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F. Buta

Bio: F. Buta is an academic researcher from University of Geneva. The author has contributed to research in topics: Critical field & Pinning force. The author has an hindex of 10, co-authored 24 publications receiving 316 citations.

Papers
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Journal ArticleDOI
TL;DR: In this article, the effect of Ta, Ti and Ta+Ti additives on the physical properties of Nb3Sn multifilamentary wires with optimized critical currents is discussed.

118 citations

Journal ArticleDOI
TL;DR: The Conductor Development Program aiming to reach a non-copper critical current density of 1500 A/mm 2 at 16 T and 4.2 K has been launched by CERN, with the involvement of industry and laboratories worldwide.
Abstract: The study of next-generation high-energy accelerators based on 16 T dipoles has emphasized the need for higher performance, cost-effective Nb 3 Sn superconducting wires. A Conductor Development Program aiming to reach a non-copper critical current density (J c ) of 1500 A/mm 2 at 16 T and 4.2 K has been launched by CERN, with the involvement of industry and laboratories worldwide. In this paper, the targets and strategy of the program are presented, with an overview of the wire layouts and development activities being pursued by each partner, and the latest characterization results are reported. Three of the four participating manufacturers have successfully reached the first-stage J c milestone, but a significant advance is still needed to achieve the final target. The next steps are briefly discussed, as the program focuses increasingly on novel alloys and methods to maximize J c .

43 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the site occupancy of Ta and Ti additives in Nb3Sn wires and showed that the Ta atoms occupied the 6c chain sites, while the Ti atoms were located on the cubic 2a sites.
Abstract: The critical current density in industrial Nb3Sn and MgB2 wires is currently optimized by introducing various kinds of additives, either Ta and/or Ti for Nb3Sn wires or SiC or C for MgB2 wires. In the following, several problems linked to the presence of additives in the two classes of compounds are discussed. A reinvestigation of the site occupancy of Ta and Ti additives in Nb3Sn wires shows that the Ta atoms occupy the 6c chain sites, while the Ti atoms are located on the cubic 2a sites. It follows that in perfectly ordered A15 compounds A1−βBβ, the relation ρo versus β exhibits a 'universal' behavior: the effect of the chemical nature of the constituents on ρo is negligible. The slopes of ρ0 versus the Ti, Ga and Ni contents in the A15 layer coincide and are much steeper than for the Ta additive, corresponding to the three times higher number of 6c sites with respect to 2a A15 lattice sites. The presence of two grain morphologies, e.g. equiaxial and columnar, is observed in Nb3Sn wires produced by the bronze route only. The nonlinearity of the Kramer plot in multifilamentary Nb3Sn bronze route wires is explained by the presence of these two different grain types, which have distinctly different Sn contents and sizes. For these wires, the total pinning force can be represented as the superposition of two contributions with different scaling fields. Simultaneous addition of different additives on 'in situ' Fe/MgB2 wires is presented as an attempt to combine different possible mechanisms influencing Jc. The substitution of boron by carbon is known to enhance the value of ρo and thus of the critical field. In addition, the pinning behavior is expected to be improved by grain boundary effects or nanosize precipitations, caused by the presence of appropriate additives during the MgB2 phase formation. Since the two mechanisms are independent, their effect on Jc is expected to be cumulative. In the present paper, the results on the additive combination B4C+LaB6 in monofilamentary Fe sheathed MgB2 wires are reported. The data are compared with the additives B4C+SiC and show that simultaneous additives could be promising in view of applications at 20 K.

37 citations

Journal ArticleDOI
TL;DR: In this article, synchrotron X-ray diffraction measurements during in-situ reaction heat treatment (HT) of a state-of-the-art high Jc Nb3 Sn internal tin strand are reported.
Abstract: The phase transformations that occur during the reaction heat treatment (HT) of Nb3Sn superconductors depend on the overall elemental composition of the strand subelements. In the case of modern high Jc strands with a relatively low Cu content, liquid phases are present during large temperature intervals and phases that can be detrimental for the microstructural and microchemical homogeneity of the fully reacted strand are formed. We report synchrotron X-ray diffraction measurements during in-situ reaction HT of a state-of-the-art high Jc Nb3 Sn internal tin strand. In this strand, Cu3Sn is formed upon Cu 6Sn 5 decomposition at 415degC, a Sn-rich ternary Cu-Nb-Sn phase is detected in the approximate temperature interval 345degC-575degC, and NbSn2 is present in the temperature interval 545degC-630degC. The formation of voids in the strand subelements has been monitored by synchrotron microtomography during in-situ reaction HT.

27 citations

Journal ArticleDOI
TL;DR: In this article, the relationship between lattice deformations and superconducting properties of Nb3Sn wires has been clarified using high-energy x-ray diffraction experiments at the European Synchrotron Radiation Facility (ESRF) in Grenoble.
Abstract: With the aim of clarifying the relationship between lattice deformations and superconducting properties of Nb3Sn technological wires we have carried out high-energy x-ray diffraction experiments at the European Synchrotron Radiation Facility (ESRF) in Grenoble on individual samples of multi-filamentary internal-tin-type Nb3Sn wires. In particular, a test probe developed at the University of Geneva allowed us to perform these experiments at 4.2 K, while applying an axial tensile load to the specimen. In this way, the lattice parameter values of all the constituents (Nb3Sn, Nb, Cu) were determined, in both the parallel and orthogonal directions with respect to the applied load axis, as a function of the applied strain. The experiments were performed on industrial wires, which were reinforced by a stainless steel outer tube, applied before the Nb3Sn reaction heat treatment, in order to evaluate the effect of an additional pre-compression strain. The relation between the microscopically determined crystalline lattice deformations and the measured applied strain is discussed as a basis for the analysis of the superconducting performances of Nb3Sn wires subject to mechanical loads.

25 citations


Cited by
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Journal ArticleDOI
A. Abada1, Marcello Abbrescia2, Marcello Abbrescia3, Shehu S. AbdusSalam4  +1496 moreInstitutions (238)
TL;DR: The third volume of the FCC Conceptual Design Report as discussed by the authors is devoted to the hadron collider FCC-hh, and summarizes the physics discovery opportunities, presents the FCC-HH accelerator design, performance reach, and staged operation plan, discusses the underlying technologies, the civil engineering and technical infrastructure, and also sketches a possible implementation.
Abstract: In response to the 2013 Update of the European Strategy for Particle Physics (EPPSU), the Future Circular Collider (FCC) study was launched as a world-wide international collaboration hosted by CERN. The FCC study covered an energy-frontier hadron collider (FCC-hh), a highest-luminosity high-energy lepton collider (FCC-ee), the corresponding 100 km tunnel infrastructure, as well as the physics opportunities of these two colliders, and a high-energy LHC, based on FCC-hh technology. This document constitutes the third volume of the FCC Conceptual Design Report, devoted to the hadron collider FCC-hh. It summarizes the FCC-hh physics discovery opportunities, presents the FCC-hh accelerator design, performance reach, and staged operation plan, discusses the underlying technologies, the civil engineering and technical infrastructure, and also sketches a possible implementation. Combining ingredients from the Large Hadron Collider (LHC), the high-luminosity LHC upgrade and adding novel technologies and approaches, the FCC-hh design aims at significantly extending the energy frontier to 100 TeV. Its unprecedented centre-of-mass collision energy will make the FCC-hh a unique instrument to explore physics beyond the Standard Model, offering great direct sensitivity to new physics and discoveries.

161 citations

Journal Article
TL;DR: In this paper, the scaling relations for the critical current density (Jc) in Nb3Sn wires and include recent findings on the variation of the upper critical field (Hc2) with temperature (T) and A15 composition.
Abstract: We review the scaling relations for the critical current density (Jc) in Nb3Sn wires and include recent findings on the variation of the upper critical field (Hc2) with temperature (T) and A15 composition. Measurements of Hc2(T) in inevitably inhomogeneous wires, as well as analysis of literature results, have shown that all available Hc2(T) data can be accurately described by a single relation from the microscopic theory. This relation also holds for inhomogeneity averaged, effective, Hc2*(T) results and can be approximated by Hc2(t)=Hc2(0) = 1-t1.52, with t = T=Tc.Knowing Hc2*(T) implies that also Jc(T) is known. We highlight deficiencies in the Summers/Ekin relations, which are not able to account for the correct Jc(T) dependence. Available Jc(H) results indicate that the magnetic field dependence for all wires from mu0H = 1 T up to about 80 percent of the maximum Hc2 can be described with Kramer's flux shear model, if non-linearities in Kramer plots when approaching the maximum Hc2 are attributed to A15 inhomogeneities. The strain (e) dependence is introduced through a temperature and strain dependent Hc2*(T,e) and Ginzburg-Landau parameter kappa1(T,e) and a strain dependent critical temperature Tc(e). This is more consistent than the usual Ekin unification of strain and temperature dependence, which uses two separate and different dependencies on Hc2*(T) and Hc2*(e). Using a correct temperature dependence and accounting for the A15 inhomogeneities leads to the remarkable simple relation Jc(H,T,e)= (C/mu0H)s(e)(1-t1.52)(1-t2)h0.5(1-h)2, where C is a constant, s(e) represents the normalized strain dependence of Hc2*(0) andh = H/Hc2*(T,e). Finally, a new relation for s(e) is proposed, which is an asymmetric version of our earlier deviatoric strain model and based on the first, second and third strain invariants. The new scaling relation solves a number of much debated issues withrespect to Jc scaling in Nb3Sn and is therefore of importance to the applied community, who use scaling relations to analyze magnet performance from wire results.

145 citations

Journal ArticleDOI
TL;DR: In this paper, the authors consider the positive role that grain boundaries (GBs) play in the metallic, low-temperature superconductors and then review the theoretical background and current experimental data relating to the properties of GBs.
Abstract: The Fe-based superconductors (FBSs) are an important new class of superconducting materials. As with any new superconductor with a high transition temperature and upper critical field, there is a need to establish what their applications potential might be. Applications require high critical current densities, so the usefulness of any new superconductor is determined both by the capability to develop strong vortex pinning and by the absence or ability to overcome any strong current-limiting mechanisms of which grain boundaries (GBs) in the cuprates are a cautionary example. In this review we first consider the positive role that GB properties play in the metallic, low-temperature superconductors and then review the theoretical background and current experimental data relating to the properties of GBs in FBS polycrystals, bicrystal thin films and wires. Based on this evidence, we conclude that GBs in FBS are weak linked in a qualitatively similar way to GBs in the cuprate superconductors, but also that the effects are a little less marked. Initial experiments with the textured substrates used for cuprate coated conductors show similar benefit for the critical current density of FBS thin films too. We also note that the particular richness of the pairing symmetry and the multiband parent state in FBS may provide opportunities for GB modification as a better understanding of their pairing state and GB properties are developed.

140 citations

Journal ArticleDOI
TL;DR: In this article, the effect of Ta, Ti and Ta+Ti additives on the physical properties of Nb3Sn multifilamentary wires with optimized critical currents is discussed.

118 citations

Journal ArticleDOI
TL;DR: In this article, the theoretical background and current experimental data relating to the properties of grain boundaries in FBS polycrystals, bi-crystal thin films, and wires are reviewed.
Abstract: The Fe-based superconductors (FBS) are an important new class of superconducting materials. As with any new superconductor with a high transition temperature and upper critical field, there is a need to establish what their applications potential might be. Applications require high critical current densities, so the usefulness of any new superconductor is determined both by the capability to develop strong vortex pinning and by the absence or ability to overcome any strong current-limiting mechanisms of which grain boundaries in the cuprates are a cautionary example. In this review we first consider the positive role that grain boundary properties play in the metallic, low temperature superconductors and then review the theoretical background and current experimental data relating to the properties of grain boundaries in FBS polycrystals, bi-crystal thin films, and wires. Based on this evidence, we conclude that grain boundaries in FBS are weak linked in a qualitatively similar way to grain boundaries in the cuprate superconductors, but also that the effects are a little less marked. Initial experiments with the textured substrates used for cuprate coated conductors show similar benefit for the critical current density of FBS thin films too. We also note that the particular richness of the pairing symmetry and the multiband parent state in FBS may provide opportunities for grain boundary modification as a better understanding of their pairing state and grain boundary properties are developed.

111 citations