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[en] The paper presents a detailed study of thermal instabilities in cooling flows associated with galaxies and clusters of galaxies. In the case of purely radiation-driven accretion onto a central object such as the cD galaxy M87, it is found that the gas is largely subject to overstability, rather than to monotonic instability. If thermal conductivity is taken into account, the flow is stabilized on scales of several kiloparsecs, even if the conductivity is appreciably reduced (e.g., about 1 percent) with respect to the Spitzer value. In no case are the globular perturbations (i.e., perturbations with comparable radial and azimuthal dimensions) found to be monotonically unstable. The paper presents numerical solutions of the local dispersion relation for the cooling flow in M87 and discusses the possible consequences of the results for a correct understanding of cooling flows. 24 references
[en] Previous numerical calculations of two-dimensional, compressible convection are extended to three dimensions, using a higher order Godunov scheme. The results show that the flow readily becomes supersonic in the upper boundary layer, where shock structures form intermittently in the vicinity of the strong downflow lanes. The convection as a whole is strongly time-dependent and evolves on a time scale comparable to the sound crossing time. The motions in the upper layers are characterized by the rapid expansion of the upward-moving fluid elements. In the interior, most of the heat is carried by a small fraction of the fluid residing in strong, highly coherent downflows. The remaining fluid is dominated by small-scale, disorganized turbulent motions. 25 refs
[en] The capability of manufacturing long superconducting MgB2 wires with already remarkable critical currents makes this material a very promising candidate for future applications. Tapes are prepared by the powder-in-tube technique. After the cold working procedure typically carried out by wire drawing and cold rolling, it has been found that a final sintering step carried out in argon atmosphere is a key process for further improving the superconducting properties of the conductors. To study the effect of the deformation and heat treatment processes, we performed neutron scattering experiment. Due to the high penetration depth of neutron inside matter, it was possible to analyse the MgB2 phase still wrapped in the Ni sheath. Our studies were carried out by a full spectra refinement by the Rietveld method. In the starting superconducting powder a large Mg deficiency was observed. In the tapes we found that the large forces applied during the cold working induced a large MgB2 lattice deformation, and that it is partly relaxed during the final sintering process. An important correlation of the residual stress with the critical temperature and the pinning properties was pointed out. We also observed the appearance of detrimental secondary phases during the sintering process. In particular, the MgB2 phase reacted with the nickel sheath and MgB2Ni2.5 was formed at temperatures higher than 850 deg C. These results are of basic importance for a further optimization of the transport properties at moderate fields where applications of MgB2 tapes are already envisageable
[en] The MgB2 superconductor has already demonstrated its potential, in particular for DC applications such as magnetic resonance imaging (MRI) magnets, thanks to the low cost of the raw materials and to its simple production process. However, further efforts have still to be made in order to broaden its employment in AC applications such as superconducting fault current limiters (SFCLs), motors, transformers etc. The main issues are related to the reduction of AC losses. Some of these can be faced by obtaining multifilamentary conductors with a large number of very fine filaments and, in this context, the powder's granulometry can play a crucial role. We have prepared MgB2 starting powders with different granulometries and by the ex situ powder-in-tube (PIT) method we have realized multifilamentary wires with a number of filaments up to 361 and an average size of each filament lowered down to 30 μm. In particular we have studied the relationship between grain and filament size in terms of transport properties and have shown that the optimization of this ratio is possible in order to obtain suitable conductors for AC industrial applications.
[en] Numerical simulations with high spatial resolution (up to 96-cubed gridpoints) are used to study three-dimensional, compressible convection. A sequence of four models with decreasing viscous dissipation is considered in studying the changes in the flow structure and transport properties as the convection becomes turbulent. 39 refs
[en] The first step towards high critical currents in Bi-2212 wires was recognizing that the supercurrent is blocked over long lengths by filament-diameter bubbles grown during the melt stage, which cause expansion of the wire diameter and dedensification of the superconducting filaments. While a succesful approach to reducing the problem of voids related to bubbles involved the application of a high overpressure during the heat treatment, we fabricated Bi-2212 wires by applying a new concept of suitably alternating groove-rolling and drawing techniques with the aim of densifying the phase during the working procedure prior to the heat treatment. We here for the first time were able to reach, in wires reacted with closed ends—i.e. with gas trapped in the wire as it happens in long length wires—the very same values of critical current shown in short wires reacted with open ends. This is the irrefutable evidence that, only by acting on the deformation technique, we were able to raise the critical current by properly densifying the superconducting powder inside the filaments already before the melt stage. Whole-conductor current densities in our long-length simulation wires already reach 400 A mm"−"2 at 4.2 K and 5 T, which can be still easily increased through architecture optimization. The actual breakthrough is that the densification is optimized without further complex treatments through a technique which can be straightforwardly applied to long length wires. (paper)
[en] In this work we investigate the effect of each different heat treatment stage in the fabrication of Bi2Sr2CaCu2O8+δ superconducting wires on intra-grain and inter-grain superconducting properties. We measure magnetic critical temperature T c values and transport critical current density J c at temperatures from 4 K to 40 K and in fields up to 7 T. From an analysis of the temperature dependence of the self-field critical current density J c( T ) that takes into account weak link behavior and the proximity effect, we study grain boundary (GB) transparency to supercurrents; we also establish a relationship between GB oxygenation in the different steps of the fabrication process and GB transparency to supercurrents. We find that GB oxygenation starts in the first crystallization stage, but it becomes complete in the plateau at 836 °C and in slow cooling stages and is further enhanced in the prolonged post-annealing step. Such oxygenation makes GBs more conductive, thus improving the inter-grain J c value and temperature dependence. On the other hand, from inspection of the T c values in the framework of the phase diagram dome, we find that grains are already oxygenated in the crystallization step up to the optimal doping, while successive slow cooling and post-annealing treatments further enhance the degree of overdoping, especially if carried out in oxygen atmosphere rather than in air. (paper)
[en] In order to use MgB2 conductors for AC applications, research and development efforts have to be carried out on their architecture and sheath material to minimize the AC losses. This paper will present the fabrication and characterization of two types of ex situ powder-in-tube processed pure MgB2 conductors with properties making them good candidates for AC industrial applications: a multifilamentary tape with 12 filaments with a twisting pitch down to 17 mm and a 361-filament wire with an average single-filament size of about 30 μm. Concerning the twisted tapes we will present values of critical current density of about 105 A cm-2 at 4.2 K and 2 T and we will show that it is possible to achieve a proper compromise between the tape size, the twisting pitch length and the critical current density to face a reduction of the critical current density as a consequence of the strain on the filaments. Concerning the 361-multifilamentary wire we will show appreciable values of critical current density of about 5 x 104 A cm-2 at 4.2 K and 2.5 T which, together with the advantages given by the high number of very thin filaments and the non-magnetic matrix, could justify their employment.
[en] The present status of ex-situ nickel-sheathed MgB2 superconducting tape development is described. Typical critical current data of short conductors are presented as a function of the temperature and of the magnetic field. The results achieved so far on longer lengths are also reported and related to the short sample data. Mono-filamentary tapes have been manufactured in pieces up to 100 meters in length and used to wind pancakes. High critical currents have been measured at temperatures up to 20 K in moderate applied magnetic fields. The lower critical currents observed in the longer tapes have been related to the damage of the superconducting core due to winding and to a residual fluctuation of the superconducting cross section observed along the tape length. Recent advances in the cold working procedure has allowed to realize multifilamentary tapes that show similar critical currents than the mono-core ones, but capable of sustaining a larger bending strain and with a lower fluctuation of the superconducting cross-section. Multifilamentary conductors, manufactured so far in short lengths, should represent the solution to solve the long length degradation