61 research outputs found

    Scaling Laws for Magnetic Energy in Superconducting Quadrupoles

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    The stored energy in superconducting magnets is one of the main ingredients needed for the quench calculation and for designing quench protections. Here we propose an analytical formula based on the Fourier transformation of the current density flowing within the winding to determine the magnetic energy stored in superconducting quadrupoles made of sector coils. Two corrective coefficients allowing to estimate the energy enhancement produced either by current grading or by the presence of an unsaturated iron yoke are respectively derived from a numerical and an analytical study. This approach is applied to a set of real quadrupoles to test the validity limits of the scaling law, which are shown to be of ~10%

    130 mm Aperture Quadrupoles for the LHC Luminosity Upgrade

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    Several studies for the LHC luminosity upgrade pointed out the need for low-beta quadrupoles with apertures larger than the present baseline (70 mm). In this paper we focus on the design issues of a 130 mm aperture quadrupole. We first consider the Nb-Ti option, presenting a magnetic design with the LHC dipole and quadrupole cables. We study the electromagnetic forces and we discuss the field quality constraints. For the Nb3_{3}Sn option, we sketch three designs, two based on the LARP 10 mm width cable, and one on a larger cable with the same strand. The issue of the stress induced by the e.m. forces, which is critical for the Nb3_{3}Sn, is discussed using both scaling laws and finite element models

    Study of fluid behaviour under gravity compensated by a magnetic field

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    International audienceFluids, and especially cryogenic fluids like Hydrogen H2 and Oxygen O2 , are widely used in space technology for propulsion and cooling. The knowledge of fluid behaviour during the acceleration variation and under reduced gravity is necessary for an efficient management of fluids in space. Such a management also asks fundamental questions about thermo-hydrodynamics and phase change once buoyancy forces are cancelled. For security reasons, it is nearly impossible to use the classical microgravity means to experiment with such cryofluids. However, it is possible to counterbalance gravity by using the paramagnetic (O2) or diamagnetic (H2) properties of fluids. By applying a magnetic field gradient on these materials, a volume force is created that is able to impose to the fluid a varying effective gravity, including microgravity. We have set up a magnetic levitation facility for H2 in which many experiments have been performed. A new facility for O2 is under construction that will enable fast change in the effective gravity by quenching down the magnetic field. The facilities and some particularly representative experimental results are presented

    A Review of the Properties of Nb3Sn and Their Variation with A15 Composition, Morphology and Strain State

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    This article gives an overview of the available literature on simplified, well defined (quasi-)homogeneous laboratory samples. After more than 50 years of research on superconductivity in Nb3Sn, a significant amount of results are available, but these are scattered over a multitude of publications. Two reviews exist on the basic properties of A15 materials in general, but no specific review for Nb3Sn is available. This article is intended to provide such an overview. It starts with a basic description of the Niobium-Tin intermetallic. After this it maps the influence of Sn content on the the electron-phonon interaction strength and on the field-temperature phase boundary. The literature on the influence of Cu, Ti and Ta additions will then be briefly summarized. This is followed by a review on the effects of grain size and strain. The article is concluded with a summary of the main results.Comment: Invited Topical Review for Superconductor, Science and Technology. Provisionally scheduled for July 200

    TCR and CD28 Concomitant Stimulation Elicits a Distinctive Calcium Response in Naive T Cells

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    T cell activation is initiated upon ligand engagement of the T cell receptor (TCR) and costimulatory receptors. The CD28 molecule acts as a major costimulatory receptor in promoting full activation of naive T cells. However, despite extensive studies, why naive T cell activation requires concurrent stimulation of both the TCR and costimulatory receptors remains poorly understood. Here, we explore this issue by analyzing calcium response as a key early signaling event to elicit T cell activation. Experiments using mouse naive CD4+ T cells showed that engagement of the TCR or CD28 with the respective cognate ligand was able to trigger a rise in fluctuating calcium mobilization levels, as shown by the frequency and average response magnitude of the reacting cells compared with basal levels occurred in unstimulated cells. The engagement of both TCR and CD28 enabled a further increase of these two metrics. However, such increases did not sufficiently explain the importance of the CD28 pathways to the functionally relevant calcium responses in T cell activation. Through the autocorrelation analysis of calcium time series data, we found that combined but not separate TCR and CD28 stimulation significantly prolonged the average decay time (ŌĄ) of the calcium signal amplitudes determined with the autocorrelation function, compared with its value in unstimulated cells. This increasement of decay time (ŌĄ) uniquely characterizes the fluctuating calcium response triggered by concurrent stimulation of TCR and CD28, as it could not be achieved with either stronger TCR stimuli or by co-engaging both TCR and LFA-1, and likely represents an important feature of competent early signaling to provoke efficient T cell activation. Our work has thus provided new insights into the interplay between the TCR and CD28 early signaling pathways critical to trigger naive T cell activation

    On the bubble shape in a magnetically compensated gravity environment

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    International audienceWe investigate the shape of bubbles in liquid oxygen under magnetic levitation conditions: a magnetic field is applied that polarizes bulk oxygen, and its spatial variation induces a body force opposed to its weight. In these conditions, bubbles appear to have a smooth ellipsoidal shape, which may be prolate (elongated in the vertical direction), oblate (elongated in the horizontal plane) or perfectly spherical. The dependence of the elongation ratio ő∑ on the volume and levitation position is explored. It is found that the bubble shape is prescribed by the minimization of the sum of surface tension, demagnetization and magnetic‚Äďgravitational potential energies

    Les effets de diffusion dans les instabilités magnétiques des supraconducteurs de seconde espèce

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    Apr√®s un bref rappel concernant les sauts de flux les auteurs √©tudient les deux m√©canismes de diffusion qui interviennent au cours de ces ph√©nom√®nes : la diffusion de la chaleur et la diffusion de l'induction magn√©tique. Les diff√©rentes m√©thodes de stabilisation sont ensuite d√©crites ainsi que les crit√®res qui leur correspondent. Dans la derni√®re partie, les auteurs soulignent le r√īle jou√© par la conductibilit√© de flots de flux dans le m√©canisme de d√©clenchement des instabilit√©s

    Les effets de diffusion dans les instabilités magnétiques des supraconducteurs de seconde espèce

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    After a short reminder of the flux jump phenomenon the authors investigate the two diffusion processes : diffusion of heat and diffusion of magnetic induction. The various stabilization methods are then described, both with the corresponding criteria. In the last part the authors emphasize the role played in the instability triggering mechanism, by the flux flow conductivity.Apr√®s un bref rappel concernant les sauts de flux les auteurs √©tudient les deux m√©canismes de diffusion qui interviennent au cours de ces ph√©nom√®nes : la diffusion de la chaleur et la diffusion de l'induction magn√©tique. Les diff√©rentes m√©thodes de stabilisation sont ensuite d√©crites ainsi que les crit√®res qui leur correspondent. Dans la derni√®re partie, les auteurs soulignent le r√īle jou√© par la conductibilit√© de flots de flux dans le m√©canisme de d√©clenchement des instabilit√©s
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