321 research outputs found
Interaction of laser radiation with the material during production powders and fibers
Воздействие лазерного излучения на твердое тело приводит к изменению температурного поля обрабатываемого вещества. Характер нагрева, определяющийся скоростями изменения температуры, температурных градиентов, оказывается различным в зависимости от свойств обрабатываемого материала и условий обработки. Основными физическими параметрами процесса лазерной обработки твердых тел являются удельная мощность поглощенного лазерного потока 104–109 Вт/см2 и время взаимодействия металла с лучом 10–5–10–8 с. При взаимодействии подобных импульсов излучения с поверхностью происходит мгновенное взрывоподобное плавление части материала и перевод окружающего поверхность вещества в плазменное состояние. Последующее расширение плазмы сопровождается возникновением ударной волны с пиковым давлением 1–10 ГПа, которая действует на материал, и имеет место диспергирование металла. Решена математическая задача нагрева и плавления цилиндрической пластины нормально падающим на ее поверхность световым потоком лазерного излучения, описываемая системой уравнений теплопроводности в трех сечениях нагреваемой пластины, которые характеризуются временным фактором воздействия лазерного излучения на вещество: 1) 0 ≤ t ≤ tm; 2) t > tm; 3) tm tm; 3) tm < t ≤ th (here tm, th is the time moment corresponding to the beginning of the formation of the liquid phase and the end of the melting of the plate, respectively). The calculated dependences of changes in the surface temperature of metal alloys X18N10T, X15N60 during the action of a laser radiation pulse with a duration of τ=5 ms are presented. The presence of a phase transition associated with metal melting (an inflection in the curves) leads to a temporary decrease in the rate of temperature growth. The distribution of temperature fields causes a significant heterogeneity in the distribution of temperature over the thickness of materials, which reaches 2000 °C or more depending on the thickness of the metal and the conditions of exposure. The temperature curves of the surface heating repeat the shape of the pulse, and the temperature of the rest of the metal has a nonlinear tendency to increase with the output to the asymptote. It is established that the process of explosive metal sputtering requires heating the volume of the material above the melting point at a thickness of 300–350 microns and an impact energy of 7–8 J. Reducing the level of energy impact to 5–6 J and increasing the thickness of the workpiece more than 500 microns does not provide the distribution of temperature fields required for the implementation of the spraying process
Strongly Hyperbolic Extensions of the ADM Hamiltonian
The ADM Hamiltonian formulation of general relativity with prescribed lapse
and shift is a weakly hyperbolic system of partial differential equations. In
general weakly hyperbolic systems are not mathematically well posed. For well
posedness, the theory should be reformulated so that the complete system,
evolution equations plus gauge conditions, is (at least) strongly hyperbolic.
Traditionally, reformulation has been carried out at the level of equations of
motion. This typically destroys the variational and Hamiltonian structures of
the theory. Here I show that one can extend the ADM formalism to (i)
incorporate the gauge conditions as dynamical equations and (ii) affect the
hyperbolicity of the complete system, all while maintaining a Hamiltonian
description. The extended ADM formulation is used to obtain a strongly
hyperbolic Hamiltonian description of Einstein's theory that is generally
covariant under spatial diffeomorphisms and time reparametrizations, and has
physical characteristics. The extended Hamiltonian formulation with 1+log
slicing and gamma--driver shift conditions is weakly hyperbolic.Comment: This version contains minor corrections and clarifications. The
format has been changed to conform with IOP styl
Macrophage polarisation affects their regulation of trophoblast behaviour
Introduction
During the first trimester of human pregnancy, fetally-derived extravillous trophoblast (EVT) cells invade into uterine decidua and remodel the uterine spiral arteries to ensure that sufficient blood reaches the maternal-fetal interface. Decidual macrophages have been implicated in the regulation of decidual remodelling and aberrant activation of these immune cells is associated with pre-eclampsia.
Methods
The monocytic cell line THP-1 was activated to induce an M1 or M2 phenotype and the conditioned media was used to treat the EVT cell line SGHPL-4 in order to determine the effect of macrophage polarisation on trophoblast behaviour in-vitro. SGHPL-4 cell functions were assessed using time-lapse microscopy, endothelial-like tube formation assays and western blot.
Results
The polarisation state of the THP-1 cells was found to differentially alter the behaviour of trophoblast cells in-vitro with pro-inflammatory M1 conditioned media significantly inhibiting trophoblast motility, impeding trophoblast tube formation, and inducing trophoblast expression of caspase 3, when compared to anti-inflammatory M2 conditioned media.
Discussion
Macrophages can regulate trophoblast functions that are critical during decidual remodelling in early pregnancy. Importantly, there is differential regulation of trophoblast function in response to the polarisation state of these cells. Our studies indicate that the balance between a pro- and anti-inflammatory environment is important in regulating the cellular interactions at the maternal-fetal interface and that disturbances in this balance likely contribute to pregnancy disorders associated with poor trophoblast invasion and vessel remodelling
Rapid post-fire re-assembly of species-rich bryophyte communities in Afroalpine heathlands
Questions
In some fire-prone ecosystems, bryophytes play a crucial role by providing the surface fuel that controls the fire return interval. Afroalpine heathlands are such an ecosystem, yet almost nothing is known about the bryophytes in this system. We do not know the level of species richness, or if there is a successive accumulation of species over time, or if some species are adapted to specific phases along the successional gradient, for example early-successional species sensitive to competition.
Location
Afroalpine heathlands in Ethiopia.
Methods
We made an inventory of all bryophytes in 48 plots of 5 m × 5 m, distributed along a chronosequence from 1 to 25 years post fire. The heathlands are located between 3500 m and 3800 m a.s.l. and are managed by traditional pasture burning with fire intervals of 8–20 years.
Results
We found in total 111 taxa of bryophytes. Post-fire mortality was almost 100%. The youngest plots had only a few cosmopolitan species often found after fire. Initially, species richness increased monotonically while starting to level off around 15 years after fire, when many plots had around 30 species and a high cover of Breutelia diffracta, which is a key ground-living species, important as surface fuel. Most species were found with sporophytes, a pattern even stronger for the most frequent species.
Conclusions
Interestingly, bryophyte diversity is already remarkably high by only 15 years after total eradication. The relatively slow accumulation of species in the first years after fire suggests that dispersal in space, and not time, is the major mechanism by which sites regain their diversity (i.e. spore banks play a smaller role than colonization of wind-borne spores). This indicates that the high species richness is built up through colonization from surrounding heathlands, and perhaps also from higher-altitude alpine grasslands and lower-altitude forests, and that the bryophyte diversity in this system is maintained by the traditional fire and grazing management
Observational diagnostics of gas in protoplanetary disks
Protoplanetary disks are composed primarily of gas (99% of the mass).
Nevertheless, relatively few observational constraints exist for the gas in
disks. In this review, I discuss several observational diagnostics in the UV,
optical, near-IR, mid-IR, and (sub)-mm wavelengths that have been employed to
study the gas in the disks of young stellar objects. I concentrate in
diagnostics that probe the inner 20 AU of the disk, the region where planets
are expected to form. I discuss the potential and limitations of each gas
tracer and present prospects for future research.Comment: Review written for the proceedings of the conference "Origin and
Evolution of Planets 2008", Ascona, Switzerland, June 29 - July 4, 2008. Date
manuscript: October 2008. 17 Pages, 6 graphics, 134 reference
Heavy Quarks and Heavy Quarkonia as Tests of Thermalization
We present here a brief summary of new results on heavy quarks and heavy
quarkonia from the PHENIX experiment as presented at the "Quark Gluon Plasma
Thermalization" Workshop in Vienna, Austria in August 2005, directly following
the International Quark Matter Conference in Hungary.Comment: 8 pages, 5 figures, Quark Gluon Plasma Thermalization Workshop
(Vienna August 2005) Proceeding
Single Electrons from Heavy Flavor Decays in p+p Collisions at sqrt(s) = 200 GeV
The invariant differential cross section for inclusive electron production in
p+p collisions at sqrt(s) = 200 GeV has been measured by the PHENIX experiment
at the Relativistic Heavy Ion Collider over the transverse momentum range $0.4
<= p_T <= 5.0 GeV/c at midrapidity (eta <= 0.35). The contribution to the
inclusive electron spectrum from semileptonic decays of hadrons carrying heavy
flavor, i.e. charm quarks or, at high p_T, bottom quarks, is determined via
three independent methods. The resulting electron spectrum from heavy flavor
decays is compared to recent leading and next-to-leading order perturbative QCD
calculations. The total cross section of charm quark-antiquark pair production
is determined as sigma_(c c^bar) = 0.92 +/- 0.15 (stat.) +- 0.54 (sys.) mb.Comment: 329 authors, 6 pages text, 3 figures. Submitted to Phys. Rev. Lett.
Plain text data tables for the points plotted in figures for this and
previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
Nuclear Modification of Electron Spectra and Implications for Heavy Quark Energy Loss in Au+Au Collisions at sqrt(s_NN)=200 GeV
The PHENIX experiment has measured mid-rapidity transverse momentum spectra
(0.4 < p_T < 5.0 GeV/c) of electrons as a function of centrality in Au+Au
collisions at sqrt(s_NN)=200 GeV. Contributions from photon conversions and
from light hadron decays, mainly Dalitz decays of pi^0 and eta mesons, were
removed. The resulting non-photonic electron spectra are primarily due to the
semi-leptonic decays of hadrons carrying heavy quarks. Nuclear modification
factors were determined by comparison to non-photonic electrons in p+p
collisions. A significant suppression of electrons at high p_T is observed in
central Au+Au collisions, indicating substantial energy loss of heavy quarks.Comment: 330 authors, 6 pages text, 3 figures. Submitted to Phys. Rev. Lett.
Plain text data tables for the points plotted in figures for this and
previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
Dilepton mass spectra in p+p collisions at sqrt(s)= 200 GeV and the contribution from open charm
The PHENIX experiement has measured the electron-positron pair mass spectrum
from 0 to 8 GeV/c^2 in p+p collisions at sqrt(s)=200 GeV. The contributions
from light meson decays to e^+e^- pairs have been determined based on
measurements of hadron production cross sections by PHENIX. They account for
nearly all e^+e^- pairs in the mass region below 1 GeV/c^2. The e^+e^- pair
yield remaining after subtracting these contributions is dominated by
semileptonic decays of charmed hadrons correlated through flavor conservation.
Using the spectral shape predicted by PYTHIA, we estimate the charm production
cross section to be 544 +/- 39(stat) +/- 142(syst) +/- 200(model) \mu b, which
is consistent with QCD calculations and measurements of single leptons by
PHENIX.Comment: 375 authors from 57 institutions, 18 pages, 4 figures, 2 tables.
Submitted to Physics Letters B. v2 fixes technical errors in matching authors
to institutions. Plain text data tables for the points plotted in figures for
this and previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
Inclusive cross section and double helicity asymmetry for \pi^0 production in p+p collisions at sqrt(s)=200 GeV: Implications for the polarized gluon distribution in the proton
The PHENIX experiment presents results from the RHIC 2005 run with polarized
proton collisions at sqrt(s)=200 GeV, for inclusive \pi^0 production at
mid-rapidity. Unpolarized cross section results are given for transverse
momenta p_T=0.5 to 20 GeV/c, extending the range of published data to both
lower and higher p_T. The cross section is described well for p_T < 1 GeV/c by
an exponential in p_T, and, for p_T > 2 GeV/c, by perturbative QCD. Double
helicity asymmetries A_LL are presented based on a factor of five improvement
in uncertainties as compared to previously published results, due to both an
improved beam polarization of 50%, and to higher integrated luminosity. These
measurements are sensitive to the gluon polarization in the proton, and exclude
maximal values for the gluon polarization.Comment: 375 authors, 7 pages, 3 figures. Submitted to Phys. Rev. D, Rapid
Communications. Plain text data tables for the points plotted in figures for
this and previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
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