Table of contents

In this letter we demonstrate the development of ternary Nb₃Sn multifilamentary conductors with artificial pinning centers (APC) which achieve high critical fields. These recently-developed conductors were tested in a 31 T magnet, and the results showed that their upper critical field (B_c2) values at 4.2 K are 27–28 T, and irreversible field (B_irr) values are above 26 T, values similar to or higher than those of best rod-restack-process (RRP) conductors. The non-Cu J_c has been brought to nearly 1200 A mm⁻² at 16 T and 4.2 K, comparable to RRP, in spite of the fact that the fine-grain Nb₃Sn fractions in filaments are still low (20%–30%) and the grain sizes are still not fully refined (70–80 nm) due to conductor designs and heat treatments that are not yet optimized. The Nb₃Sn layer J_c at 4.2 K, 16 T is 4710 A mm⁻² for the APC wire with 1%Zr, about 2.5 times higher than RRP conductors, in spite of the fact that its grain size is not yet fully refined due to insufficient oxygen and unoptimized heat treatment. An analysis is presented about the non-Cu J_c that can be achieved by further optimizing the APC conductors and their heat treatments.

Synchrotron light sources employ undulators to produce brilliant photon beams. Most synchrotrons and all free electron lasers use undulators made by permanent magnets. Superconducting undulators can produce, for the same geometry, a higher peak magnetic field on axis with respect to permanent magnet ones, and have the additional advantage of being radiation hard. Recently superconducting undulators have been successfully applied in the Karlsruhe Institute of Technology synchrotron and in the Advanced Photon Source at the Argonne National Laboratory. While precise measurement techniques to characterize the magnetic field for permanent magnet undulators are well established, new developments have been necessary in the past few years to magnetically characterize superconducting undulators. After an introduction on undulators and the measurements needed to be performed before operating such devices with electron beams, this paper reviews magnetic field measurements of full scale conduction cooled superconducting undulator coils. Current and future methods used for the characterization of the magnetic field, after installation of such coils in the final cryostat, are also discussed.

Iron-based superconductors (IBSs), discovered in 2008, formed the second high-T_c superconductor family after cuprate superconductors, and over the past decade have been the subject of extensive research into their physical nature and application potential. With their attractions of very high upper critical fields and small electromagnetic anisotropy, tremendous advances have been made in wire research and development (R&D) to explore the potential of IBSs for high-field applications. In recent years, rapid progress was made on the critical current density (J_c) of the 122-type IBS wires based on a powder-in-tube technique. Encouraging breakthroughs were made, including a high transport J_c exceeding the practical level of 10⁵ A cm⁻² (at 4.2 K, 10 T) and the first 100 meter-class wire. This review covers the state-of-the-art techniques and their mechanism in realizing high transport J_c with respect to the grain connectivity, grain texture and flux pinning for IBS wires and tapes, as well as the temperature and field angle dependence of critical currents. The mechanical properties, AC losses and magneto-thermal stability of IBS wires are investigated, and further improvements in IBS conductors for large-scale applications are proposed. In addition to long wire fabrication, this review also highlights some remarkable advances relevant to practical applications, including scalable process optimization, copper sheaths, multifilamentary fabrication, and superconducting joints. Finally, a summary and outlook for R&D for IBS wires are presented.

We report on atomic-scale analyses of the microstructure of an Nb₃Sn coating on Nb, prepared by a vapor diffusion process for superconducting radiofrequency (SRF) cavity applications using transmission electron microscopy, electron backscatter diffraction and first-principles calculations. Epitaxial growth of Nb₃Sn on a Nb substrate is found and four types of orientation relationships (ORs) at the Nb₃Sn/Nb interface are identified by electron diffraction or high-resolution scanning transmission electron microscopy (HR-STEM) analyses. Thin Nb₃Sn grains are observed in regions with a low Sn flux and they have a specific OR: Nb₃Sn $(1\bar{2}0)$//Nb $(\bar{1}11)$ and Nb₃Sn $(002)$//Nb $(0\bar{1}1).$ The Nb₃Sn/Nb interface of thin grains has a large lattice mismatch, 12.3%, between Nb $(0\bar{1}1)$ and Nb₃Sn (002) and a high density of misfit dislocations as observed by HR-STEM. Based on our microstructural analyses of the thin grains, we conclude that the thin regions are probably a result of a slow interfacial migration with this particular OR. The Sn-deficient regions are seen to form initially at the Nb₃Sn/Nb interface and remain in the grains due to the slow diffusion of Sn in bulk Nb₃Sn. The formation of Sn-deficient regions and the effects of interfacial energies on the formation of Sn-deficient regions at different interfaces are estimated by first-principles calculations. The finding of ORs at the Nb₃Sn/Nb interface provides important information about the formation of imperfections in Nb₃Sn coatings, such as large thin-regions and Sn-deficient regions, which are critical to the performance of Nb₃Sn SRF cavities for accelerators.

Superconducting joints are one of the crucial components to make Sr_1−xK_xFe₂As₂ (Sr-122) superconducting wires or tapes successful for future high-field, high-homogeneity magnetic application. In this paper, the hot-pressing process for an iron-based superconducting joint of Sr-122 tapes was optimized. The microstructures, superconducting properties and element distribution in the connection areas were researched. The transport properties of the iron-based superconducting joints were enhanced by prolonging the holding time at high pressure, which prevents potassium loss in the iron-based superconducting joint. The best transport critical current that was achieved was 57 A at 4.2 K and 10 T; meanwhile, the critical current ratio (CCR = I_c^joint/I_c^non-joint) of the joint was realized to be 63.3%, which is the highest value of an iron-based superconducting joint reported so far. These results clearly demonstrate that iron-based superconductors are very promising for high-field magnet applications.

For the first time a mobile underwater full tensor magnetic gradiometer (FTMG) system based on low-T_c superconducting quantum interference devices (SQUIDs) has been deployed in order to scan the sea floor for magnetized targets. The application is mainly focused on waste deposits and unexploded ordnance (UXO), but could also include shallow geological features as well as archaeological remains. The main methods for detection and localisation of underwater UXO and waste deposits are side sonar scanning and magnetic mapping. While modern sonar scanners can achieve a very high spatial resolution and long detection range, they still have problems detecting targets under cover—for magnetic sensors a layer of non-magnetic sand or ooze does usually not have an effect on the signal apart from an increased distance. In this paper we discuss the setup of the mobile underwater FTMG SQUID system, its challenges and main performance features. It also illustrates its detection and localization capabilities in tests on known magnetic targets.

A Nb₃Sn wire which was manufactured for the ITER toroidal field coil conductor by a bronze route process was prepared for this study to investigate the effect of neutron irradiation on the critical current in a high magnetic field. The critical current of the virgin wire was measured in liquid helium with a 28 T hybrid superconducting magnet at the High Field Laboratory for Superconducting Materials in Tohoku University. It was also measured in vacuum with a heat conduction type variable temperature insert (VTI) at the International Research Center for Nuclear Materials Science at Tohoku University. The wire was irradiated at below 100 °C by fission neutrons at up to 4.9 × 10²² neutrons m⁻² (>0.1 MeV) at BR2 in Belgium, and the critical current after the irradiation was evaluated with a VTI in the range of 8–15.5 T. The difference of the critical current measured with two facilities was discussed, focussing on Joule heating of the sample holder which was made of pure copper, and the neutron irradiation effect on the critical current was investigated in the range of up to 15.5 T. The results show that the critical current measured in vacuum becomes lower than that in liquid helium because of the temperature rise of the sample holder where the sample was soldered, the critical current was increased by the neutron irradiation, and the current ratio (I_C/I_C0) was almost constant of 1.75 in the range of 8–15.5 T at around 4 K.

Within the framework for establishing standards of test methods for superconducting technical wires, various standards have been issued by the International Electrotechnical Commission (IEC) (standard documents IEC 61788-1 to -20). Following the successful round robin test (RRT) for tensile testing REBCO wires at room temperature (Osamura K et al 2014 Supercond. Sci. Technol. 27 085009), this effort is extended to tensile test HTS wires at cryogenic temperatures and is coordinated by the CryoMaK lab at Karlsruhe Institute of Technology. Five different commercially available REBCO wires from five different manufacturers and one BiSCCO wire from another supplier were provided by the Versailles Project on Advanced Materials and Standards (http://vamas.org) for testing. Samples were distributed between eight participating labs from five different countries for testing according to the specified guidelines. After the test results were delivered by all participants, the data were evaluated with statistical tools to investigate the main source of scatter and its magnitude in the test results. The final goal of the RRT is issuing an ISO/IEC standard for a cryogenic temperature tensile test for REBCO wires. In this report the results of the RRT for tensile testing REBCO wires at cryogenic temperatures are presented and discussed.

The engineering of the microstructure of a superconducting FeSe thin film by the choice of a proper substrate and its crystallographic orientation is demonstrated. To this end epitaxial c-axis oriented FeSe thin films are grown on (110) oriented single-crystalline SrTiO₃ substrates by means of sputtering. The extraordinary microstructure is characterized by the uniaxial alignment of plate-like crystalline grains along the ${\left[001\right]}_{{\rm{S}}}$ substrate direction. The alignment of the elongated grains eventuates in a direction-dependent grain-boundary structure associated with an anisotropic grain-boundary density along the two perpendicular ${\left[001\right]}_{{\rm{S}}}$ and ${\left[1\bar{1}{\rm{0}}\right]}_{{\rm{S}}}$ substrate directions. The effect of the grain-boundary structure on the observed anisotropies of the temperature dependent in-plane resistivities and critical current densities in transverse magnetic fields is extensively studied for current transport along ${\left[001\right]}_{{\rm{S}}}$ and ${\left[1\bar{1}{\rm{0}}\right]}_{{\rm{S}}}.$

Superconducting material parameters of the Nb film coatings on the quarter-wave resonator for the HIE-ISOLDE project were studied by fitting experimental results with the Bardeen–Cooper–Schrieffer Mattis–Bardeen (BCS-MB) theory. We pointed out a strong correlation among fitted estimators of material parameters in the BCS-MB theory, and used a procedure to reduce the uncertainty by merging two χ² distributions of the surface resistance and effective penetration depth. In this procedure, unlike previous studies, BCS coherence length (ξ₀) and London penetration depth (λ_L) were not fixed at their literature values in the calculation because in our film, whose thickness is from 2 to 13 μm, they may take altered values as a consequence of microstructual defects. Since surface resistance and penetration depth have similar dependencies on coherence length and mean free path, the effects of the correlation between the estimators of these two parameters could not be mitigated by just combining surface resistance and penetration depth data. We used upper critical field measurement by SQUID magnetometry to provide a complementary constraint to these RF measurements, and this allowed all the material parameters to be obtained by fitting the experimental data. In the best performing cavity, the determined parameters are ξ₀ = 29 nm, λ_L = 26 nm, mean free path l = 99 nm, and coupling strength Δ₀/k_BT_c = 1.7. The coherence length is slightly shorter than for clean bulk Nb (39 nm) in the literature although the Nb film is thick and rather bulk-like. A poorly performing cavity showed weaker coupling constant Δ₀/k_BT_c = 1.5 which may indicate film contamination during the coating process.

A Conductor on Round Core (CORC^®) cable wound with a high temperature superconductor is an important cable concept for high current density applications. The design of a CORC cable makes understanding its electromagnetic performance—for example its AC losses—challenging. This paper presents a thorough study of CORC cables by combining experimental and numerical methods. In particular, it focuses on understanding how the cable structure influences the magnetization losses and on how these can be reduced. A novelty of this paper lies in the use of a new T-A formulation, which, for the first time, is employed for three-dimensional modelling of a CORC cable with real geometry. The use of the new T-A formulation in finite element software enables the study of how the winding direction and multiple-layer structure affect the magnetization losses of CORC cables. Moreover, influence of striation in CORC cables is studied as an effective way to reduce their losses. A CORC cable with striated tapes shows a significant magnetization loss reduction at high magnetic fields, in comparison to its counterpart without striated tapes. At low magnetic fields, tape striation leads to an increase in loss when the number of filaments is low, then the loss drops with a further increase in the number of filaments, but this loss reduction is much weaker than that at high fields. This paper provides an efficient tool for investigating the electromagnetic behaviour of CORC cables, which can provide valuable guidance in designing CORC cables with minimized losses for high energy physics and energy conversion applications.

The demand for coated conductors with high cost performance has triggered the search for effective artificial pinning centers by the low cost and high-scalability route. In this work, YBa₂Cu₃O_7−x (YBCO) nanocomposite films with BaMnO₃ nanosized particles induced by manganese (Mn) addition were prepared by low-fluorine metal-organic deposition. A strategy of rapid heating during the medium-temperature stage was adopted to reduce the size of the BaMnO₃ nanoparticles and improve the quality of the YBCO films. An enhanced critical current density and pinning force were found in the high-quality YBCO film embedded with extraordinarily small nanoparticles (average 3 ± 1 nm).

Bulk, single grain (RE)Ba₂Cu₃O_7−δ [(RE)BCO, where RE is a rare earth element or yttrium] high temperature superconductors exhibit significant potential for use in a variety of engineering applications due to their ability to trap large magnetic fields, which can be up to ten times greater than those generated by conventional, iron-based magnets. Limitations on the maximum size to which single grains can be grown, however, are a major obstacle to the further development of these materials. Indeed, multiple samples are often required to achieve the required superconducting properties in particular applications. The geometry of bulk (RE)BCO single grain samples plays an important role in determining the superconducting properties of a given technical arrangement. In order to gain a better understanding of the full application potential of bulk single grain superconductors, three relatively long, cylindrical YBCO single grains of different diameters were fabricated and their trapped field and total trapped flux measured at 77 K as a function of sample height. The effects of size and aspect ratio of YBCO single grains on these key applied properties have been investigated experimentally and the results compared qualitatively with the predictions of an established theoretical model. Conclusions based on the trapped field measurements on a variety of single grain samples are presented in this study and the possibilities of using assemblies of smaller samples for engineering devices, in particular, are discussed.

We investigate the influence of carbon-ion irradiation on the superconducting (SC) critical properties of MgB₂ thin films. MgB₂ films of two thicknesses, 400 nm (MB400nm) and 800 nm (MB800nm), were irradiated by 350 keV C ions having a wide range of fluence, 1 × 10¹³–1 × 10¹⁵ C atoms cm⁻². The mean projected range (R_p) of 350 keV C ions in MgB₂ is 560 nm, thus the energetic C ions will pass through the MB400nm, whereas the ions will remain into the MB800nm. The SC transition temperature (T_c), upper critical field (H_c2), c-axis lattice parameter, and corrected residual resistivity (ρ_corr) of both the films showed similar trends with the variation of fluence. However, a disparate behavior in the SC phase transition was observed in the MB800nm when the fluence was larger than 1 × 10¹⁴ C atoms cm⁻² because of the different T_cs between the irradiated and non-irradiated parts of the film. Interestingly, the SC critical properties, such as T_c, H_c2, and critical current density (J_c), of the irradiated MgB₂ films, as well as the lattice parameter, were almost restored to those in the pristine state after a thermal annealing procedure. These results demonstrate that the atomic lattice distortion induced by C-ion irradiation is the main reason for the change in the SC properties of MgB₂ films.

In this study, the effects of heat treatment temperature on superconducting properties were systematically studied by sintering silver-sheathed Ba_1−xK_xFe₂As₂ tapes at different temperatures. The transport J_c increases as the sintering temperature rises from 700 °C to 850 °C. The optimized J_c values of tapes sintered at 900 °C and 850 °C reach 6.2 × 10⁴ A cm⁻² (4.2 K and 10 T) which represent a very high J_c level for flat rolled mono-core silver-sheathed iron-based superconducting wires and tapes. The properties of tapes sintered at different temperatures were investigated, including grain crystallinity, core density, c-axis texture and elemental distribution. The results demonstrate that 850 °C is a suitable temperature to fabricate silver-sheathed Ba_1−xK_xFe₂As₂ superconducting tapes.

We present the crystallographic analysis, superconducting and spectroscopic characterization, and theoretical modeling of CeIr₃. Lattice parameters a = 5.2945(1) Å and c = 26.219(1) Å are found for the R-3m symmetry crystal structure, which are close to the literature values. CeIr₃ is a moderate type-II superconductor (κ_GL = 17, λ_e–p = 0.65) below 2.5 K. Ce ions exhibit a strongly intermediate valence character as evidenced by x-ray photoelectron spectroscopy. The normal state magnetic susceptibility is weakly temperature dependent and follows the inter-configuration fluctuation model with a singlet Ce−4 f⁰ ground state. Theoretical calculations support a non-magnetic ground state of the system and reveal that Ir−5d states are dominant at the Fermi level.

Understanding vortex behaviour at microscopic scales is of extreme importance for the development of higher performance coated conductors with larger critical currents. Here, we study and map the critical state in a YBCO-based coated conductor at different temperatures using two distinct operation modes of scanning Hall microscopy. An analytical Bean critical state model for long superconducting strips is compared with our measurements and used to estimate the critical current density. We find several striking deviations from the model; pronounced flux front roughening is observed as the temperature is reduced below 83 K due to vortex-bundle formation when strong broadening of the flux front profile is also seen. In higher magnetic fields at the lower temperature of 65 K, fishtail-like magnetization peaks observed in local magnetization measurements are attributed to flux-locking due to an increase in the critical current density near the edges of the tape, which we tentatively link to vortex pinning matching effects. Our measurements provide valuable insights into the rich vortex phenomena present in coated conductor tapes at the microscopic scale.

We investigated the transport properties of a curved iron-based superconducting whisker, a geometry which is considered to be interesting for electronic applications. The current–voltage characteristics (IVCs) are bi-stable at low temperatures, exhibit multiple branches at intermediate temperatures and are continuous but nonlinear at temperatures close to the superconducting transition temperature T_c. Low temperature scanning laser microscopy revealed that the multiple branches arise from a localized hot spot which for the different branches appears at different positions along the whisker. By contrast, at low temperatures Joule heating completely overheats the sample to temperatures above T_c in the resistive state. Close to T_c, the hot spot disappears and the IVCs are continuous. In the multiple-branch region, the whisker can be switched between different states by laser irradiation, provided that the laser is positioned at the location of the hot spot. The effect could be interesting for using the whisker as a photon detector or as a position-sensitive opto-electronic switch.