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.

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HE-LHC : The High-Energy Large Hadron Collider Future Circular Collider Conceptual Design Report Volume 4

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.

A general scaling relation for the critical current density in Nb3Sn

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.

https://iopscience.iop.org/article/10.1088/0034-4885/74/12/124511/pdf

The behavior of grain boundaries in the Fe-based superconductors

Microstructure, composition and critical current density of superconducting Nb3Sn wires

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.

The invention concerns a device for the high pressure densification of superconducting wire out of compacted superconductor material or superconductor precursor powder particles, said device comprising four hard metal anvils (5, 6, 7, 8) with a total length (L2) parallel to the superconducting wire, said hard metal anvils relied to external independent pressure blocks (9, 10, 11), which are in turn either fixed or connected to high pressure devices, preferably hydraulic presses, which is characterised in that at least one of the hard metal anvils is a free moving anvil (6) having clearances of at least 0,01 mm up to 0,2 mm towards the neighbouring hard metal anvils (5, 8), so that no wall friction is occurring between the free moving anvil and the neighbouring anvils. This allows for high critical current densities Jc at reduced pressure applied to the hard metal anvils.

Device and method for the densification of filaments in a long superconductive wire

Low temperature calorimetry was used to determine the distribution of the superconducting transition temperature in binary and Ta-doped wires fabricated by the Bronze route and by the Powder-In-Tube (PIT) method. From this analysis we were able to discern the effects of Sn and Ta compositions on the distribution of the superconducting parameters Tc and Bc2 in the different samples. The influence of the heat treatment conditions on the superconducting properties was investigated for the PIT wires. In particular we determined the field dependence of the distribution for a commonly used reaction schedule (675 /84 h) and for an optimized heat treatment (625 /320 h). For the first time, we show that the wire reacted at 625/320 h exhibits two separated contributions in the distribution directly related to the grain morphology of the A15 layer: a narrow peak determined by the large grains, with a lower Bc2, and a broad peak due to the fine grains, with a higher Bc2. The kinetics of the Sn diffusion in Nb and the growth rate of the A15 layer were experimentally studied. The influence of Ta doping on the A15 phase formation was analysed by electron microscopy, the growth rate and the grain morphology in binary and Ta-alloyed Bronze route wires with the same filament layout being compared at different stages of the heat treatment. At the end of reaction, the well known microstructure comprising equiaxed and columnar regions was observed in the filaments of both the binary and the Ta-alloyed wires. Based on these observations and from the growth rate analysis we conclude that Ta does not affect the Sn diffusion rate.

Phase Formation, Composition and ${\rm T}_{\rm c}$ Distribution of Binary and Ta-Alloyed ${\rm Nb}_{3}{\rm Sn}$ Wires Produced by Various Techniques

PROBLEM TO BE SOLVED: To provide a device and a method for generating a high critical current density Junder a condition that a pressure applied to a hard metal anvil is reduced.SOLUTION: The device makes a superconducting wire material into high pressure and high density from a compressed superconducting material or a superconducting precursor powder particle. The device is provided with four hard metal anvils (5, 6, 7, 8) with an entire length (L2) parallel to the superconducting wire material, and the hard metal anvils are supported by outside independent pressure blocks (9, 10, 11), and are finally fixed or connected to a high pressure device, preferably a liquid pressure press. At least one of the hard metal anvils is a free movement anvil (6) having a gap of at lest 0.01 mm-0.2 mm relative to the adjacent hard metal anvils (5, 8), so as to prevent wall friction between the free movement anvil and the adjacent anvils.

Device and method for making filament in long superconducting wire material into high density

We have analyzed the influence of Ta on the superconducting properties of Ti doped bronze processed multifilamentary wires. The study was carried out on wires with identical configuration, consisting of 14,641 filaments with 4.5 diameter embedded in a Ti bearing Osprey bronze and externally stabilized by copper. Several wires were made using either Nb7.5Ta or pure Nb as a filament material, some of them containing a core of NbTi as a supplementary Ti source. All wires were manufactured by three hot extrusion steps, cold drawing and several intermediate anneals. Wire samples with round and rectangular cross sections have been prepared and reacted with various heat treatments between 550 and 730 . The residual Sn content in the Cu-Sn-Ti bronze after reaction and the degree of reaction of the filaments were determined by means of EDS and SEM analysis, respectively. AC susceptibility has been used to determine the effect of prestrain and doping on the superconducting transition temperature . At 21 T and 4.2 K, similar non-Cu values up to 90 (overall : 63 ) and values up to 25 have been observed on both compositions and . These results suggest that Ta as a quaternary dopant does not lead to further improvements of and of ternary .

F. Buta

Papers

Device and method for the densification of filaments in a long superconductive wire

Phase Formation, Composition and ${\rm T}_{\rm c}$ Distribution of Binary and Ta-Alloyed ${\rm Nb}_{3}{\rm Sn}$ Wires Produced by Various Techniques

Device and method for making filament in long superconducting wire material into high density

Investigation on the Effect of Ta Additions on and of Bronze Processed Multifilamentary Wires at High Magnetic Fields