow, which
is one of the most exciting phenomenon in heavy ion collisions at relativistic
energies. Next we discuss how to formulate the hydrodynamic model to describe
dynamics of heavy ion collisions. Finally, we conclude the third part of the
lecture note by showing some results from ideal hydrodynamic calculations and
by comparing them with the experimental data.Comment: 40 pages, 35 figures; lecture given at the QGP Winter School, Jaipur,
India, Feb.1-3, 2008; to appear in Springer Lecture Notes in Physic
Experimental and Theoretical Challenges in the Search for the Quark Gluon Plasma: The STAR Collaboration's Critical Assessment of the Evidence from RHIC Collisions
We review the most important experimental results from the first three years
of nucleus-nucleus collision studies at RHIC, with emphasis on results from the
STAR experiment, and we assess their interpretation and comparison to theory.
The theory-experiment comparison suggests that central Au+Au collisions at RHIC
produce dense, rapidly thermalizing matter characterized by: (1) initial energy
densities above the critical values predicted by lattice QCD for establishment
of a Quark-Gluon Plasma (QGP); (2) nearly ideal fluid flow, marked by
constituent interactions of very short mean free path, established most
probably at a stage preceding hadron formation; and (3) opacity to jets. Many
of the observations are consistent with models incorporating QGP formation in
the early collision stages, and have not found ready explanation in a hadronic
framework. However, the measurements themselves do not yet establish
unequivocal evidence for a transition to this new form of matter. The
theoretical treatment of the collision evolution, despite impressive successes,
invokes a suite of distinct models, degrees of freedom and assumptions of as
yet unknown quantitative consequence. We pose a set of important open
questions, and suggest additional measurements, at least some of which should
be addressed in order to establish a compelling basis to conclude definitively
that thermalized, deconfined quark-gluon matter has been produced at RHIC.Comment: 101 pages, 37 figures; revised version to Nucl. Phys.
Phi meson production in Au+Au and p+p collisions at sqrt (s)=200 GeV
We report the STAR measurement of Phi meson production in Au+Au and p+p
collisions at sqrt (s)=200 GeV. Using the event mixing technique, the Phi
spectra and yields are obtained at mid-rapidity for five centrality bins in
Au+Au collisions and for non-singly-diffractive p+p collisions. It is found
that the Phi transverse momentum distributions from Au+Au collisions are better
fitted with a single-exponential while the p+p spectrum is better described by
a double-exponential distribution. The measured nuclear modification factors
indicate that Phi production in central Au+Au collisions is suppressed relative
to peripheral collisions when scaled by the number of binary collisions. The
systematics of versus centrality and the constant Phi/K- ratio versus beam
species, centrality, and collision energy rule out kaon coalescence as the
dominant mechanism for Phi production.Comment: 6 pages, 3 figures, submitted to Phys. Rev. Let
Longitudinal scaling property of the charge balance function in Au + Au collisions at 200 GeV
We present measurements of the charge balance function, from the charged
particles, for diverse pseudorapidity and transverse momentum ranges in Au + Au
collisions at 200 GeV using the STAR detector at RHIC. We observe that the
balance function is boost-invariant within the pseudorapidity coverage [-1.3,
1.3]. The balance function properly scaled by the width of the observed
pseudorapidity window does not depend on the position or size of the
pseudorapidity window. This scaling property also holds for particles in
different transverse momentum ranges. In addition, we find that the width of
the balance function decreases monotonically with increasing transverse
momentum for all centrality classes.Comment: 6 pages, 3 figure
New insights into the genetic etiology of Alzheimer's disease and related dementias
Characterization of the genetic landscape of Alzheimer's disease (AD) and related dementias (ADD) provides a unique opportunity for a better understanding of the associated pathophysiological processes. We performed a two-stage genome-wide association study totaling 111,326 clinically diagnosed/'proxy' AD cases and 677,663 controls. We found 75 risk loci, of which 42 were new at the time of analysis. Pathway enrichment analyses confirmed the involvement of amyloid/tau pathways and highlighted microglia implication. Gene prioritization in the new loci identified 31 genes that were suggestive of new genetically associated processes, including the tumor necrosis factor alpha pathway through the linear ubiquitin chain assembly complex. We also built a new genetic risk score associated with the risk of future AD/dementia or progression from mild cognitive impairment to AD/dementia. The improvement in prediction led to a 1.6- to 1.9-fold increase in AD risk from the lowest to the highest decile, in addition to effects of age and the APOE ε4 allele
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