Superconductors can carry very high current densities without resistive loss. This makes them highly suitable for the construction of compact high field magnets of several tens of tesla. However, with increasing current density and magnetic field come high Lorentz loads and high strain levels, and it is empirically known that superconducting properties are affected by strain. The superconducting properties of the most commonly used high field material, Nb3Sn, are particularly badly affected by strain. Here we demonstrate that strain causes significant sub-lattice distortion in the A15 lattice structure of Nb3Sn, and show how this leads to the relatively large reduction of its superconducting properties. The changes are found to be primarily due to changes in the electron density of states, with a lesser contribution due to changes in the phonon spectrum. The amount of sub-lattice distortion further depends on crystal orientation. These findings suggest that it is possible to mitigate the reduction of the current carrying capacity by strain through crystal alignment and sub-lattice stabilization. This would enable superconducting magnets to reach a significantly higher magnetic field, and ease their construction by simplifying react-and-wind coil fabrication.

This paper presents a recently developed Code for Heater Delay Analysis (CoHDA), which is a tool for modeling protection heater induced quenches in superconducting Nb3S high-field accelerator magnets. The CoHDA thermal model numerically computes the heat diffusion from the heater to the coil and estimates the time delay to quench initiation by comparing the coil temperature with its critical surface. The model takes into account heater geometry, power, and various insulation layers and coil properties. Computational heater delays are compared with experimental data from the U.S. Large Hadron Collider Accelerator Research Program Nb3S High-Gradient Quadrupole magnet with good agreement. Based on the results, CoHDA provides a useful tool for quench protection design in impregnated magnets. Copyright © 2014 IEEE.

The effect of disorder on the normal state resistivity and the superconducting properties of Nb3Sn Sn is explored in a combination of ab initio calculations and microscopic theory. The crystal symmetry is calculated to be preferentially tetragonal at a normal state resistivity below 27.0 ±1.4 μcm, and preferentially cubic above this value, which is shown to be consistent with the experimentally observed transition point. The phonon density of states, the Eliashberg spectrum a2 (w)F (w), the electronphonon coupling constant, the characteristic frequency, the critical temperature Tc, and the upper critical magnetic field at 0 K Hc2 (0) are calculated over a large normal state resistivity range and shown to be consistent with experimental observations. The high degree of consistency between the calculation results and experimental observations is a strong indication that the calculation approach utilized here, a combination of ab initio calculations and microscopic theory, is a useful tool for understanding the superconducting and normal state properties of Nb3Sn.

Lawrence Berkeley National Laboratory (LBNL) is developing Wind-and-React (W&R) Bi2sr2cacu2o8+δ (Bi-2212) accelerator magnet technology for insert coils, to surpass the intrinsic limitations of Nb-based magnets, and eventually develop hybrid systems that can approach 20 T dipole fields. The Bi-2212 technology is being developed in close collaboration with industry, and has been partly supported by the US Very High Field Superconducting Magnet Collaboration (VHFSMC). Steady improvements were made over the last several years, with coil HTS-SC08 reaching 2636 A, or about 85% of its witness sample critical current (Ic). Though this is still a factor 3 to 4 too low to be competitive with Nb-based materials, it is expected that the required Ic can be achieved through further conductor optimizations. Recent developments include the commissioning of infrastructure for the reaction of coils at LBNL. Earlier coils were fabricated and tested at LBNL, but were reacted at the wire manufacturer. We describe in detail the furnace calibrations and heat treatment optimizations that enable coil reactions at temperatures approaching 890 °C with a homogeneity of ± 1 °C in a pure oxygen flow. We reacted two new coils at LBNL, and tested the performance of coil HTS-SC10 at 4.2 K in self-field using a superconducting transformer system. We find that its performance is consistent with witness samples, and comparable to coil HTS-SC08, which is an identical coil that was reacted at Oxford Instruments Superconductor Technology (OST), thereby validating the in-house reaction process.

Binary Nb-Sn thin film samples were fabricated and characterized in terms of their composition, morphology, and superconducting properties. Nb-Sn was magnetron-sputtered onto heated R-plane sapphire substrates at 700 ^{\circ}\hbox{C}, 800 ^{ \circ}\hbox{C}, and 900 ^{\circ}\hbox{C}, using a custom-built heater assembly. Samples were cut into strips, where each strip has a unique composition. For a subset of the samples, Nb-Sn was selectively etched away at an etching rate of 6 \pm 1 nm/s using an aqueous solution of 3 vol.\% hydrofluoric and 19 vol.\% nitric acid. The sample composition was investigated with a scanning electron microscope with an X-ray energy dispersive spectroscopy detector. Surface and cross-section morphologies were investigated using scanning electron microscopy and scanning transmission electron microscopy, revealing a dense columnar poly-crystalline grain structure. X-ray diffraction measurements indicate a highly textured film that is (100) oriented out-of-plane and random in-plane. The critical temperature Tc (ranging from 9.8 to 17.9 K), critical magnetic field \mu-{0} Hc2 (ranging from 12.5 to 31.3 T), residual resistivity ratio (RRR), and normal state resistivity \rho0 were measured and found to be broadly consistent with literature data on bulk Nb-{3}\hbox{Sn}. © 2002-2011 IEEE.