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Search Results: 1 - 10 of 13462 matches for " X. Buffat "
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Beam-beam and impedance
S. White,X. Buffat
Physics , 2014, DOI: 10.5170/CERN-2014-004.217
Abstract: As two counter-rotating beams interact they can give rise to coherent dipole modes. Under the influence of impedance these coherent beam-beam modes can couple to higher order head-tail modes and lead to strong instabilities. A fully self-consistent approach including beam-beam and impedance was used to characterize this new coupled mode instability and study possible cures such as a transverse damper and high chromaticity.
Operational considerations on the stability of colliding beams
X. Buffat,W. Herr,T. Pieloni
Physics , 2014, DOI: 10.5170/CERN-2014-004.193
Abstract: While well studied in the absence of beam-beam and while colliding head-on, the stability of the LHC beams can be very critical in intermediate steps. During the squeeze, the long-range beam-beam interaction becomes a critical component of the beam's dynamics. Also, while the transverse separation at the interaction points is collapsed, the beam-beam forces change drastically, possibly deteriorating the beam's stability. Finally, during luminosity production, the configuration of the LHC in 2012 included few bunches without head-on collision in any of the interaction points having different stability properties. Stability diagrams are being evaluated numerically in these configurations in an attempt to explain instabilities observed in these phases during the 2012 proton run of the LHC.
Stability diagram of colliding beams
X. Buffat,W. Herr,N. Mounet,T. Pieloni
Physics , 2014, DOI: 10.5170/CERN-2014-004.57
Abstract: The effect of the beam-beam interactions on the stability of impedance mode is discussed. The detuning is evaluated by the means of single particle tracking in arbitrarily complex collision configurations, including lattice non-linearities, and used to numerically evaluate the dispersion integral. This approach also allows the effect of non-Gaussian distributions to be considered. Distributions modified by the action of external noise are discussed.
Observations of beam-beam effects at the LHC
G. Papotti,X. Buffat,W. Herr,R. Giachino,T. Pieloni
Physics , 2014,
Abstract: This paper introduces a list of observations related to the beam-beam interaction that were collected over the first years of LHC proton physics operation (2010-12). Beam-beam related effects not only have been extensively observed and recorded, but have also shaped the operation of the LHC for high-intensity proton running in a number of ways: the construction of the filling scheme, the choice of luminosity levelling techniques, measures to mitigate instabilities, and the choice of settings for improving performance (e.g. to reduce losses), among others.
Coherent beam-beam mode in the LHC
X. Buffat,R. Calaga,R. Giachino,W. Herr,G. Papotti,T. Pieloni,S. White
Physics , 2014, DOI: 10.5170/CERN-2014-004.227
Abstract: Observations of single bunch beam-beam coherent modes during dedicated experiments in the LHC are presented. Their role in standard operation for physics is discussed and, in particular, candidates of beam-beam coherent mode driven unstable by the machine impedance are presented.
Long Range Beam-beam Effects in the LHC
W. Herr,X. Buffat,R. Calaga,R. Giachino,G. Papotti,T. Pieloni,D. Kaltchev
Physics , 2014, DOI: 10.5170/CERN-2014-004.87
Abstract: We report on the experience with long-range beam--beam effects in the LHC, in dedicated studies as well as the experience from operation. Where possible, we compare the observations with the expectations.
Les réformes au prisme de l’autonomie et du contr le des agents publics de base : le cas de la politique suisse du ch mage
Aurélien Buffat
Pyramides : Revue du Centre d'Etudes et de Recherches en Administration Publique , 2011,
Abstract: Cet article se propose de réfléchir aux réformes de l’administration en étudiant ce qu’elles deviennent au niveau des agents publics de base ou ‘street-level bureaucrats’. Si la littérature qui leur est consacrée a jusqu’ici mis en avant à la fois l’autonomie décisionnelle de ces acteurs ainsi que les stratégies de contr le de celle-ci, nous proposons ici de revisiter la problématique de l’autonomie et du contr le à ce niveau en rendant compte de la manière dont elle est affectée par les réformes de nouvelle gestion publique d’une part, et les nouvelles technologies de l’information et de la communication d’autre part. Pour ce faire, la partie empirique présente les résultats préliminaires d’une recherche de l’auteur portant sur un type d’agent public aux premières lignes de la mise en uvre de la politique suisse du ch mage, à savoir les taxateurs d’une caisse cantonale d’indemnisation, laquelle fonctionne selon une logique néo-managériale . Les données analysées montrent que si les éléments du dispositif NGP/TIC (NDLR : une liste des acronymes se trouve en fin d’article) conduisent globalement à un renforcement considérable du contr le hiérarchique et organisationnel d’une autonomie qui reste néanmoins importante, ceci s’accompagne également de modes d’appropriation spécifiques et intéressants dans ce qu’ils ont d’inattendu, de créatif voire de subversif au niveau des agents sur le terrain.
Analysis of transient growth using an orthogonal decomposition of the velocity field in the Orr-Sommerfeld Squire equations
Marc Buffat,Lionel Le Penven
Physics , 2013,
Abstract: Despite remarkable accomplishment, the classical hydrodynamic stability theory fails to predict transition in wall-bounded shear ow. The shortcoming of this modal approach was found 20 years ago and is linked to the non-orthogonality of the eigenmodes of the linearised problem, de noted by the Orr Sommerfeld and Squire equations. The associated eigenmodes of this linearised problem are the normal velocity and the normal vorticity eigenmodes, which are not dimensionally homogeneous quantities. Thus non-orthogonality condition between these two families of eigenmodes have not been clearly demonstrated yet. Using an orthogonal decomposition of solenoidal velocity fields, a velocity perturbation is expressed as an L2 orthogonal sum of an OrrSommerfeld velocity field (function of the perturbation normal velocity) and a Squirevelocity field (function of the perturbation normal vorticity). Using this decomposition,a variational formulation of the linearised problem is written, that is equivalent to the Orr Sommerfeld and Squire equations, but whose eigenmodes consist of two families of velocity eigenmodes (thus dimensionally homogeneous). We demonstrate that these two sets are non-orthogonal and linear combination between them can produce large transient growth. Using this new formulation, the link between optimal mode and continuous mode transition will also be clari fied, as the role of direct resonance. Numerical solutions are presented to illustrate the analysis in the case of thin boundary layers developing between two parallel walls at large Reynolds number. Characterisations of the destabilizing perturbations will be given in that case.
Electron diffraction and high resolution transmission electron microscopy in the characterization of calcium phosphate precipitation from aqueous solutions under biomineralization conditions
Suvorova E. I.,Buffat P. A.
European Cells and Materials (ECM) , 2001,
Abstract: Calcium phosphate precipitation obtained from aqueous solutions at room and body temperature and pH 5.5-7.5 were investigated by high-resolution transmission electron microscopy (HRTEM), transmission electron diffraction, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Supersaturated solutions of calcium phosphates were prepared by different methods of mixing of the stock solutions: diffusion-controlled mixing in space, convection-controlled mixing on earth and forced mixing on earth and with typical physiological parameters (pH and temperature). Concentrations of the stock solutions, rate of solution mixing and duration of precipitation influence very strongly the chemical composition of the precipitation, the phase composition of individual crystals, their sizes, morphology and structure. Microdiffraction and HRTEM techniques showed an incontestable advantage on other techniques like SEM and XRD in the investigation of small particles and mixtures of calcium phosphates (hydroxyapatite and octacalcium phosphate) with different proportions.
Developmental Origins of Chronic Renal Disease: An Integrative Hypothesis
F. Boubred,M. Saint-Faust,C. Buffat,I. Ligi,I. Grandvuillemin,U. Simeoni
International Journal of Nephrology , 2013, DOI: 10.1155/2013/346067
Abstract: Cardiovascular diseases are one of the leading causes of mortality. Hypertension (HT) is one of the principal risk factors associated with death. Chronic kidney disease (CKD), which is probably underestimated, increases the risk and the severity of adverse cardiovascular events. It is now recognized that low birth weight is a risk factor for these diseases, and this relationship is amplified by a rapid catch-up growth or overfeeding during infancy or childhood. The pathophysiological and molecular mechanisms involved in the “early programming” of CKD are multiple and partially understood. It has been proposed that the developmental programming of arterial hypertension and chronic kidney disease is related to a reduced nephron endowment. However, this mechanism is still discussed. This review discusses the complex relationship between birth weight and nephron endowment and how early growth and nutrition influence long term HT and CKD. We hypothesize that fetal environment reduces moderately the nephron number which appears insufficient by itself to induce long term diseases. Reduced nephron number constitutes a “factor of vulnerability” when additional factors, in particular a rapid postnatal growth or overfeeding, promote the early onset of diseases through a complex combination of various pathophysiological pathways. 1. Introduction Cardiovascular diseases ((CVD) hypertension, coronary disease and stroke, and heart failure) are one of the leading causes of mortality in industrialized countries, and the prevalence is increasing in emerging societies. All cardiovascular diseases account for 4.3 million deaths per year in the European Union, and the prevalence of chronic heart failure in the United States of America is approximately 6 million [1, 2]. In industrialized countries, hypertension (HT) affects 25% to 35% of the global population and reaches 60% to 70% of the population aged 60 or more. Hypertension is the principal risk factor of death worldwide [3]. It increases the severity of ischemic vascular diseases and, with obesity and type 2 diabetes, is one of the important risk factors for chronic kidney disease (CKD). Chronic kidney disease is defined as reduced glomerular filtration rate (GFR) up to end-stage renal disease (ESRD), proteinuria, or both. Prevalence of ESRD, estimated to be 0.5–2.5‰ worldwide, is increasing in several countries [4]. In turn, impaired renal factor favors the development of and amplifies the severity of CVD [5–7]. During the last two decades, it has been raised the concept of developmental programming of adult chronic
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