Nb3Al Strands Research Articles

Study of RHQT-processed Nb3Al multifilamentary rectangular tape strand to be applied to a fusion magnet

Nb3Al superconducting wire shows an excellent strain tolerance compared with Nb3Sn wire and is promising conductor for a future fusion magnet. We investigated about the changes of the superconducting property and the wire flexibility by rectangular tape-shaped deformation on the Rapid Heating/Quenching and Transformation (RHQT) processed Nb3Al multifilamenatry wires. The improvement of the wire flexibility will be enable to the future complicated coil winding through the "React and Winding" process. The Rapid Heating/Quenching (RHQ) treatment was performed on a round-shaped Nb/Al precursor wire, and Nb/Al filaments has converted to the ductile bcc Nb-Al supersaturated solid solution (bcc-SS) phase after the RHQ (Rapid heating and quenching) treatment. The round-shaped wire after the RHQ treatment could be easily cold flat-rolled to the rectangular tape-shape (width: 4.6 mm, thickness: 0.2 mm) at room temperature. The phase transformation from bcc-SS phase to stoichiometric A15-Nb3Al phase by the post-annealing was confirmed. Critical current (Ic) and critical current density (Jc) at 4.2 K and 18 T of the RHQT processed Nb3Al rectangular tape strand was obtained approximately 180 A and 200 A/mm2, respectively. Jc property above 18 T in the rectangular tape strand were clearly improved compared to the round wire strand, and the upper critical magnetic field (Hc2) above 26 T was also obtained. In addition, no Ic deterioration by applying a bending strain up to 0.66% was observed. The wire flexibility of the RHQT processed Nb3Al strand could be improved by the change of wire shape from round wire to rectangular tape. We found that the Nb3Al rectangular tape stand will be desirable and suitable material to apply to a future huge fusion magnet through the React & Winding process.

Difference of Irreversible Strain Limit in Technical RHQT Nb3Al Superconductors

Strain limit for critical current degradation in various technical rapid-heating, quenching, and transformation-processed Nb3Al strands has been studied with respect to the matrix material and filament diameter. This property is one important factor as well as the strain sensitivity to $J_{{\rm{c}}}$ characteristics, especially when we consider to apply the so-called react and wind technique to large magnets such as DEMO reactors. We compared five strands, including a standard Nb matrix, standard Ta matrix, Nb/Ag/Nb three-layered barrier structure, and other two fine filament diameter strands prepared by the restacking method. In normal Nb and Ta matrix strands with a filament diameter of more than 30 μ m, the irreversible strain limit is about 0.3%. Refinement of the filament diameter enables us to significantly improve the irreversible strain limit up to more than 0.7%. The Nb/Ag/Nb three-layered barrier structure appears more attractive from a practical point of view. The irreversible strain limit was improved up to nearly 0.6%, even if the filament diameter is more than 20 μ m. The Ag layer between the Nb3Al filaments seems to act as a cushion to mitigate the stress concentration in the wire. The Ag barrier also seems to be effective for current bypass in existence of cracks, resulting in suppressing $I_{{\rm{c}}}$ degradation in a higher strain range.

Thermal strain exerted on superconductive filaments in practical Nb3Sn and Nb3Al strands

Practical superconductive (SC) wires such as the Nb3Sn and Nb3Al strands used in fusion reactors are typical composite materials consisting of brittle superconducting intermetallic compounds. Thermally induced strain is inevitably generated in the composite due to the different coefficients of thermal expansion and different moduli of elasticity among the constituent components. In order to evaluate quantitatively the thermal strain, local strain measurements during heating by means of quantum beams, and room temperature tensile tests were carried out. The stress versus strain curves of both Nb3Sn and Nb3Al strands showed a typical elasto–plastic behavior, which could be numerically evaluated on the basis of the rule of mixtures. The local strain exerted on SC filaments along the axial direction was compressive at room temperature and tensile at high temperatures, which is common for Nb3Sn and Nb3Al strands. Their temperature dependence was numerically evaluated by means of the iteration method. As a whole, it has been established that the temperature dependence of thermal strain can be reproduced well by the numerical calculation proposed here. It is pointed out that the thermal strain in SC filaments is affected by the creep phenomenon at high temperatures above a threshold temperature.

Feasibility Studies of 0.7 mm $\hbox{Nb}_{3}\hbox{Al}$ Strands and Rutherford Cable

We are planning to demonstrate a quadrupole magnet with magnetic mirror structure by using 0.7-mm Nb3Al strands. As feasibility studies for this program, we investigated an influence of the diameter reduction of Cu-stabilized Nb3Al strands from 1.0 to 0.7 mm. Wire breakages and Cu separations did not happen with applying the cold die-drawing. The non-Cu Jc and n-values of the 0.7 mm strands did not degrade and could keep the same performance of the 1.0 mm strands. Although irregular deformations of Nb-Al filaments slightly occurred, magnetization properties of the 0.7 mm strands are almost the same as those of the 1.0 mm strands. Ta interfilament matrix of the 0.7 mm strands was also effective to improve the low field instability at 4.2 K. In addition, 27 strands Rutherford cable has been made by using the 0.7 mm F1 strand without any troubles. All of 27 extracted strands taken from the F1 cable showed very uniform Ic performance at 4.2 K.

Development of Ta-matrix ${\rm Nb}_{3}{\rm Al}$ Strand and Cable for High-Field Accelerator Magnet

Research and development of Nb3Al strands and cables for a high field accelerator magnet is ongoing under the framework of the CERN-KEK collaboration. In this program, new Ta-matrix Nb3Al strands were developed and their mechanical properties and superconducting properties were studied. The non-Cu Jc values of these strands were 750 ~ 800 A/mm2 at 15 T and 4.2 K. Using these strands, test fabrication of 27-strand Rutherford cable was carried out in collaboration with NIMS and Fermilab. The properties of the strands extracted from the cable were examined and it was found that there was no degradation of the superconducting properties of the strands. In this paper, we report the fabrication of the strands and the cable in brief and present some of the results obtained by studying their properties.

Critical Current and Fracture Behavior of Nb3Al Superconducting Strands under Cyclic Electromagnetic Force

To obtain basic information on fracture behavior and its critical current properties in superconducting strands with long-term periodic excitation, a unique experiment of cyclic strain loading was carried out. The strain was applied to a superconducting filament directly at 4.2 K using an electromagnetic (Lorenz) force instead of the usual mechanical load. The sample strands were Jelly-roll process Nb3Al with a copper ratio of 2.1. A new magnet system to evaluate long-term cyclic strain loading was set up. It was able to generate external fields up to 14 T with 58 mm bore. Cyclic strain was applied by the sample current control: 500 A, 0.1 Hz. In the experiment, critical current measurements and applied cyclic strain were carried out in alternation and the variation of critical current was obtained during the cyclic test. In this paper, the experimental results of cyclic strain loading on the critical current of Nb3Al strand and mirostructural observation of fracture surface of the filaments is described. Our results show that degradation of critical current properties was not observed at cycle number n=1500 under the applied axial strain of 0.17%. A noxious sample, from which the outside copper stabilizer was deliberately removed, broke after n 10, and crack propagation was observed at the fracture surface of the Nb3Al filaments. Under a microstructural observation of the break sample, the fracture points of the filaments were seen to be different in a longitudinal direction. A striped pattern like a beach mark due to crack propagation was also observed at the Nb3Al fracture surface. It was found that the cross-sectional homogeneity of the strand is important to avoid the damage and breaking from cyclic loading.

Recent Status of Studies on the Neutron Irradiation Effect Focusing on Nb3Sn and Nb3Al Strands

A fusion reactor generates a lot of 14 MeV neutrons, some of which penetrate shielding blankets, stream out of ports and reach superconducting magnets. Some important studies were performed in the 1970s and a basic understanding of the mechanisms of neutron irradiation effect was established. Advances in the design concept of nuclear fusion reactors led to the need for consistent studies on the neutron irradiation effect of A-15 strands such as Nb3Sn and Nb3Al, which are strong candidates for fusion reactors. In the early 2000s, a progressive attempt to organize the collaborative research of universities and national institutes was started using a 14 MeV neutron source at Japan Atomic Energy Agency. This paper outlines the neutron irradiation issues related to superconducting magnets for fusion, and a brief history of research on the neutron irradiation effect is provided. In addition, experimental results regarding changes in the superconducting properties of Nb3Sn and Nb3Al strands by neutron irradiation obtained in the newly established collaborative framework are presented, and general mechanisms for the property changes are introduced.

Stability Test of RHQT Nb3Al Cable-in-conduit Conductor

The Japan Atomic Energy Agency (JAEA) and National Institute for Material Science (NIMS) have been collaborating in the development of high-performance (i.e., more than 16 T and 80 kA) Nb3Al cable-in-conduit (CIC) conductor prepared using the rapid-heating, quenching and transformation (RHQT) process, and aiming for the application of this conductor to a demonstration plant. The technical issue relating to application of the RHQT Nb3Al strands to a fusion magnet is stabilization against perturbation. NIMS has developed a technique to attach a copper stabilizer using electroplating, and a sub-scale CIC conductor is developed using this conductor. JAEA performed a stability test of the developed CIC conductor to demonstrate the efficiency of this copper stabilization technique. The initial perturbation was applied via inductive heating, whose energy was calibrated using a calorimetric method. The measured stability margin is sufficiently high compared to that of a similar NbTi CIC conductor previously tested by the authors. In addition, a heat transfer coefficient to define the so-called limiting current is estimated to be about 1 kW/m2K, a sufficiently high value that is almost the same as that of a CIC conductor consisting of normal Cu stabilized strands. From these experimental results, it can be concluded that the copper stabilizer works efficiently from the viewpoint of stability, thus offering a solution to the remaining technical issues relating to the RHQT CIC conductor. On this basis, we can say that the RHQT Nb3Al CIC conductor is the most promising candidate for application to a magnet in the demonstration plant.

Comparison Between ${\hbox {Nb}}_{3}{\hbox {Al}}$ and ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Strands and Cables for High Field Accelerator Magnets

The Nb3Al small racetrack magnet, SR07, has been successfully built and tested to its short sample limit beyond 10 Tesla without any training. Thus the practical application of Nb3Al strands for high field accelerator magnets is established. The characteristics of the representative F4 strand and cable, are compared with the typical Nb3Sn strand and cable. It is represented by the OST high current RRP Nb3Sn strand with 108/127 configuration. The effects of Rutherford cabling to both type strands are explained and the inherent problem of the Nb3Sn strand is discussed. Also the test results of two representative small racetrack magnets are compared from the stand point of Ic values, and training. The maximum current density of the Nb3Al strands is still smaller than that of the Nb3Sn strands, but if we take into account of the stress-strain characteristics, Nb3Al strands become somewhat favorable in some applications.

Strand and Cable Development for a High Field ${\rm Nb}_{3}{\rm Al}$ Common Coil Magnet

Research and development of copper stabilized Nb3Al strands and cables for a high field Nb3Al common coil magnet is ongoing with the cooperation of KEK-NIMS-Fermilab. In this work, newly designed K1 and K2 strands and cables were fabricated. The Kl strand used tantalum for the inter-filament matrix only and the K2 strand used it for the entire strand matrix. Wire breakage happened on the drawing of both precursors, and it may be caused by the de-bonding of the inter-filament as well as poorer cold-workability of the tantalum itself. Non Cu Jc's at 12 T were 1,300 A/mm2 at 4.2 K and 1,600 A/mm2 at 1.9 K, an increase of about 20%. The low field magnetic instabilities at 4.2 K could be suppressed with K1 and K2 strands because of the tantalum inter-filament matrix. Similar superconducting characteristics were obtained on K1 and K2 strands. However, K2 strands showed excessive separation of the copper stabilizer when made into Rutherford cables, although Kl strands did not. The copper stabilizer prefers niobium as an interface material to obtain good bonding.

Characteristics of Cu Stabilized ${\rm Nb}_{3}{\rm Al}$ Strands With Low Cu Ratio

Characteristics of recently developed F4-Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strand with low Cu ratio are described. The overall <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strand could be easily increased by decreasing of the Cu ratio. Although the quench of a pulse-like voltage generation is usually observed in superconducting unstable conductor, the F4 strand with a low Cu ratio of 0.61 exhibited an ordinary critical transition of gradual voltage generation. The F4 strand does not have magnetic instabilities at 4.2 K because of the tantalum interfilament matrix. The overall <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the F4 strand achieved was 80-85% of the RRP strand. In the large mechanical stress above 100 MPa, the overall <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the F4 strand might be comparable to that of high <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> RRP-Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands. The Rutherford cable with a high packing factor of 86.5% has been fabricated using F4 strands. The small racetrack magnet, SR07, was also fabricated by a 14 m F4 cable. The quench current, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</sub> , of SR07 were obtained 22.4 kA at 4.5 K and 25.2 kA at 2.2 K. The tantalum matrix Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strands are promising for the application of super-cooled high-field magnets as well as 4.2 K operation magnets.

Characterization of ${\rm Nb}_{3}{\rm Al}$ Strands Subjected to an Axial-Strain for a Fusion DEMO Reactor

A Nb3Al strand, produced by the ldquorapid-heating, quenching and transformation annealing (RHQT)rdquo process, has been developed in collaboration between JAEA and NIMS aiming at application to the conductor in a fusion DEMO reactor. A detailed description of critical current characteristics of the Nb3Al RHQT strand may assist in predicting accurate conductor performance. Calculations of the critical current density are adjusted to 5 scaling laws. Advantages of the newly developed strand compared to the Nb3Sn ITER strand and the Nb3Al Insert strand include improved strain tolerance and a higher current density at around 16 T. Performance parameters of the Nb3Al RHQT strand include 450 A/mm2 at 15.9 T, 5.7 K and -0.4%. These performance parameters fulfill the requirements of the TF conductor of the fusion DEMO reactor. Therefore, the Nb3Al RHQT strand represents an improved performance with prospective application in the fusion DEMO reactor. In addition, the Nb3Al RHQT strand has the potential to enhance further the critical current density.

Magnetization, Low Field Instability and Quench of RHQT ${\rm Nb}_{3}{\rm Al}$ Strands

Since 2005, we made and tested three RHQT Nb3Al strands, one with Nb matrix and two with Ta matrix, which are fully stabilized with Cu electroplating. We observed anomalously large magnetization curves extending beyond 1 to 1.5 Tesla with the F1 Nb matrix strand at 4.2 K, when we measured its magnetization with a balanced coil magnetometer. This problem was eliminated with the Ta matrix strands operating at 4.2 K. But with these strands a similar but smaller anomalous magnetization was observed at 1.9 K. We studied these phenomena with FEM. With the F1 Nb matrix strand, it is explained that at low external field, inter-filamentary coupling currents in the outer layers of sub-elements create a shielding effect. It reduces the inside field, keeps the inside Nb matrix superconductive, and stands against a higher outside field beyond the Hc of Nb. At an even higher external field, the superconductivity of the whole Nb matrix collapses and releases a large amount of energy, which may cause a big quench. Depending on the size of the energy in the strand or the cable, a magnet could quench, causing the low field instability. Some attempt to analyze the anomaly with FEM is presented.

Status and perspective of the Nb 3Al development

Nb 3Al has advantages of better tolerance to strain/stress and a higher critical magnetic field (30 T at 4.2 K) for stoichiometric composition over Nb 3Sn. The rapid-heating, quenching and transformation annealing (RHQT) process enables to form a stoichiometric Nb 3Al with fine grain structures via metastable bcc supersaturated-solid-solution. As a result a large critical current density of Nb 3Al is achieved over the whole range of magnetic fields without trading off the excellent strain tolerance. A long-length of RHQ processing has been established, and a rectangular but Cu stabilized Nb 3Al strand is about be commercially available for NMR uses. Ag or Cu internal stabilization and Cu ion-plating/electroplating techniques have been also developed to enable the stabilized round wire for accelerator and fusion magnets. Successfully energized test coils that were manufactured with a wind-and-react technique have demonstrated that a long piece of Cu stabilized RHQT Nb 3Al wire is really available for practical applications.

Characteristics of Round and Extracted Strands of ${\hbox{Nb}}_{3}\hbox{Al}$ Rutherford Cable

Long Nb3Al strands with copper stabilizer are promising for future high field accelerator magnets. A 1.2 kilometer Nb3Al strand with Cu stabilizer was fabricated at the National Institute for Materials Science in Japan. Using this strand a 30 meter Cu stabilized Nb3Al Rutherford cable was made for the first time by a collaboration of NIMS and Fermilab. The Nb3Al strands extracted from cable with a relatively low packing factor showed almost no Jc degradation. But the extracted strands from the highly compacted cable showed some degradation in both Jc and n value, which may be caused by local separation of the copper stabilizer. Still, its Jc degradation is lower than that of typical Nb3Sn strands. The current limit due to magnetic instability in low field is about 500 A at 4.2 K. The magnetization of the strands, which was measured with balanced coils at 4.2 K, showed large flux jumps, usually around 1.5 T. This value is much larger than the Bc2 (4.2 K) of the Nb matrix, which is around 0.4 Tesla. The magnetic instability of the Nb3Al strand at low field is not completely understood, but it might be explained by the superconducting coupling current through the Nb matrix.

Feasibility Study of ${\rm Nb}_{3}{\rm Al}$ Rutherford Cable for High Field Accelerator Magnet Application

Feasibility study of Cu stabilized Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strand and Rutherford cable for the application to high field accelerator magnets are being done at Fermilab in collaboration with NIMS. The Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strand, which was developed and manufactured at NIMS in Japan, has a non-copper Jc of about 844 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 15 Tesla at 4.2 K, a copper content of 50%, and filament size of about 50 microns. Rutherford cables with 27 Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strands of 1.03 mm diameter were fabricated and tested. Quench tests on a short cable were done to study its stability with only its self field, utilizing a high current transformer. A pair of 2 meter long Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al cables was tested extensively at CERN at 4.3 and 1.9 K up to 11 Tesla including its self field with a high transport current of 20.2 kA. In the low field test we observed instability near splices and in the central region. This is related to the flux-jump like behavior, because of excessive amount of Nb in the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strand. There is possibility that the Nb in Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al can cause instability below 2 Tesla field regions. We need further investigation on this problem. Above 8 Tesla, we observed quenches near the critical surface at fast ramp rate from 1000 to 3000 A/sec, with quench velocity over 100 m/sec. A small racetrack magnet was made using a 14 m of Rutherford cable and successfully tested up to 21.8 kA, corresponding to 8.7 T.

Evaluation of bending strain dependence of critical current of Nb 3Al conductor for coils with react-and-wind method

In order to clarify the limitation of bending strain in conductor to fabricate the toroidal field coils of DEMO reactor by react-and-wind (R&W) method, the effect of bending strain on the critical currents ( I c) of Nb 3Al cable-in-conduit (CIC) conductor was investigated using the newly developed apparatus which can bend cooled sample from outside of cryostat. The Nb 3Al strand manufactured by jelly roll process was 0.74 mm in diameter with the copper/non-copper ratio of 4.0. The CIC conductor sample which consisted of 54 Nb 3Al strands and 27 copper wires inserted into a stainless steel conduit was 12.4 mm in diameter and 2.3 m in length. The CIC conductor was wound into a torsion coil spring shape with 4.5 turns and 150 mm in diameter. The bending strain and external magnetic field ranges for I c measurement were from 0% to 0.6% and from 9.2 T to 11.3 T, respectively. It was clearly shown that the I c decrease of the CIC conductor by bending is considerably low compared with that of strand, indicating the relaxation of intrinsic strain in Nb 3Al filaments in CIC conductor due to the cabling effect. The I c was not decreased up to 0.4% in bending strain and slightly decreased to 96% of initial I c at 0.55% in bending strain. This indicates that the R&W method with bending strain of 0.6% is fully applied to the coil fabrication method. The furnace size for heat-treatment for R&W method becomes half of wind-and-react (W&R) method.

Effects of Tensile and Compressive Strain on Critical Currents of&lt;tex&gt;$rm Nb_3rm Al$&lt;/tex&gt;Strand and Cable-in-Conduit Conductor

Effects of tensile and compressive strain on critical currents (I c) of Nb3Al strand and cable-in-conduit (CIC) conductor were investigated using a newly developed apparatus. The Nb 3Al strand manufactured by jelly roll process was 0.74 mm in diameter with a copper/noncopper ratio of 4.05. The CIC conductor sample which consists of two Nb3Al strands and one copper wire inserted into a stainless steel conduit was prepared. The effective strain and magnetic field ranges for the CIC conductor sample were from -0.91% to +0.26% and from 6 T to 11 T, respectively. It is clearly shown that the Ic decrease of the CIC conductor sample by longitudinal strain is relieved considerably compared with the strand sample, indicating the relaxation of intrinsic strain in Nb3Al filaments in CIC conductor due to the cabling effect. The relaxation effect is about +0.1% for -0.8% in effective strain of CIC conductor

Feasibility Study for 15-Tesla Magnet using Rapid-Heating Quenching and Transformation Nb3Al Strand

A new practically long copper stabilized Rapid-Heating Quenching and Transformation (RHQT) Nb3Al strand is presented, which is being developed and manufactured at the National Institute of Material Science (NIMS) in Japan. It has achieved a non-copper Jc of 1000 A/mm2 at 15 T at 4.2K, with a copper over non-copper ratio of 1.04, and a filament size about 50 µm. Using this strand the feasibility study of a 15 T dipole magnet is presented. For this study a Rutherford cable with 28 Nb3Al strands of 1 mm diameter will be fabricated early in 2006. A block-type magnet is designed using ROXIE, and the stress and strain in the coil is estimated and studied with the characteristics of the Nb3Al strand. The advantages and disadvantages of the Nb3Al cable are compared with the prevailing Nb3Sn cable from the point of view of stress-strain, and possible effects on Jc due to cabling is presented. The Nb3Al coil of the magnet, which will be made by wind and react method, has to be heat treated at 800 degree C for 10 hours. As preparation for the 15 T magnet design, a series of tests on strands and Rutherford cables is presented.

Development of advanced Nb3Al superconductors for a fusion demo plant

A Nb3Al superconductor inherently has the outstanding features of a high critical field and an excellent strain tolerance in critical current performance. The Japan Atomic Energy Research Institute developed the world's first large Nb3Al coil using jelly-roll processed Nb3Al strands. This coil was successfully operated up to the nominal current and field of 46 kA and 13 T, respectively. The test results demonstrated that a Nb3Al conductor is suitable for application in high field, large magnets, such as the toroidal field (TF) coils in a fusion reactor. In parallel with this work, a Nb3Al strand having a high critical current at high field has been developed by the National Institute of Materials Science. This strand cannot incorporate enough copper stabilizer, resulting in poor stability, since it is heat-treated at a much higher temperature than the melting temperature of copper. In order to find a solution to this issue, we performed an analytical study, and it showed that the externally incorporated copper after the high temperature heat treatment is effective for stabilization when the electric conductance and heat transfer coefficient between the Nb3Al strand and the external copper are more than 10 MS m−1 and 10 kW m−2 K−1, respectively. Since they seem to be attained by using present conductor technologies, the development of a TF coil operated at a high field around 16 T seems promising.