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Search Results: 1 - 10 of 240603 matches for " P-Y. Beauvais "
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An ECR hydrogen negative ion source at CEA/Saclay: preliminary results
R. Gobin,P-Y. Beauvais,O. Delferriere,R. Ferdinand,F. Harrault,J-M. Lagniel
Physics , 2000,
Abstract: The development of a high intensity negative ion source is part of a considerably larger activity presently undergoing at CEA Saclay in the field of high intensity linear accelerators. Preliminary studies toward the construction of a 2.45 GHz ECR H- ion source have been performed for few months. This new test bench takes advantage of our experience on the French high intensity proton source SILHI. In the new source, the high-energy electrons created in the ECR zone are trapped by a dipole magnetic filter. A rectangular 200 mm long plasma chamber and an intermediate iron shield are used to minimize the magnetic field in the extraction region. A second magnetic filter separates electrons and negative ions in a 10 kV extraction system. To reduce the electron/H- ratio, the plasma electrode is slightly polarized. The design allows future evolutions such as cesium injection, higher energy extraction and plasma diagnostics. The installation of the source is now in progress. The first helium plasma has been produced for few weeks to verify the electron separator behavior. The design, computations and the first results of the source are presented.
Status Report on the 5 Mev Iphi RFQ
R. Ferdinand,P-Y. Beauvais,R. Duperrier,A. France,J. Gaiffier,J-M. Lagniel,M. Painchault,F. Simoens,CEA-Saclay,DSM-DAPNIA-SEA,P. Balleyguier,CEA-Bruyeres le Chatel,DAM
Physics , 2000,
Abstract: A 5-MeV RFQ designed for a proton current up to 100-mA CW is now under construction as part of the High Intensity Proton Injector project (IPHI). Its computed transmission is greater than 99 %. The main goals of the project are to verify the accuracy of the design codes, to gain the know-how on fabrication, tuning procedures and operations, to measure the output beam characteristics in order to optimise the higher energy part of the linac, and to reach a high availability with minimum beam trips. A cold model has been built to develop the tuning procedure. The present status of the IPHI RFQ is presented.
Turbulent resistivity evaluation in MRI generated turbulence
G. Lesur,P-Y. Longaretti
Physics , 2009,
Abstract: (abriged) MRI turbulence is a leading mechanism for the generation of an efficient turbulent transport of angular momentum in an accretion disk through a turbulent viscosity effect. It is believed that the same process could also transport large-scale magnetic fields in disks, reshaping the magnetic structures in these objects. This process, known as turbulent resistivity, has been suggested and used in several accretion-ejection models and simulations to produce jets. Still, the efficiency of MRI-driven turbulence to transport large-scale magnetic fields is largely unknown. We investigate this problem both analytically and numerically. We introduce a linear calculation of the MRI in the presence of a spatially inhomogeneous mean magnetic field. We show that, in this configuration, MRI modes lead to an efficient magnetic field transport, on the order of the angular momentum transport. We next use fully non linear simulations of MRI turbulence to compute the turbulent resistivity in several magnetic configurations. We find that the turbulent resistivity is on the order of the turbulent viscosity in all our simulations, although somewhat lower. The turbulent resistivity tensor is found to be highly anisotropic with a diffusion coefficient 3 times greater in the radial direction than in the vertical direction. These results support the possibility of driving jets from turbulent disks; the resulting jets may not be steady.
Subcritical turbulent transition in rotating and curved shear flows
P-Y. Longaretti,O. Dauchot
Physics , 2005,
Abstract: The effects of global flow rotation and curvature on the subcritical transition to turbulence in shear flows are examined. The relevant time-scales of the problem are identified by a decomposition of the flow into a laminar and a deviation from laminar parts, which is performed for rotating plane Couette and Taylor-Couette flows. The usefulness and relevance of this procedure are discussed at the same time. By comparing the self-sustaining process time-scale to the time-scales previously identified, an interpretation is brought to light for the behavior of the transition Reynolds number with the rotation number and relative gap width in the whole neighborhood (in parameter space) of the non-rotating plane Couette flow covered by the available data.
On the relevance of subcritical hydrodynamic turbulence to accretion disk transport
G. Lesur,P-Y. Longaretti
Physics , 2005, DOI: 10.1051/0004-6361:20053683
Abstract: Hydrodynamic unstratified keplerian flows are known to be linearly stable at all Reynolds numbers, but may nevertheless become turbulent through nonlinear mechanisms. However, in the last ten years, conflicting points of view have appeared on this issue. We have revisited the problem through numerical simulations in the shearing sheet limit. It turns out that the effect of the Coriolis force in stabilizing the flow depends on whether the flow is cyclonic (cooperating shear and rotation vorticities) or anticyclonic (competing shear and rotation vorticities); keplerian flows are anticyclonic. We have obtained the following results: i/ The Coriolis force does not quench turbulence in subcritical flows; ii/ The resolution demand, when moving away from the marginal stability boundary, is much more severe for anticyclonic flows than for cyclonic ones. Presently available computer resources do not allow numerical codes to reach the keplerian regime. iii/ The efficiency of turbulent transport is directly correlated to the Reynolds number of transition to turbulence $Rg$, in such a way that the Shakura-Sunyaev parameter $\alpha\sim 1/Rg$. iv/ Even the most optimistic extrapolations of our numerical data show that subcritical turbulent transport would be too inefficient in keplerian flows by several orders of magnitude for astrophysical purposes. v/ Our results suggest that the data obtained for keplerian-like flows in a Taylor-Couette settings are largely affected by secondary flows, such as Ekman circulation.
Muscimol increases the survival rate and inhibits the inflammatory response in endotoxemic mice
D-Z Hsu, Y-H Li, P-Y Chu, M-Y Liu
Critical Care , 2009, DOI: 10.1186/cc8075
Abstract: C57BL6 mice, lipopolysaccharide (derived from Escherichia coli, serotype O55:B5), and muscimol were used in this study.Mice endotoxemia was induced by 10 mg/kg lipopolysaccharide intraperitoneally. Muscimol ranging from 0 to 3 mg/kg were given subcutaneously 30 minutes before lipopolysaccharide administration. Serum TNFα, IL-1β, IL-10, and IL-12 were determined using ELISA.Muscimol significantly increased the survival rate in sub-lethal dose of lipopolysaccharide-treated mice (from 7% to 100%) (P < 0.0001) within 72 hours. Muscimol inhibited serum TNFα, IL-1β, and IL-12 production in a dose-dependent manner. Furthermore, muscimol significantly increased serum IL-10 levels (P < 0.001) in lipopolysaccharide-treated mice.Muscimol potently increased the survival rate and inhibited inflammatory response in endotoxemic mice.
Sesamol attenuates septic hypotension through peroxisome proliferator-activated receptor activation after the onset of systemic inflammatory response
P-Y Chu, D-Z Hsu, M-Y Liu
Critical Care , 2009, DOI: 10.1186/cc8076
Abstract: Wistar rats, lipopolysaccharide (LPS) (derived from Escherichia coli, serotype O55:B5), and sesamol were used in this study.Hypotension was induced by injecting LPS intravenously. Mean arterial pressure was measured using an invasive blood pressure system. Serum nitrite and cytokine levels were determined using the Griess reaction and ELISA, respectively. Peroxisome proliferator-activated receptor (PPAR) activation was measured using a PPAR assay kit.LPS administration significantly increased the serum TNFα level at 1 hour. Sesamol treated 1 hour after LPS administration inhibited the LPS-associated blood pressure decrease. Sesamol failed to decrease the LPS-induced nitrite production, but decreased the LPS-induced TNFα and IL-1β production after the onset of systemic inflammation. Sesamol enhanced the IL-10 production in serum and the PPAR activation in white blood cells.Sesamol may attenuate septic hypotension through alternating cytokine production by PPAR activation after the onset of systemic inflammatory response.
Stability and turbulent transport in rotating shear flows: prescription from analysis of cylindrical and plane Couette flows data
B. Dubrulle,O. Dauchot,F. Daviaud,P-Y. Longaretti,D. Richard,J-P. Zahn
Physics , 2011, DOI: 10.1063/1.2008999
Abstract: This paper provides a prescription for the turbulent viscosity in rotating shear flows for use e.g. in geophysical and astrophysical contexts. This prescription is the result of the detailed analysis of the experimental data obtained in several studies of the transition to turbulence and turbulent transport in Taylor-Couette flow. We first introduce a new set of control parameters, based on dynamical rather than geometrical considerations, so that the analysis applies more naturally to rotating shear flows in general and not only to Taylor-Couette flow. We then investigate the transition thresholds in the supercritical and the subcritical regime in order to extract their general dependencies on the control parameters. The inspection of the mean profiles provides us with some general hints on the mean to laminar shear ratio. Then the examination of the torque data allows us to propose a decomposition of the torque dependence on the control parameters in two terms, one completely given by measurements in the case where the outer cylinder is at rest, the other one being a universal function provided here from experimental fits. As a result, we obtain a general expression for the turbulent viscosity and compare it to existing prescription in the literature. Finally, throughout all the paper we discuss the influence of additional effects such as stratification or magnetic fields.
Dissipative effects on the sustainment of a magnetorotational dynamo in Keplerian shear flow
A. Riols,F. Rincon,C. Cossu,G. Lesur,G. I. Ogilvie,P-Y. Longaretti
Physics , 2014, DOI: 10.1051/0004-6361/201424324
Abstract: The magnetorotational (MRI) dynamo has long been considered one of the possible drivers of turbulent angular momentum transport in astrophysical accretion disks. However, various numerical results suggest that this dynamo may be difficult to excite in the astrophysically relevant regime of magnetic Prandtl number (Pm) significantly smaller than unity, for reasons currently not well understood. The aim of this article is to present the first results of an ongoing numerical investigation of the role of both linear and nonlinear dissipative effects in this problem. Combining a parametric exploration and an energy analysis of incompressible nonlinear MRI dynamo cycles representative of the transitional dynamics in large aspect ratio shearing boxes, we find that turbulent magnetic diffusion makes the excitation and sustainment of this dynamo at moderate magnetic Reynolds number (Rm) increasingly difficult for decreasing Pm. This results in an increase in the critical Rm of the dynamo for increasing kinematic Reynolds number (Re), in agreement with earlier numerical results. Given its very generic nature, we argue that turbulent magnetic diffusion could be an important determinant of MRI dynamo excitation in disks, and may also limit the efficiency of angular momentum transport by MRI turbulence in low Pm regimes.
Fast and Chaotic Fiber-Based Nonlinear Polarization Scrambler
M. Guasoni,P-Y. Bony,M. Gilles,A. Picozzi,J. Fatome
Physics , 2015,
Abstract: We report a simple and efficient all-optical polarization scrambler based on the nonlinear interaction in an optical fiber between a signal beam and its backward replica which is generated and amplified by a reflective loop. When the amplification factor exceeds a certain threshold, the system exhibits a chaotic regime in which the evolution of the output polarization state of the signal becomes temporally chaotic and scrambled all over the surface of the Poincar\'e sphere. We derive some analytical estimations for the scrambling performances of our device which are well confirmed by the experimental results. The polarization scrambler has been successfully tested on a single channel 10-Gbit/s On/Off Keying Telecom signal, reaching scrambling speeds up to 250-krad/s, as well as in a wavelength division multiplexing configuration. A different configuration based on a sequent cascade of polarization scramblers is also discussed numerically, which leads to an increase of the scrambling performances.
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