Sn Superconductors Research Articles

Structural and Superconducting Proximity Effect of SnPb Bimetallic Nanoalloys

We report the superconducting properties between a conventional strong-coupled Pb and weak-coupled Sn superconductor. A series of SnrPb1-r nanoalloys with various compositions r were synthesized, and their superconducting properties were measured using superconducting quantum interference devices (SQUIDs) magnetometer. Our results reveal a superconducting proximity effect (SPE) between immiscible Sn and Pb granules in the range of r = 0.2~0.9, as a weak superconducting coupling can be established with the coexistence of phonon hardening and increased Ginzburg-Landau coherence length. Furthermore, our results provide new insights into improving the study of the superconducting proximity effect introduced by Sn doping.

Towards 20 T Hybrid Accelerator Dipole Magnets

The most effective way to achieve very high collision energies in a circular particle accelerator is to maximize the field strength of the main bending dipoles. In dipole magnets using Nb-Ti superconductor the practical field limit is considered to be 8-9 T. When Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductor material is utilized, a field level of 15-16 T can be achieved. To further push the magnetic field beyond the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn limits, High Temperature Superconductors (HTS) need to be considered in the magnet design. The most promising HTS materials for particle accelerator magnets are Bi2212 and REBCO. However, their outstanding performance comes with a significantly higher cost. Therefore, an economically viable option towards 20 T dipole magnets could consist in an “hybrid” solution, where both HTS and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn materials are used. We discuss in this paper preliminary conceptual designs of various 20 T hybrid magnet concepts. After the definition of the overall design criteria, the coil dimensions and parameters are investigated with finite element models based on simple sector coils. Preliminary 2D cross-section computation results are then presented and three main layouts compared: cos-theta, block, and common-coil. Both traditional designs and more advanced stress-management options are considered.

Properties of Nb3Sn Superconductors of Various Designs

To manufacture a high field magnet it is required to have Nb₃Sn superconductors with high current-carrying capability in magnetic fields up to 18 T. For example, CERN has chosen Nb₃Sn together with HTS as potential material for high field magnet of Future Circular Collider (FCC) project. Also, these superconductors must have an RRR of more than 150 and a low level of energy loss. One of the ways to achieve high current density is to use Internal Tin method to manufacture Nb₃Sn superconductors. In this paper, Internal Tin Nb₃Sn strands with 37, 54, 84 and 120 sub-elements are studied. Using TEM and SEM analysis, the structure and elemental composition of the superconducting layer were investigated. The analysis of the measurement results of electrophysical parameters showed that with an increase in the number of subelements from 37 to 120, at the same heat treatment mode of 370&#x00B0;C, 100 h &#x002B; 665&#x00B0;C, 40 h, the critical current density measured in the range of magnetic fields from 12 to 23 T for strands of different designs practically does not changes significantly, and the RRR parameter decreases from 280 to 110 units and the effective filament diameter decreases from 139 to 70 micrometers.

Study on Internal Strain Distribution of the High-Field Nb₃Sn Superconducting Accelerator Magnets With Homogenization Theory

To fabricate 10–16 Tesla high-field accelerator magnets, the high-Jc Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductor is one of the most practical choices. However, it is a brittle material and is strain-sensitive. To reduce the potential Jc degradation of the conductor, it is crucial to accurately evaluate the stress of the coils owing to the large Lorentz force during magnet excitation. In the traditional stress analysis of magnets, the coils are usually regarded as homogenized structures using isotropic material coefficients; however, most superconducting (SC) coils have hierarchical and complicated structures. To obtain a more accurate estimation, it is necessary to consider the detailed internal structure of the coils during stress analysis. In this study, the homogenization method was applied to multi-scale stress analysis of a high-field accelerator SC magnet. The effective material coefficients of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coils were calculated using this method. Stress analysis of the magnet was performed using the effective material coefficients and isotropic material coefficients for comparison. Compared with the result obtained using isotropic material coefficients, the Von-Mises stress of the coil using effective material coefficients is larger at different load steps. The stress level of most Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn filaments in the strand was higher than that of the coil. The maximum stress of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn filaments appeared in the strand contact regions. In addition, there was a significant difference in the average stress of all materials in the strand. These findings will help develop more accurate analysis and simulation models for high-field SC magnets.

Superconducting Properties of Nb3Sn Superconductors, Doped With Ti, Zr and Ti, Ti and Ta

The paper presents the results of a study of three types of internal tin Nb3Sn strands of the same design, differed by the way of doping with titanium, titanium and zirconium, titanium and tantalum. The diameter of the fabricated strands was 1.0 mm. For the formation of the superconducting phase, a series of two steps heat treatments (HT) were carried out. The temperature of the reaction second stage of HT was varied from 665 to 720 °C. The microstructure of the Nb3Sn layer after HT was studied by scanning electron microscopy. Transport properties of the strands samples were measured in magnetic fields up to 24 T. The interrelation of the HT regimes and superconducting properties of the strands investigated, including RRR, Tc and Jc has been analyzed. It has been shown that with an increase of the temperature of the second stage of heat treatment the critical temperature of samples doped with two elements tends to increase more rapidly compared to a sample doped only with titanium. Studies of the critical current density of the samples in the wide magnetic field range as a function of the HT second stage temperature are presented.

WITHDRAWN: An Analytical Model of Grain Size Effects on Critical Current Density of Nb3Sn Superconducting Nano-polycrystals

Abstract The critical current density of Nb 3 Sn superconductors is a key parameter for its applications in high magnetic field engineering and is affected intensively by the grain size. This study proposes a model describing the grain size effects on the effective critical current density of Nb 3 Sn nano-polycrystals based on the electron scattering pinning mechanism and the collective pinning theory, in which the electron effective mean free path affected by the grain size and grain boundary scattering properties is obtained by solving Boltzmann transport equation. The investigation indicates that the effective critical current density is not only depended on the grain size but also on the electron transmissivity at grain boundaries; when the transmissivity is less than a specific value, there exists the optimal grain size for the effective critical current density. The results predicted by the proposed BT model – small bundle pinning mechanism agree with the experimental results.

Strain Measurements with Fiber Bragg Grating Sensors in the Short Models of the HiLumi LHC Low-Beta Quadrupole Magnet MQXF

The replacement of the LHC Nb-Ti inner triplet magnets with more powerful quadrupole magnets based on Nb3 Sn superconductor, as foreseen by the HiLumi LHC Project, has pushed the technology advancement not only in the manufacturing process and tooling but also in the instrumentation. For the short model quadrupole magnet MQXFS-5, fabricated and tested at CERN, fiber optic sensors, based on fiber Bragg grating (FBG) technology, have been proposed for monitoring the magnet mechanical behavior from its assembly to its operation in cryogenic conditions. After the experience gained with the FBG integration in short model racetrack coils, the paper deals with the first integration of 16 FBG sensors arranged into eight fibers and glued on the coils and on the support structure of the Nb3 Sn quadrupole. The advantages offered by the FBGs' multiplexing capability has allowed the integration improvement of the FBG-based measuring system in the magnet structure as well as in the test facility. Examples of strain measurements during magnet assembly, cool-down and powering are presented in the paper and compared with strain gauges data.

Testing Mechanical Behavior of Nb3Sn Rutherford Cable During Coil Winding

In the framework of the development of high field magnets made of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductor for projects such as high luminosity upgrade of the large hadron collider and future circular collider studies, it is needed to refine the understanding of the coil winding process and its impact on the overall mechanical behavior of the conductor. For this purpose, a new cable winding setup has been developed in order to compare the windability of different Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Rutherford cables. In addition, various geometrical cable inspection methods were tested and compared. First experimental results obtained with the new set-up for winding tests are summarized.

Composite Technical Superconductors

The main applications and trends in the development of modern high-current superconductors, the characteristics of niobium-titanium superconductors for the FAIR (Germany) and NICA (Dubna, Russia) accelerator complexes, medical tomographs, and Nb 3 Sn superconductors for high-resolution spectrometers and modernization of the large hadron collider are examined. Advanced composite superconductors developed at VNIINM for different applications are described. The main results of development work on technologies for the fabrication of targets for spraying buffer and superconducting layers and for substrate tapes of different types for the fabrication of second-generation high-temperature superconductors are examined.

Research and Development of Wires and Cables for High-Field Accelerator Magnets

The latest strategic plans for high energy physics endorse steadfast superconducting magnet technology R&D for future energy frontier facilities. This includes 10 to 16 T Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn accelerator magnets for the luminosity upgrades of the Large Hadron Collider and eventually for a future 100 TeV-scale proton-proton (pp) collider. This paper describes the multi-decade R&D investment in the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductor technology, which was crucial to produce the first reproducible 10 to 12 T accelerator-quality dipoles and quadrupoles, as well as their scale-up. We also indicate prospective research areas in superconducting Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires and cables to achieve the next goals for superconducting accelerator magnets. Emphasis is on increasing performance and decreasing costs while pushing the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn technology to its limits for future pp colliders.

Morphology and Structure of Diffusion Layers in Nb&lt;sub&gt;3&lt;/sub&gt;Sn-Based Superconductors of Different Geometry

The evolution of structure and morphology of superconducting Nb3Sn layers forming under various regimes of the diffusion annealing in multifilamentary Nb/Cu–Sn superconductors of different design is briefly reviewed based on the authors’ (with their coauthors) original studies and the available publications. Different modes of the solid-state diffusion formation of the superconducting phase are compared. Possible mechanisms of the Nb3Sn nucleation upon the diffusion reaction of the Nb filaments and bronze matrixes are discussed. The effects of different doping elements, especially Ti, on the formation of the superconducting phase, its structure and morphology, and, consequently, on the current-carrying capacities of multifilamentary Nb3Sn-based wires are considered. Special attention is paid to the effect of the mode of Ti doping, the content of this alloying element and the regimes of the diffusion annealing on the structure, morphology and resulting performance of the bronze-processed wires with coupled Nb filaments worked out at Bochvar Institute of Inorganic Materials for the creation of International Thermonuclear Experimental Reactor.

The Study of Nb&lt;sub&gt;3&lt;/sub&gt;Sn Phase Formation Process in Multifilamentary Superconductors for High-Magnetic-Field Systems

Recent investigations of the connection between the microstructure and the properties of Nb 3 Sn superconductors produced by the bronze method, for the large ITER magnets, have enabled us to develop wire fabrication technologies and to manufacture large quantities of such material that met all the technical requirements. The next stage is to study the possibility of increasing the critical current properties in a wide range of magnetic fields with the aim to apply them in the next generation of magnetic systems. In this study, the superconducting phase formation process, its structure, and the composition and critical properties of various layouts of Nb 3 Sn wire used under magnetic fields of 12-16 T have been investigated. It is shown that the intermetallic compound structure heavily depends on the grain structure of niobium filaments produced by multiple deformation of the composite wire with intermediate heat treatments. It is concluded that the improvement of the superconducting phase structure and the increase in current-carrying capability of wire in high magnetic fields are possible by the fine grain structure of initial niobium filament and an increase in the tin content of the bronze matrix.

Conceptual Design of the CFETR Toroidal Field Superconducting Coils

The China Fusion Engineering Test Reactor (CFETR) superconducting tokamak is a national scientific research project of China with plasma major and minor radii of 5.7 and 1.6 m, respectively. The magnetic field in the center of plasma at a radius of R = 5.7 m is 5.0 T. The major objective of the project is to build a fusion engineering tokamak reactor with fusion power within the range of 50-200 MW and should be self-sufficient by blanket. Sixteen toroidal field (TF) superconducting coils of CFETR have been arrayed toroidally and are made up of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductor. The maximum magnetic field of the TF coil is 10.8 T under the operation current of 67.4 kA. The TF coil should have strong stability margin under the condition of high field and strain. This paper discusses the design parameters, electromagnetic distribution, structure, and thermal and stability analysis of the TF superconducting magnet for CFETR.

Threshold Field for Runaway Instability of Bilayer Hard Type-II Superconductor

We have solved the problem of the critical state stability of a bilayer type-II superconductor, whose layers have different physical properties (critical current density, specific heat, and critical temperature). We have found the optimal thickness for the outer layer, which results in a maximal increase in the stability threshold—the magnetic field at which the first thermomagnetic avalanche occurs (the jump of the magnetic flux and other physical properties). The calculations were performed for a Nb\(_3\)Sn superconductor coated with a layer of NbTi. It is shown that by choosing the optimum coating thickness, the stability threshold of the magnetic field increases by more than 60 %.

A Dipole Magnet for the FRIB High Radiation Environment Nuclear Fragment Separator

Magnets in the fragment separator region of the Facility for Rare Isotope Beams (FRIB) would be subjected to extremely high radiation and heat loads. The critical elements of FRIB are the dipole magnets, which are used to select the desired isotopes. Since conventional NiTi and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductors must operate at ~ 4.5 K, the removal of the high heat load generated in these magnets using these superconductors would be difficult. High-temperature superconductors have been shown to be radiation resistant and can operate in the 40 K temperature range where heat removal is an order of magnitude more efficient than at 4.5 K. The coils of this magnet must accommodate the large curvature from the 30° bend that the magnet will subtend. This paper will describe the magnetic and conceptual design for these magnets.

Novel Type of Aluminum-Alloy Jacketed ${\rm Nb}_{3}{\rm Sn}$ Superconductors Manufactured by Friction Stir Welding Technique

New process to compose a reacted Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconducting cable with aluminum-alloys has been developed by using friction stir welding (FSW) technique. The FSW joining process, which takes place in the solid phase below the melting point of the materials, has considerable advantages such as a high strength of welding zone and much lower distortion due to both small heat affects and reliable control of joining depth as compared with conventional welding method. In order to realize practical aluminum-alloy jacketed Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductors, we have carried out elemental researches by using FSW technique. For a fabrication of aluminum-alloy jacketed Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductor of a dimension 17 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</sup> × 4.9 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sup> a Rutherford type superconducting cable having 18 reacted Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands of diameter 1.0 mm was assembled into a set of channel and fitting-cover made of aluminum-alloy A6061-T6 with filling material indium. Then, their joining parts were connected by using the FSW technique. The measurement on critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) of the fabricated Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductor achieved over 9.5 kA at 8 T, 4.26 K even after bending with a radius of 150 mm in a flatwise direction, which showed no degradation in comparison with I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values multiplied by number of strands in the cable. As the results, we succeeded in developing novel type of aluminum-alloy jacketed Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductors manufactured by FSW technique, which have large current capacity, sufficient allowable bending strains for a coil winding and improved contact between a cable and an aluminum-alloy jacket. The developed superconductors will be applicable to react-and-wind superconducting magnets having very large current at high magnetic field such as nuclear fusion reactors and detectors for high energy physics of the next generation.

Optimization and Test of 120 mm LARP Nb$_{3}$Sn Quadrupole Coils Using Magnetic Mirror Structure

The US-LARP collaboration is developing a new generation of large-aperture high-field quadrupoles based on Nb Sn superconductor for the LHC upgrades. The development and implementation of this new technology involves the fabrication and testing of series of model magnets, coils and other components with various design and processing features. New 120-mm HQ coils made of Rutherford cable, one with an interlayer resistive core, and both with optimized reaction processes, were fabricated and tested using a quadrupole mirror structure under operating conditions similar to those in a real magnet. The coils were instrumented with voltage taps and strain gauges to study the mechanical and quench performance. Quench antenna and temperature gauges were installed in the mirror structure to measure the coil temperature and locate quench origins. This paper presents details of the coil design and fabrication procedures, coil assembly and pre-stress in the quadrupole mirror structure, and coil test results.

Design Aspects of Dual Nb-Ta Sheet Diffusion Barriers for ${\rm Nb}_{3}{\rm Sn}$ Conductors

Tantalum (Ta) diffusion barriers used in Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductors deform nonuniformly during wire drawing. A strategy sometimes used to mitigate this problem is to place a sacrificial layer of niobium (Nb) next to the Ta to act as a mechanical transition zone between the softer Cu-Nb composite and the harder Ta. This approach can promote more uniform co-deformation of the Ta layer enabling thinner Ta layers and a reduced chance of layer rupture, and has the added benefit of providing additional Nb for conversion to Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn. A thinner Ta layer translates to less Ta volume and less cost. In the current work the authors find that a layer of Nb adjacent to a Ta layer embedded in Cu significantly increases the composite wire strain to fracture, and appears to decrease the Ta layer interface roughness.

Titanium Doped ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Superconductor Fabricated by the Internal Tin Tube (ITT) Approach

Multifilament Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductor has been produced by the internal tin tube (ITT) approach. The process employs tubular niobium filaments with cores consisting of tin or high tin alloy inside a copper sheath. The niobium annulus of each filament contained eight Nb-47Ti ldquoislands.rdquo During the subsequent reaction heat treatments the titanium was diffused out of these islands and throughout the niobium annulus resulting in a calculated titanium concentration of 1.5 wt%. The process holds the promise to minimize the amount of non-superconducting area in the conductor cross section and thereby maximize non-copper critical current density. Niobium tube fabrication, wire processing, filament quality, and microstructure are discussed, and superconducting properties are presented.

A15 Phase Formation Kinetics Study of Mono-Element Internal-Sn Nb$_{3}$Sn Superconducting Wires

Four sets of mono-element internal-Sn (MEIT) wires which have different Sn-Cu ratio and 1 at% Zr addition in one wire were designed and fabricated for investigating A15 phase formation kinetics of ND3 Sn superconductors. All samples underwent a 210degC/50 h + 340degC/25 h thermal duration for Cu-Sn alloying prior to the A15 phase formation heat treatment (HT). Four reaction temperatures of 650degC, 675degC, 700degC, and 725degC were chosen to study the temperature and time influence. All the heat- treated samples were examined by scanning electronic microscope for A15 layer thicknesses that were then plotted versus HT time at different temperatures and were nonlinearly fitted. Superconducting quantum interference device magnetization measurements were used to determine the inductive JC variations for the heat- treated wires. The obtained results demonstrate that the A15 phase growth is promoted by four factors: reaction temperature elevation, reaction time extension, Sn-Cu ratio enlargement, and the alloyed Zr addition. The phase formation kinetics of MEIT Nb3Sn superconducting wires is in agreement with Yn = K(T)t relation and the A15 growth exponent n is affected by HT temperature and Zr alloying.