26,964 research outputs found
Density Fluctuations in the Quark-Gluon Plasma
Using the kinetic theory we discuss how the particle and energy densities of
the quark-gluon plasma fluctuate in a space-time cell. The fluctuations in the
equilibrium plasma and in that one from the early stage of ultrarelativistic
heavy-ion collisions are estimated. Within the physically interesting values of
the parameters involved the fluctuations appear sizeable in both cases.Comment: 8 pages, no macro
Atmospheric pressure glow discharge for CO2 conversion : model-based exploration of the optimum reactor configuration
We investigate the performance of an atmospheric pressure glow discharge (APGD) reactor for CO2 conversion in three different configurations, through experiments and simulations. The first (basic) configuration utilizes the well-known pin-to-plate design, which offers a limited conversion. The second configuration improves the reactor performance by employing a vortex-flow generator. The third, "confined" configuration is a complete redesign of the reactor, which encloses the discharge in a limited volume, significantly surpassing the conversion rate of the other two designs. The plasma properties are investigated using an advanced plasma model
Turbo-FLASH based arterial spin labeled perfusion MRI at 7 T.
Motivations of arterial spin labeling (ASL) at ultrahigh magnetic fields include prolonged blood T1 and greater signal-to-noise ratio (SNR). However, increased B0 and B1 inhomogeneities and increased specific absorption ratio (SAR) challenge practical ASL implementations. In this study, Turbo-FLASH (Fast Low Angle Shot) based pulsed and pseudo-continuous ASL sequences were performed at 7T, by taking advantage of the relatively low SAR and short TE of Turbo-FLASH that minimizes susceptibility artifacts. Consistent with theoretical predictions, the experimental data showed that Turbo-FLASH based ASL yielded approximately 4 times SNR gain at 7T compared to 3T. High quality perfusion images were obtained with an in-plane spatial resolution of 0.85×1.7 mm(2). A further functional MRI study of motor cortex activation precisely located the primary motor cortex to the precentral gyrus, with the same high spatial resolution. Finally, functional connectivity between left and right motor cortices as well as supplemental motor area were demonstrated using resting state perfusion images. Turbo-FLASH based ASL is a promising approach for perfusion imaging at 7T, which could provide novel approaches to high spatiotemporal resolution fMRI and to investigate the functional connectivity of brain networks at ultrahigh field
Quasiquarks in two stream system
We study the collective quark excitations in an extremely anisotropic system
of two interpenetrating streams of the quark-gluon plasma. In contrast to the
gluon modes, all quark ones appear to be stable in such a system. Even more,
the quark modes in the two-stream system are very similar to those in the
isotropic plasma.Comment: 4 pages, 2 figures, minor corrections, to appear in Phys. Rev.
Chromodynamic Weibel instabilities in relativistic nuclear collisions
Employing a previously derived formulation, and extending the treatment from
purely transverse modes to wave vectors having a longitudinal component, we
discuss the prospects for the occurrence of Weibel-type color-current
filamentation in high-energy nuclear collisions. Numerical solutions of the
dispersion equation for a number of scenarios relevant to RHIC and LHC suggest
that modes with (predominantly transverse) wave numbers of several hundred MeV
may become moderately agitated during the early collision stage. The emergence
of filamentation helps to speed up the equilibration of the parton plasma and
it may lead to non-statistical azimuthal patterns in the hadron final state.Comment: 11 pages, RevTex, 13 (e)ps files (revised for PRC
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Influence of convection and biomass burning outflow on tropospheric chemistry over the tropical Pacific
Observations over the tropics from the Pacific Exploratory Mission-Tropics A Experiment are analyzed using a one-dimensional model with an explicit formulation for convective transport. Adopting tropical convective mass fluxes from a general circulation model (GCM) yields a large discrepancy between observed and simulated CH3I concentrations. Observations of CH3I imply the convective mass outflux to be more evenly distributed with altitude over the tropical ocean than suggested by the GCM. We find that using a uniform convective turnover lifetime of 20 days in the upper and middle troposphere enables the model to reproduce CH3I observations. The model reproduces observed concentrations of H2O2 and CH3OOH. Convective transport of CH3OOH from the lower troposphere is estimated to account for 40-80% of CH3OOH concentrations in the upper troposphere. Photolysis of CH3OOH transported by convection more than doubles the primary HOx source and increases OH concentrations and O3 production by 10-50% and 0.4 ppbv d-1, respectively, above 11 km. Its effect on the OH concentration and O3 production integrated over the tropospheric column is, however, small. The effects of pollutant import from biomass burning regions are much more dominant. Using C2H2 as a tracer, we estimate that biomass burning outflow enhances O3 concentrations, O3 production, and concentrations of NOx and OH by 60%, 45%, 75%, and 7%, respectively. The model overestimates HNO3 concentrations by about a factor of 2 above 4 km for the upper one-third quantile of C2H2 data while it generally reproduces HNO3 concentrations for the lower and middle one-third quantiles of C2H2 data. Copyright 2000 by the American Geophysical Union
Magnetosphere-Ionosphere Coupling Through E-region Turbulence: Anomalous Conductivities and Frictional Heating
Global magnetospheric MHD codes using ionospheric conductances based on
laminar models systematically overestimate the cross-polar cap potential during
storm time by up to a factor of two. At these times, strong DC electric fields
penetrate to the E region and drive plasma instabilities that create
turbulence. This plasma density turbulence induces non-linear currents, while
associated electrostatic field fluctuations result in strong anomalous electron
heating. These two effects will increase the global ionospheric conductance.
Based on the theory of non-linear currents developed in the companion paper,
this paper derives the correction factors describing turbulent conductivities
and calculates turbulent frictional heating rates. Estimates show that during
strong geomagnetic storms the inclusion of anomalous conductivity can double
the total Pedersen conductance. This may help explain the overestimation of the
cross-polar cap potentials by existing MHD codes. The turbulent conductivities
and frictional heating presented in this paper should be included in global
magnetospheric codes developed for predictive modeling of space weather.Comment: 13 pages, 5 figures, 2nd of two companion paper
Ultimate strength equation for pultruded CFRP plates in fire
The paper presents a model for the ultimate strength calculation of fibre-reinforced polymer (FRP) pultruded plates in fire. The model is calibrated from a series of coupon tests on pultruded CFRP plates carried out using steady state and transient state test methods. In the steady state tests, temperatures ranging from ambient to approximately 700℃ were considered. The test specimens were heated to a specified temperature then loaded until failure while the same temperature was maintained throughout the test. Different periods of times of 5 and 30 minutes were maintained to investigate the effect of heating duration. In the transient state tests, the test specimens were loaded to a specified stress level prior to heating, and then the temperature was increased until failure of the test specimens. Three different stress levels applied to the pultruded CFRP plates were considered. Based on the test results, an ultimate strength equation is proposed which has been inspired from a model developed for metallic materials subjected to elevated temperatures. Application of the model is finally demonstrated in a worked example.postprin
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