Nb3Sn Strands Research Articles

A comparative study of compositions and microstructures of two types internal-tin process Nb3Sn wires

In the present work, microstructural and microchemical of Nb3Sn strands with the internal tin process produced by the Western Superconductor Technology co., Ltd (WST) and Oxford Instruments co., Ltd (OST) were analyzed. The internal 3D-view of two Nb3Sn strands were detected with a (sub-)micrometric resolution in a non-invasive, non-destructive way, i.e. high-resolution 3D X-ray micro tomography. The distribution of filaments and voids induced by the functional heat treatment of two strands were investigated. The elements distribution after heat reaction of two strands was investigated through elemental mapping by scanning electron microscope (SEM) and energy dispersive X-ray analysis. The filaments of the WST Nb3Sn strand distributed uniformly, whereas the filaments separated with clear copper boundary among filaments. However, the filaments of the OST Nb3Sn strand have serious overlap phenomenon and the copper isolation interface was not clear among filaments. In order to investigate the distinction of microstructure, the selected area electron diffraction was also performed by transmission electron microscopy (TEM) and the results indicated that obvious impurities were observed in the Nb3Sn phase of the WST strand compared to the OST strand.

Effect of Nb3Sn Coarse Grains on Critical Current Densities of Internal Tin Nb3Sn Strand

Nb3Sn strands have significant applications in constructing high-field (>10 T) magnets and have great potential value in the next 20 years. Internal-tin (IT) Nb3Sn strand has been developed by many methods for the future fusion reactor after ITER with low hysteresis loss. However, how to improve the current densities of Nb3Sn strand has become very important with the increasing demand for high Jc Nb3Sn strand. In our study, we found that more than double Nb content should be added within Internal Tin Nb3Sn strand and 2 times critical current densities can be enhanced compared with ITER Nb3Sn strand. With this Nb3Sn strand, the phase transition during heat treatment was investigated and new phase (Cu-Nb-Sn) can be formed at 400 °C due to higher ratio of Sn/Cu and more Cu6Sn5 phase decomposition. The coarse Nb3Sn grains were analyzed and a composite structure (Cu and Nb3Sn) can be formed as Nb element diffusion during Cu-Nb-Sn phase formation. We also found that phase-balance (less Cu-Nb-Sn produced), which can control the formation of Nb3Sn coarse during filaments, can be obtained through element ratio optimization. With this method, coarse grains among filaments can be refined from micron size into nano-size. Effect of varisized coarse grains on the critical current densities was also studied and we found that micron sized Nb3Sn (1∼2 μm) grains reduced the critical current densities severely.

Status of Internal-Tin Nb3Sn Strand for Divertor Tokamak Test Facility

Divertor Tokamak Test facility (DTT) is going to be built in Italy, and is expected to be operated along with ITER to explore the potential solution for the power exhaust to be eventually implemented in DEMO. 77 tons of Nb3Sn strand in two types are required by DTT. For type one, the weight is 55 tons. The dominant performance requirements are I c >285 A (4.2 K, 12 T) and hysteresis loss <; 1000 mJ/cm 3 (4.2 K, ± 3 T). For type two, the weight is 22 tons. The dominant performance requirements are I c >260 A (4.2 K, 12 T) and hysteresis loss <; 400 mJ/cm 3 (4.2 K, ± 3 T). These performance requirements are quite challenging in the respect of performance stability in Nb 3 Sn strand production. WST has successfully developed three types of Nb 3 Sn strand to meet these requirements especially for DTT. For DTT type one, considering the ultra-high level of I c , Nb and Sn volume was increased based on the structure of WST Nb 3 Sn strand for ITER. I c >300 A (4.2 K, 12 T) and hysteresis loss <; 700 mJ/cm 3 (4.2 K, ± 3 T) have been obtained from both 164-filament structure and larger-filament structure. For DTT type two, Cu split structure was selected to guarantee low hysteresis level. For the structure with 160 filaments and one Cu split, I c >260 A (4.2 K, 12 T) and hysteresis loss <; 400 mJ/cm 3 (4.2 K, ± 3 T) have been achieved. Finally, all the performance of DTT Nb 3 Sn strand has been accomplished by WST.

Impact of 440 GeV Proton beams on Superconductors in a Cryogenic Environment

The superconducting magnets used in high energy particle accelerators such as CERN’s LHC can be impacted by the circulating beam in case of specific failures. This leads to interaction of the beam particles with the magnet components, like the superconducting coils, through direct beam impact or via secondary particle showers. The interaction causes energy deposition in the timescale of microseconds and induces large thermal gradients within the superconductors, which are in the order of 100 K/mm. To investigate the effect on the superconductors, an experiment at CERN’s HiRadMat facility was designed and executed, exposing short samples of Nb-Ti and Nb3Sn strands in a cryogenic environment to microsecond 440 GeV proton beams. The irradiated samples were extracted and analyzed for their critical transport current I c. This paper describes the results and analysis of the measurements of I c as well as thermo-mechanical simulations of the Nb3Sn strands to evaluate the degradation of I c as a function of the mechanical strain present during and after the beam impact.

Estimation of performance of Nb3Sn CICC with thermal strain distribution

In the last few years, the critical temperature (T c) of several ITER cable-in-conduit conductors (CICC) was determined by magnetization measurements at zero current. The distribution of the critical temperatures (T c), caused by variation of strain in the Nb3Sn strands, was found to vary with the number of load cycles. A comparison with mechanical modelling requires the strain distribution, while the T c distribution is sufficient to determine the CICC performance. The current sharing temperature (T cs) is calculated supposing that the measured distribution of T c is representative of its variation along a single strand and that the current is uniformly distributed among the strands. The T cs values, found for background and peak magnetic field, are compared with the results of the DC test in the SULTAN facility. The presented estimation techniques lead to an overestimation of T cs. The differences of the estimated and measured T cs values are discussed including the effect of different current-voltage characteristics of single strands and CIC conductors.

Impact of Transverse Compression on the Sub-Element RRP Nb3Sn Strand

For developing a 14 Tesla whole-body superconducting magnetic resonance imaging (MRI) magnet, a new kind of a Nb3Sn composite conductor with Rutherford cable-in-channel (RIC) structure is adopted. This structure of the conductor has a high current carrying capacity with an increased thermal and mechanical stability. The Rutherford cable is the core component of the RIC conductor, and controlling the deformation and performance degradation of Nb3Sn strand are the key points in Rutherford cable development. The strand would experience plastic deformation during the compaction process, which causes sub-elements damage and degrades the transport performance and residual resistivity ratio (RRR). In this paper, the impact of transverse compression on the sub-element of RRP 84/91 Nb3Sn strand before heat treatment was studied with experiments and finite element model (FEM), the experiment results and FEM results were compared and analyzed. Experiment results show that the transverse deformation of the strand should be kept below 20% to avoid the Nb barrier from breaking. The deformation analysis result of the FEM simulation shows good agreement with experimental results.

Study on performance variation of Nb3Sn strand caused by heat treatment failure

The TF and CS coils in the superconducting magnet system of China Fusion Engineering Test Reactor (CFETR) will be using Nb3Sn strands in conduit. Nb3Sn Cable-in-Conduit Conductor (CICC) must heat-treated after winding to allow solid diffusion reaction and forming the A15 superconducting phase. Heat treatment is the key process of Nb3Sn coil manufacturing. However, the operating cycle of heat treatment is very long, which may cause some unforeseeable faults in the heat treatment process. During heat treatment, the most serious failure may be a power failure or other system failure. To study the effect of failure during the heat treatment procedure, eight kinds of heat treatment power failure scenarios were simulated. Each set of experiments included two critical current samples and two RRR samples to evaluate the effect of different power failure scenarios on the performance of Nb3Sn strand. This paper compares the performance of Nb3Sn strand samples given the recommended heat treatment with samples subjected to simulated power failure. The superconductivity of all experimental samples did not degrade significantly.

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.

The use of Nb3Sn in fusion: lessons learned from the ITER production including options for management of performance degradation

The overall 30 year Nb3Sn conductor program for ITER provides an unusual opportunity to look at the issues created by the application of novel Nb3Sn technology on a large scale. ITER design criteria have evolved to make use of the features of the industrialized material, exploiting advantages as well as managing the disadvantages. There are lessons from the successes and failures to be learned for the future application of very high current Nb3Sn strands in fusion and high energy physics. The behaviour of Nb3Sn strands in the large compound ITER conductors is still producing surprises after 25 years of development, industrial production and testing, with the Nb3Sn strain sensitivity and brittleness producing many more subtle design impacts than originally foreseen. The results now obtained at the end of the ITER production suggest that, as a novelty for Nb3Sn, there are options for conditioning the strands in the ITER conductors that can be applied during commissioning and which could reduce the impact of brittleness by as much as one third.

Study of the superconducting layer microstructure and (Nb,Ti,Ta)3Sn bronze strands properties

Superconducting materials with high current carrying capacity operating in magnetic fields up to 18 T at 4.2 K are required for promising applications in magnetic systems of high-energy physics devices, NMR-spectroscopes and others. The paper presents the study results of the Nb3Sn strands manufactured by the bronze route differed in the filament composition. The filaments were made from the alloys Nb-3.5 wt. %Ta and Nb-7.0wt.%Ta and the matrix was produced from the Cu-15.5wt.%Sn-0.25wt.%Ti alloy. To form the superconducting Nb3Sn phase a series of reaction heat treatments of conductor samples were carried out by two-stage regimes with the temperature of the second stage varying from 650 to 720 °C. The microstructure of the Nb3Sn layer was studied by scanning electron microscopy and their electro-physical characteristics were also measured. It was shown that after reaction at 575 °C, 100 h + 650 °C, 200 h samples with different Ta content had almost the same current -carrying capacity in the field range 12-18 T at 4.2 K. The critical temperature of superconductors decreased from 17.5 to 16.5 K with an increase of the second stage of reaction temperature from 650 to 720 °C.

Mechanical Behavior Laws for Multiscale Numerical Model of Nb3Sn Conductors

Low-temperature superconductors are widely used in high field magnets, mostly within Rutherford-type cables. Cables are multiscale composite structures including strands composed of superconducting filaments twisted in a metallic matrix. Mechanical effects at the filament scale affect magnetic performance, especially with strain-sensitive superconductors such as Nb3Sn. During operation, the conductor is subject to a complex combination of axial and transverse loads. It is thus necessary to build 3-D models at the cable scale. Previous work has presented a numerical approach for 3-D Finite-Elements (FE) modeling of Rutherford cables at the scale of the strand considered bi-metallic. In this paper, representative behavior laws for the different components of the strand model are identified, based on experimental results. Copper is modeled with a non-linear elastic-plastic behavior accounting for isotropic and kinematic hardenings. Homogenization of the superconducting filamentary area is carried out numerically using a FE representative volume element. Underlying parameters are identified from microstructural characterization and tensile tests on superconducting strands using an optimization routine. The strand model is compared to monotonic and cyclic tensile tests performed on Nb3Sn strand manufactured with the Powder-In-Tube method.

Properties of Toroidal Field Nb3Sn Strands Made for the ITER Chinese Domestic Agency

China was responsible for supplying 7.5% of ITER toroidal field (TF) Nb 3 Sn conductors. It has delivered all the required conductors unit lengths and fulfilled the TF conductor package. The conductor package delivery consisted of four phases, in terms of phase I (call for tender), II (qualification process), III (pre-production process), and IV (production process). This paper focuses on work from phases II to IV. The Superconducting Strand Test Laboratory of the Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) was the reference laboratory of the Chinese Domestic Agency and undertook the task of Nb 3 Sn strands verification based on the requirements of ITER procurement arrangements. The verification results showed that the strands properties fulfill ITER specifications. In addition, in order to know well the strand properties and supply essential data for conductors' performance prediction and analysis, the mechanical properties and critical current Ic as a function of axial strain, temperature, and magnetic field for the TF strands have been tested and summarized.

Summary of the Mechanical Performances of the 1.5 m Long Models of the Nb&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$_{3}$&lt;/tex-math&gt; &lt;/inline-formula&gt;Sn Low-&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\beta$&lt;/tex-math&gt; &lt;/inline-formula&gt; Quadrupole MQXF

Author(s): Vallone, G; Ambrosio, G; Anderssen, EC; Bajas, H; Bourcey, N; Cheng, DW; Chlachidze, G; Ferracin, P; Grosclaude, P; Guinchard, M; Bermudez, SI; Juchno, M; Pan, H; Perez, JC; Prestemon, S; Strauss, T | Abstract: The Nb3Sn quadrupole MQXF is being developed as a part of the large hadron collide (LHC) High Luminosity upgrade. The magnet design was tested on 1.5-m-long short models, sharing the same cross section with the full-length magnets. Various azimuthal and longitudinal preloads were applied, studying the impact on the magnet training and on its mechanical performances. The experiments demonstrated the possibility to control the magnet prestress. However, various factors, coil size among the others, may affect the stress variation between and within each winding. This variation could prevent the magnets from reaching the magnet performances, as for example as a result of the critical current reduction of the Nb3Sn strands. This paper analyzes the mechanical performances of the short models, studying in particular the stress variation on different coils. The measured coil size was used as input in the numerical simulations, and the results were then compared with the strain gauge measurements. Finally, the short model experience was used to evaluate the feasibility of a loading operation that does not rely on the strain measurements.

AC losses calculation of Bi-2212 Cable-in-Conduit Conductor used in CFETR

Chinese fusion engineering test reactor (CFETR) is designed to bridge gaps between the fusion experimental reactor ITER and the demonstration reactor (DEMO). Phase II of CFETR aims for DEMO validation with a fusion power over 1 GW, H-mode physics. In order to get this high operation parameters, the superconducting magnet system of CFETR has been designed based on the larger machine with R = 7.2 m, a = 2.2 m, BT = 6.5 T, and the maximum field at CS (Central Solenoid) Coils will be over 15 T. A hybrid superconducting CS magnet employing Bi-2212 round wires and Nb3Sn strands is considered for CFETR. In this paper, a design of 10 kA CICC twisted by Bi-2212 round wires at 15 T field and 4.5 K temperature was introduced, and a method was study to calculate AC losses of Bi-2212 cable in different indentation and strain conditions. In the CS coils, AC losses were calculated in detail, hysteresis loss was sensitive to indentation and strain, however, coupling losses were sensitive to indentation depth only.

An Analysis of AC Loss of Bi-2212 CICC

With the development of magnetic confinement fusion research, higher magnetic fields are needed. Commonly used is Nb 3 Sn superconductor in high-field magnets, which does not have much potential for improvement. It is necessary to develop new conductor with new superconducting materials. Bi 2 Sr 2 CaCu 2 Ox is such a promising material. One sub-size conductor with 42 wires has been designed and manufactured at Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) and its ac loss has been tested in the standard magnetization magnet for transverse magnetic fields at the University of Twente. The phenomenological model was used to analyze the test results. And the results showing that the coupling loss of Bi-2212 conductor is greater than that of Nb 3 Sn conductor under the same condition. This result may due to the fact that the silver matrix used in the Bi-2212 strands has a lower electrical resistivity than the copper matrix of the Nb 3 Sn strands.

Electromagnetic Design and Challenges for 14 T High-Tc Toroidal Field Superconducting Magnet for CFETR

The design work of the China Fusion Engineering Test Reactor (CFETR) was started in 2012. Related concept design work for its superconducting magnet system was carried out in the past years. In order to get higher plasma operation parameters, the system was optimized with the major radius (R) of 7.2 m, the minor radius (r) of 2.2 m, and the plasma center magnetic field ($B_{t}$) of 6.5 T. With these higher parameters, the fusion power may exceed 1 GW and the maximum magnetic field of the toroidal field superconducting magnet system will reach to 14.3 T. Due to the fact that the performance of the Nb3Sn strand used for ITER project is very difficult to be applied for such a high field of CFETR, new cable-in-conduit conductors (CICC) structure and conductor design work become the key point of CFETR. This paper illustrates the design and electronic magnetic analysis work of the toroidal field (TF) coils of CFETR. In addition, such high operation current and magnetic field bring to huge electromagnetic force, which will cause lots of challenges for the design work of the TF coil such as the stability margin, mechanical safety, and so on. It is not easy to solve all the issues for such a big superconducting coil for the fusion device in a short time, but we hope the analysis results and database can be good references for the next step R&D work.

Multi-scale 3D modelling of a DEMO prototype cable from strand to full-size conductor based on X-ray tomography and image analysis

The design of a superconducting magnet system of a fusion reactor candidate is usually based on the Cable in Conduit Conductor (CICC) concept. CICC consist of complex structures with several hundreds of highly packed, multistage twisted superconductor and copper strands, cooling structures – all wrapped in thin steel foils and jacketed in relatively thick stainless-steel pipes. As recently demonstrated, such complex morphology can be captured by fully 3D X-ray tomography. One of the three alternative winding pack (WP) options for the toroidal field coils of the European DEMO superconducting magnet system as proposed by the Italian ENEA, features a layer-wound WP design adopting a wind-and-react conductor with high aspect ratio rectangular cross section. A multi-physics modelling requiring high accuracy from the strand to the full-size conductor scale was tested on two X-ray tomography systems. For the microstructural integrity analysis of the internal tin one mm diameter Nb3Sn strand, manufactured by Western Superconducting Technologies, a microtomograph with ˜1 μm feature recognition was used. For the full-size conductor (66 x 25 mm2 internal dimensions) a high energy (> 300 kVp) microtomograph with voxel resolution of ˜20 μm was employed. Voids and/or geometrical imperfections were detected, accurately pin-pointed and locally investigated at high resolution before and after heat treatment. Dimensional measurements on the 3D internal structure of the strand showed the small dimensional changes of the strand after the heat treatment. A significant result was the determination of the twist-pitch factor in a purely non-invasive way. The 3D reconstructions of the CICC cables were used to verify the cabling and compaction processes as well as the structural integrity of the strands, wrapping foils and spiral cooling structures. Also, 3D reconstruction of strand trajectories provided a strong input for a multiphysical modelling/interpretation of mechanical–electrical properties of CICC in cryogenic–electromagnetic environments.

Structural assessment of TF superconducting magnet of the DTT device

The paper presents the multi-physics study of the Toroidal Field (TF) coils of the DTT device, as defined according to the design of the “Divertor Tokamak Test facility Interim Design Report” also known as the “green book”. TF coils winding packs are constituted by 5 Double-Pancakes (DP), enclosed into a SS316LN casing for structural reinforcement. The conductor design is based on a 0.82 mm Nb3Sn strand similar to what already produced for the ITER TF coils.The structural assessment has been performed for the Single Null (SN) reference scenario, adopting Ansys® Workbench™ as numerical tool. Electromagnetic load has been evaluated by calculating with the TOSCA® code the magnetic field components on a cloud of points within the TF winding pack. The pointwise information on Lorentz density of forces has been then transferred onto the FEA mesh using the software embedded interpolation tool. The 3D FEM analysis of the TF coil has been accomplished considering the Winding Pack (WP) as a homogeneous body. The steel casing was the object of the stress assessment, evaluated along a path through the most critical location on the equatorial cross section.

Indentation effects on strain sensitivity of critical current for internal-tin Nb3Sn strand

The cable-in-conduit conductor (CICC) technology has been widely used in large scale superconducting magnet. The Short-Twist-Pitch (STP) design was aimed to avoid the conductor degradation during electromagnetic and thermal cycling. But manufacture of STP CICC deforms the cabled strands significantly. Indentations appear on superconducting strands after cabling and compaction. The conductor used for China Fusion Engineering Test Reactor (CFETR) magnet will be subjected to much higher Lorentz force than ITER. The STP CICC will be the first choice for the conductor design, since it showed no observed current sharing temperature Tcs degradation after electromagnetic cycling. In order to estimate the effects of indentation on the superconducting and mechanical properties of Nb3Sn strands, series measurements have been done on an internal-tin (IT) Nb3Sn strand, fabricated by Western Superconducting Technologies Co., Ltd (WST). The strand was artificially indented and cut to several sections for different kinds of measurements, which mainly included the critical current Ic versus axial strain measurements with Pacman device and stress-strain tests. The results show that the indentation has less impact on Ic, but more impact on mechanical performance. The sample preparation, test results and analysis have been described in details.

Computation of the Reversible Critical Current Degradation in Nb $_{3}$Sn Rutherford Cables for Particle Accelerator Magnets

Superconducting coils limit current is usually measured on unloaded short samples of strands extracted from the same cable used to wound the coil. However, the critical current density of Nb 3 Sn strands is strongly dependent on the applied strain. This dependence is well reproduced by the so-called exponential scaling laws on strands subject to uniaxial loading. Unfortunately, no standard procedure exists to apply these laws under the complex stress condition experienced by the strands inside Rutherford cables for particle accelerator magnets. As a consequence, an empirical limit from magnet tests was set in the past: 150-200 MPa of maximum equivalent stress within the coil, considered as a block of uniform material. It is not clear how this number relates with the actual stress/strain state in the strands and if it is possible in magnet applications to infer the coil degradation from single strands tests. In this context, this paper presents a model to estimate the critical current degradation of superconducting coils as a function of the applied strain. The model results were compared against the critical current degradation measured on impregnated cable stacks subject to transverse pressure.