NbTi Wires Research Articles

A Multiple Layers Superconducting Magnet Design for 9.4T Magnetic Resonance Imaging

A five-layer superconducting magnet design for 9.4-T whole-body magnetic resonance imaging using an NbTi wire is presented in this paper. In order to control the peak field within the winding region and reduce the consumption of the high-field NbTi superconducting wire, four different conductors were used in four field grades: 9.6, 8.2, 7.0, and 5.2 T, respectively. The 0-1 integer linear programming optimization method was adopted to obtain the final coils' geometries. The design has an inner diameter of 0.95 m and a length of 2.9 m. The field uniformity is better than 5 ppm over a diameter spherical volume of 30 cm with the maximum magnetic field of 9.57 T in the coil region when the center magnetic induction is 9.4 T.

Experimental and Numerical Analysis of Interfilament Resistances in <formula formulatype="inline"><tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex></formula> and NbTi Strands

The transverse resistivity among filaments is a crucial parameter of composite superconducting wires, since it affects the coupling losses in ac regime, the current transfer length of the wire subjected to bending strain, and the current redistribution among filaments in the case of a quench. A direct measurement of the transverse interfilament resistance as a function of temperature in several Nb3 Sn multifilamentary wires was performed by means of a 4-probe method. This paper deals with the numerical analysis of the measurements performed on a OCSI wire manufactured by Luvata, containing 82 superconducting subelements each made of 150 filaments. The measurements are analyzed by applying a 2-D FEM of the wire cross-section and a 3-D electrical circuit model of the wire sample. The results obtained have been compared with those found on two other Nb3Sn wires with different architecture and a NbTi wire taken as a reference.

A Test System for Superconducting Joints Based on GM Cryocooler

A helium-free superconducting joints test system based on a 1.5 W GM cryocooler has been built in the Institute of Modern Physics, CAS. The resistance and the critical-current of a joint at a particular magnetic field and a particular temperature can be measured synchronously. This system consists of a device on the second stage of the cryocooler and an instrumentation system. The basic method of this system is inductive technique and the current-decay method. The main components of the device include a drive coil, a sample holder, and a background magnet. The measurement process is operating by a LabView program. It has been used to test the NbTi wire joints of the Lanzhou penning trap magnet; it seems reliable and convenient. The principles of main design and some results are presented in this paper.

Numerical Simulation of Superconducting Coil Wound With No-Insulation NbTi Wire

A no-insulation winding technique for an NbTi superconducting coil is expected to improve its dynamic stability during charging or discharging. Experimental results, recently reported, demonstrated the charging stability enhancement of an NI coil. In an NI winding, an amount of current can bypass to adjacent wires to avoid the generation of a local hot spot and, as a result, the dynamic stability is improved. In spite of the potential technical merits of the NI technique, the charging responses of an NI coil, including the non-uniform current distribution under a time-varying condition, have not been investigated analytically yet. In this paper, we propose a simulation method using an equivalent circuit equation for an NI coil with its inductance and the contact turn-to-turn resistances taken into consideration. The simulation results were analyzed in three aspects: terminal voltage, loss energy, and stored magnetic energy.

Intrawire resistance, AC loss and strain dependence of critical current in MgB2 wires with and without cold high-pressure densification

The intrawire resistance and alternating current (AC) loss of two MgB2 wires with filaments surrounded by Nb barriers have been measured and analyzed. Relatively high values of filament-to-matrix contact resistivity are found in the MgB2 wires; the values are two or three orders higher than those commonly found in NbTi or Nb3Sn wires. Considering the high porosity of the MgB2 filaments, cold high-pressure densification has been applied on the two MgB2 wires to investigate its impact on intrawire resistance and AC loss. The intrawire resistance is measured with a direct four-probe voltage-current method at various temperatures. The AC loss is measured by vibrating sample magnetometer measurements at 4.2 K. In addition to the intrawire resistance measurements, the critical current of MgB2 wires before and after densification is measured with a U-shaped bending spring at 4.2 K as function of axial strain. The critical current in densified MgB2 wires is found to be higher than that in the same wire without densification; it is also less sensitive to the applied axial strain.

A Time-Harmonic Approach to Numerically Model Losses in the Metal Matrix in Twisted Superconductors in External Magnetic Field

The first NbTi superconductor was developed in 1962 at Westinghouse. During 50 years, the manufacturing process of NbTi wires became highly optimized, and complex wire structures, which are needed in reducing ac losses, can be produced these days. Twisted multifilamentary structures generate many challenges from the modeler's point of view. Considering numerical modeling, NbTi wires are too complicated to model with filament-level details. This is due to the high number and very nonlinear resistivity of filaments. Consequently, simplified approaches are needed for wire modeling. In this paper, a time-harmonic approach to model losses in the metal matrix is introduced. It is based on the linear approximation of filaments. The method to determine the linear resistivity of filaments is based on the definition of the skin depth and the radius of filaments. This can be done in advance without solving a nonlinear problem in 3-D. The suitability of the time-harmonic approach is benchmarked against the H-formulated 3-D eddy current model (ECM) with power-law resistivity. According to simulations, the losses in the metal matrix predicted by the time-harmonic approach agreed well with the nonlinear ECM when the filaments were uncoupled. The results achieved with the proposed approach similarly followed the effects of geometrical changes in the wire structure on losses as the nonlinear ECM and predicted the circumstances where the filaments became partially coupled. The simulation times were considerably lower with the new approach. From the manufacturer's perspective, it is important to design conductors where the filaments stay uncoupled and have low ac losses in such a situation. Thus, the new approach can provide an effective design tool in developing new superconductors.

High-performance MgB2 superconducting wires for use under liquid-helium-free conditions fabricated using an internal Mg diffusion process

MgB2 has a superconducting transition temperature (Tc) of 39 K, which is much higher than that for practical metallic superconductors. Thus, it is hoped that MgB2 can not only replace metallic superconductors, but can be used under liquid-helium-free conditions, for example, at temperatures of 10–20 K that can easily be achieved using cryocooling systems. However, to date, the reported critical current density (Jc) for MgB2 wires is not high enough for large-scale applications in liquid-helium-free conditions. In the present study, successful fabrication of high-performance MgB2 superconducting wires was carried out using an internal Mg diffusion (IMD) process, involving a p-dimethylbenzene (C8H10) pre-treatment of carbon-coated B powder with nanometer-sized particles. The resulting wires exhibited the highest ever Jc of 1.2 × 105 A cm−2 at 4.2 K and 10 T, and an engineering critical current density (Je) of about 1 × 104 A cm−2. Not only in 4.2 K, but also in 10 K, the Jc values for the wires fabricated in the present study are in fact higher than that for Nb–Ti wires at 4.2 K for the magnetic fields at which the measurements were carried out. At 20 K and 5 T, the Jc and Je were about 7.6 × 105 A cm−2 and 5.3 × 103 A cm−2, respectively, which are the highest values reported for MgB2 wires to date. The results of a detailed microstructural analysis suggested that the main reason for the superior electrical performance was the high density of the MgB2 layer rather than just the small grain size, and that the critical current could be further increased by suitable control of the microstructure. These high-performance IMD-processed MgB2 wires are thus promising superconductors for applications such as magnetic resonance imaging and maglev trains that can operate under liquid-helium-free conditions.

Experimental investigation of the transverse resistivity in Nb3Sn wires through ac susceptibility

In this paper we compare two different experimental methods to estimate the transverse resistivity in superconducting NbTi and Nb3Sn multifilament wires. The first method is based on direct measurements of the resistance through micro wires bonded on the cross section of the strand. The second method exploits the physical meaning of the ac susceptibility of the multifilament wires. Both methods have been previously used: the first on Nb3Sn and NbTi strands, while the second has been applied only to NbTi wires. Here we first compare and analyze results in a NbTi system, then we extend the ac susceptibility method to Nb3Sn wires. The advantages and the limits of both techniques, when used on wires with different layouts, are analyzed. The results of the transverse resistivity obtained by using the two experimental methods are finally compared to the estimation based on a numerical approach.

Magic-Angle-Spinning NMR Magnet Development: Field Analysis and Prototypes

We are currently working on a program to complete a 1.5 T/75 mm RT bore magic-angle-spinning nuclear magnetic resonance magnet. The magic-angle-spinning magnet comprises a z-axis 0.866-T solenoid and an x-axis 1.225-T dipole, each to be wound with NbTi wire and operated at 4.2 K in persistent mode. A combination of the fields creates a 1.5-T field pointed at 54.74 degrees (magic angle) from the rotation (z) axis. In the first year of this 3-year program, we have completed magnetic analysis and design of both coils. Also, using a winding machine of our own design and fabrication, we have wound several prototype dipole coils with NbTi wire. As part of this development, we have repeatedly made successful persistent NbTi-NbTi joints with this multifilamentary NbTi wire.

The Manufacture and Test of a GdBCO Insert Coil for an 11 T Superconducting Magnet

In this paper, an 11 T hybrid (LTS/HTS) magnet was designed and developed. The LTS background magnet generates 9 T and the HTS insert coil generates 2 T. The inner diameter of the HTS coil is 40 mm. The outer diameter of the LTS magnet is 176 mm. The height of the inner HTS magnet and the outer LTS magnet are 100 and 200 mm, respectively. The LTS magnet and the HTS coil were fabricated using NbTi wire and GdBCO tape, respectively. The LTS background magnet was fabricated without mandrel, thus reducing shear stress, and using a pre-preg material between each layer. Pre-preg is a thin, glass-woven fabric impregnated with a resin. The electromagnetic field analysis presented here was performed with respect to the structure and the operating current of both the LTS magnet and HTS coil. The hybrid (LTS/HTS) magnet was operated at liquid helium temperature 4.2 K. After several quench, the maximum magnetic field of the center was 10.51 T.

Finite Element Simulations of Twisted NbTi Conductors

Low-temperature superconducting wires, NbTi and Nb3Sn, designed for ac applications, such as CERN and ITER magnets, are composed of twisted multifilament structures. Under time-varying applied magnetic field, twisting decreases the induced electromotive forces between the filaments and is therefore an effective method to reduce interfilamentary coupling. In order to study coupling losses computationally with high precision, 3-D numerical models are needed. In this work, we use 3-D finite-element method simulations to study hysteresis and coupling losses in NbTi superconductors. We investigate the effect of twist pitch on ac losses. In practice, NbTi wires cannot be studied at the filament level due to the extremely complex geometries. However, the manufacturing of these wires is done by using filament bundles. Therefore, we consider wires that consist of homogenized filament bundles embedded in normal conducting matrix. In particular, we consider the effect of barriers around filament bundles and how filaments should be arranged in bundles to minimize the losses. The simulations show that the qualitative behavior of the model is consistent with the analytical results and it can be used, e.g., in optimization processes, where a comparison of wire geometries is needed. Additionally, when considering the coupling of filaments, the barrier plays a very important role in minimizing the losses.

Design Process for a NbTi Wire With New Specification Objectives: Technical Design Constraints and Optimization of a Wire Layout Considering Critical Current and AC Losses

New ac applications set high requirements for the performance of superconductors. In particular, in particle accelerators, where large magnets are frequently charged and discharged, low ac losses and high stability must be provided. Otherwise, the cooling costs become intolerable, or the reliability cannot be guaranteed. Although the NbTi conductors have been available for a long time, new wire configurations are needed to fulfill new high performance requirements. Development work of a superconducting wire requires adjusting many parameters, such as filament size and zones and the composition of matrix metal. In addition, there are some technical limits for different design parameters due to the manufacturing process. In this paper, we consider the design process and the design parameters and their limitations from the manufacturer's point of view. As a part of this NbTi wire design, the use of numerical ac loss computation and an optimization method is demonstrated in designing of a cross-section layout for minimizing the ac losses and to achieve a high critical current. This optimization method is among the first steps of numerical optimization procedures, which considers the geometrical optimization, and more studies are needed before it could become a method of practical use. However, in such a case, substantial savings in time and costs in a design process of a new wire can be achieved.

Monofilament MgB2 Wire for a Whole-Body MRI Magnet: Superconducting Joints and Test Coils.

This paper presents recent results from our continued development of a 0.5 T whole-body MRI magnet at the Francis Bitter Magnet Laboratory. HyperTech Research Corp. (Columbus, OH) manufactures the MgB2 conductor for this project. During the past year, we have found that our technique, originally developed successfully to splice unreacted multifilament MgB2 wires, works much better, i.e., of higher reliability, with unreacted monofilament MgB2 wires. This has led us to wind the entire coil components in our persistent-mode MRI magnet with unreacted monofilament MgB2 wire, having a MgB2 core of 0.4 mm in diameter, an overall diameter of 0.8 mm bare, 1 mm S-glass insulated. To verify that these coils would not suffer from flux jumping, as they would if wound with monofilament NbTi wire, magnetization studies were performed on monofilament wires of MgB2 and NbTi (as a reference) at 4.2 K. For the monofilament MgB2 wire, the results were affirmative. To further ensure the absence of flux jumping that may quench these current-carrying coils, two test coils were wound with unreacted monofilament MgB2 wire. One MgB2 coil was operated in driven mode, while the other MgB2 coil, equipped with a persistent current switch and terminated with a superconducting joint, was operated in persistent mode. The operating temperature range was 4.2-15 K for these MgB2 coils. The driven mode coil was operated in self-field. The persistent mode coil achieved a persistent current of 100 A, corresponding to a self-field of ~1 T in the winding, for 1 hour with no measurable decay. Both test coils were operated quench free.

Intra-wire resistance and AC loss in multi-filamentary MgB2 wires

Intra-wire resistance and AC loss of various multi-filamentary MgB2 wires with filaments surrounded by Nb barriers have been measured and analyzed. The intra-wire resistance is measured with a direct four-probe voltage–current method at various temperatures. The AC loss is acquired by both vibrating sample magnetometer (VSM) and magnetization measurements at 4.2 K. With the aid of finite element method simulations, the filament-to-matrix contact resistance and effective transverse resistivity are derived from direct intra-wire resistance measurements. The effective transverse resistivity values are in good agreement with those analytically derived from the AC coupling loss measurements. Surprisingly, very high values of filament-to-matrix contact resistivity are found, being 2 or 3 orders higher than commonly found for NbTi or Nb3Sn wires.

Stability of superconducting coils made of NbTi wires internally doped with large heat capacity Gd2O3 powder

During the past decade we have been carrying out R&D aimed at increasing the stability of low temperature superconducting magnets. The main idea is to enhance the superconductor temperature margin by means of introduction into the winding of several volume per cent of specific substances with enormously large heat capacities (LHCS) at liquid helium temperatures. Two doping techniques have been developed at the Kurchatov and Bochvar institutes: the ‘external’ one—introduction of a dopant into the epoxy compound—and the ‘internal’ one—the introduction of doping filaments directly into NbTi and Nb3Sn wires. Up to now, our experiments with LHCS internal doping have been carried out on short samples only. In this paper we report the first experimental and numerical investigation of the stability of small-scale internally doped coils made of NbTi wires of a new type, with Gd2O3 ceramic filaments. The coil wound from doped wire and an identical control coil without any doping were subjected to electromagnetic pulses with 1.4–7.4 ms duration. Minimum quench energies for the doped coil turned out to be about 80% larger than those for the undoped one. The gain is especially pronounced in the range of large transport currents (∼0.9Ic). Comparative effectiveness analyses of the two (external and internal) LHCS doping techniques are also presented.

Preliminary Design of Superconducting Dipole Coils for HIAF in IMP

Heavy-Ion Advanced Research Facility (HIAF) is a project in the twelfth five-year plan for heavy ion research in China. The dipole magnet system of the low energy beam line on this project plans to use superconducting magnets. This type of magnet is similar to the prototype dipole for Super-FRS of FAIR, and its features are warm iron and warm bore, only the coils are housed in one cryostat and cooled with liquid helium. The coils have a trapezoidal-shaped configuration which will be wound with NbTi wires of high Cu/Sc ratio. This paper introduces the preliminary design of the superconducting coil system which including the stability analysis of the superconductor, selection of the superconducting wire, quench analysis, mechanical structure analysis of coils and coil box based on 3D finite-element analysis. According to the results of the analysis, more details of quench protection and structure should be taken into account in future research.

A Superconducting Magnetization System With Hybrid Superconducting Wire for the Study and Application of Superconductivity

For the study of high-temperature superconductors and for the application of superconductivity, a superconducting high-field magnetization system with a hybrid superconducting coil was designed. This coil is composed of superconducting wires of three kinds-HTS YBCO 2G-wire, high-field and low-field NbTi wires. Three principal purposes of building this system are to inspect the characteristics of disk-shape bulk YBCO, to develop a HTS-bulk undulator, and to magnetize a portable HTS-bulk high-field magnet for application to a resonant X-ray scattering experiment. With four steps of temperature decrease, a two-stage GM-type cryocooler and liquid nitrogen provide cooling for the magnetization system. The hybrid superconducting coil, two cryogenic systems for the magnetization system and the portable HTS-bulk magnet, and the control and monitor systems that were designed are discussed.

A new structure of superconducting magnetic system for 50 GHz operations (invited)

High field and high frequency have been leading the development of electron cyclotron resonance ion sources (ECRISs) in the past decade as demonstrated by the achieved great performance. The present superconducting magnet structures built with NbTi wires have reached an axial field of 3.5-4.0 T and a radial field of 2.0 T for operating frequency up to 28 GHz. Further increase of the magnetic field strength will require higher current superconductor, i.e., Nb(3)Sn wires. This paper will present the features of a new superconducting magnet structure and review of the existing structures. Using NbTi wires, the new magnet structure could be able to produce maximum fields of 7.0 T on axis and radial field of 3.7 T at a hexagonal plasma chamber wall for ECRIS operations up to 50 GHz. If this new magnet can be built with Nb(3)Sn wires, much higher fields can be expected.

Possible Application of NbTi Artificial Pinning Centers Wire for Insertion Devices

Abstract Superconductive insertion devices (IDs) allow higher fields for a given gap and period length compared to the classical permanent magnet IDs. This technological concept enables an increase in the brilliance and/or the photon energy. The workhorse for superconducting magnets are multifilament NbTi wires, which are nowadays also used for superconducting insertion devices. Even higher magnetic fields can be reached by using a conductor with enhanced critical current density.Here, we propose a possible application for superconducting undulators, wound with NbTi wire with artificial pinning centers, developed by SupraMagnetics, Inc. We report the critical current characteristic, Jc(B), of a short wire measured in a liquid helium bath, and the load-line of a racetrack coil, designed to simulate the field configuration on the conductor as in a superconducting undulator. Based on the measured load line, we report on the simulations of the magnetic field on axis and of the spectrum in a third generation light source of a possible undulator wound with a wire having similar properties to the measured one.

Microscopic role of carbon on MgB2 wire for critical current density comparable to NbTi

Increasing dissipation-free supercurrent has been the primary issue for practical application of superconducting wires. For magnesium diboride, MgB2, carbon is known to be the most effective dopant to enhance high-field properties. However, the critical role of carbon remains elusive, and also low-field critical current density has not been improved. Here, we have undertaken malic acid doping of MgB2 and find that the microscopic origin for the enhancement of high-field properties is due to boron vacancies and associated stacking faults, as observed by high-resolution transmission electron microscopy and electron energy loss spectroscopy. The carbon from the malic acid almost uniformly encapsulates boron, preventing boron agglomeration and reducing porosity, as observed by three-dimensional X-ray tomography. The critical current density either exceeds or matches that of niobium titanium at 4.2 K. Our findings provide atomic-level insights, which could pave the way to further enhancement of the critical current density of MgB2 up to the theoretical limit. Hiroaki Kumakura, Shi Xue Dou and co-workers have uncovered the role of carbon doping at the microscopic scale on magnesium diboride (MgB2). This inexpensive superconductor — a material with no electrical resistance below a certain temperature — is also sensitive to magnetic fields. The researchers show that incorporating an organic molecule into MgB2 (‘carbon doping’) improves its critical current density under low magnetic field — an effect previously known only for high fields. Detailed characterizations revealed that the number of voids within the material increased, yet their size decreased, so that MgB2 was denser after doping. The team suggests that a boron deficiency causes microscopic defects within the superconductor's structure, improving its properties, while a particular arrangement of the doping molecules can explain the increased density. These findings may lead to further enhancement of the properties of MgB2, which has recently attracted interest in various fields. In medical magnetic resonance imaging, for example, MgB2-based magnets are promising alternatives to the current NbTi-based technology that relies on expensive cooling with liquid helium. Where does carbon go when it is doped into magnesium diboride (MgB2) and why the superconducting properties are improved? In this work, malic acid-doped MgB2 was investigated and it was shown that carbon encapsulates boron powder, prevents agglomeration and as a result reduces void fraction as was confirmed by the first detailed X-ray tomogram analysis. It was also found that carbon induces a lot of stacking faults within MgB2 grains. The critical current density is now comparable to commercial niobium titanium (NbTi) wire and further improvements are expected.