Taking the relay of the large Hadron collider (LHC) at CERN, ITER has become the largest project in applied superconductivity. In addition to its technical complexity, ITER is also a management challenge as it relies on an unprecedented collaboration of seven partners, representing more than half of the world population, who provide 90% of the components as in-kind contributions. The ITER magnet system is one of the most sophisticated superconducting magnet systems ever designed, with an enormous stored energy of 51?GJ. It involves six of the ITER partners. The coils are wound from cable-in-conduit conductors (CICCs) made up of superconducting and copper strands assembled into a multistage cable, inserted into a conduit of butt-welded austenitic steel tubes. The conductors for the toroidal field (TF) and central solenoid (CS) coils require about 600?t of Nb3Sn strands while the poloidal field (PF) and correction coil (CC) and busbar conductors need around 275?t of Nb?Ti strands. The required amount of Nb3Sn strands far exceeds pre-existing industrial capacity and has called for a significant worldwide production scale up. The TF conductors are the first ITER components to be mass produced and are more than 50% complete. During its life time, the CS coil will have to sustain several tens of thousands of electromagnetic (EM) cycles to high current and field conditions, way beyond anything a large Nb3Sn coil has ever experienced. Following a comprehensive R&D program, a technical solution has been found for the CS conductor, which ensures stable performance versus EM and thermal cycling. Productions of PF, CC and busbar conductors are also underway. After an introduction to the ITER project and magnet system, we describe the ITER conductor procurements and the quality assurance/quality control programs that have been implemented to ensure production uniformity across numerous suppliers. Then, we provide examples of technical challenges that have been encountered and we present the status of ITER conductor production worldwide.

https://iopscience.iop.org/article/10.1088/0953-2048/27/4/044001/pdf

Challenges and status of ITER conductor production

With the first tokamak designed for full nuclear operation now well into final assembly (ITER), and a major new research tokamak starting commissioning (JT60SA), nuclear fusion is becoming a mainstream potential energy source for the future. A critical part of the viability of magnetic confinement for fusion is superconductor technology. The experience gained and lessons learned in the application of this technology to ITER and JT60SA, together with new and improved superconducting materials, is opening multiple routes to commercial fusion reactors. The objective of this roadmap is, through a series of short articles, to outline some of these routes and the materials/technologies that go with them.

Superconductors for fusion: A roadmap

As a milestone in the 1-GHz NMR magnet project that is being carried out at the Tsukuba Magnet Laboratory (TML), a 920-MHz high-resolution NMR magnet was successfully manufactured. It is made of 15%Sn-bronze-processed (Nb, Ti)/sub 3/Sn, Ta-reinforced (Nb, Ti)/sub 3/Sn, and NbTi conductors. The room-temperature bore of the cryostat is 54 mm in diameter. All the coils are cooled with pressurized superfluid helium at 1.55 K. The magnet was moved to a new building of the TML in December 2001. A persistent operation at 920 MHz started in April 2002. The field homogeneity after correcting with superconducting shim coils was less than 0.1 ppm in a sample volume. Field decay decreased to below 1.3 Hz/h at the beginning of July. NMR measurements using this magnet started in July.

Operation of a 920-MHz high-resolution NMR magnet at TML

High critical current density (Jc) Nb3Sn A15 multifilamentary wires require a large volume fraction of small grain, superconducting A15 phase, as well as Cu stabilizer with high Residual Resistance Ratio (RRR) to provide electromagnetic stabilization and protection. In Powder-in-Tube (PIT) wires the unreacted Nb7.5wt.%Ta outer layer of the tubular filaments acts as a diffusion barrier and protects the interfilamentary Cu stabilizer from Sn contamination. A high RRR requirement generally imposes a restricted A15 reaction heat treatment (HT) to prevent localized full reaction of the filament that could allow Sn to reach the Cu. In this paper we investigate recent high quality PIT wires that achieve a Jc(12 T, 4.2 K) up to ~2500 A/mm-2 and find that the minimum diffusion barrier thickness decreases as the filament aspect ratio increases from ~1 in the inner rings of filaments to 1.3 in the outer filament rings. We found that just 2-3 diffusion barrier breaches can degrade RRR from 300 to 150 or less. Using progressive etching of the Cu we also found that the RRR degradation is localized near the external filaments where deformation is highest. Consequently minimizing filament distortion during strand fabrication is important for reducing RRR degradation. The additional challengemore » of developing the highest possible Jc must be addressed by forming the maximum fraction of high Jc small-grain (SG) A15 and minimizing low Jc large-grain (LG) A15 morphologies. Finally, in one wire we found that 15% of the filaments had a significantly enhanced SG/LG A15 ratio and no residual A15 in the core, a feature that opens a path to substantial Jc improvement.« less

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Evaluation of critical current density and residual resistance ratio limits in powder in tube Nb3Sn conductors

We succeeded in upgrading the 920-MHz nuclear magnetic resonance (NMR) superconducting magnet (21.6 T) to 1020 MHz (24.0 T) by replacing the innermost Nb3Sn coil with a (Bi,Pb)2Sr2Ca2Cu3O10 (Bi-2223) coil. The 920-MHz NMR spectrometer had been installed in the National Institute for Materials Science, Tsukuba, Japan, in 2001. It has been operated in the persistent mode for six years. The upgrading project started in 2006. A Bi-2223 coil was developed as the innermost coil instead of the Nb3Sn one. The newly installed Bi-2223 innermost coil is connected to Nb3Sn and NbTi coils in series. The upgraded NMR magnet was seriously damaged by the Great East Japan Earthquake in March 2011. After more than two years of restoration and additional improvements of current leads and the power supply system, the magnet was cooled down to below 1.8 K in August 2014. The magnet successfully generated 24.0 T, corresponding to 1020 MHz, in October 2014. To achieve the required homogeneity and stability of the magnetic field, not only superconducting and room-temperature shim coils but also ferromagnetic shims were used. The 1020-MHz superconducting NMR magnet has been operated in a power-supply-driven mode for six months.

Successful Upgrading of 920-MHz NMR Superconducting Magnet to 1020 MHz Using Bi-2223 Innermost Coil

Bronze processed Nb/sub 3/Sn superconducting wire with Cu-16wt%Sn-0.3wt%Ti (16%Sn bronze) matrix has been developed. For the development, two-step approach was taken. At the first step, two kinds of small size round wires, having a bronze to niobium ratio (bronze ratio) of 2.5 and 1.9, were prepared. Heat treatment was made on these wires at 650/spl deg/C or 700/spl deg/C and the effects of bronze ratio and heat treatment temperature on critical current density (J/sub c/) in magnetic field region from 18 to 25 T were examined. A sample with a bronze ratio of 2.5 and heated at 700/spl deg/C was found to show the highest J/sub c/ of all. In the second step, we manufactured commercial scale rectangular cross-sectioned wire using 16%Sn bronze. J/sub c/ n-value in high magnetic field region are estimated for this wire. As a result, the wire showed J/sub c/ and n-value of 115 A/mm/sub 2/ and 37, respectively, at 22 T, 2.1 K, demonstrating the possibility as a candidate superconducting wire for high field magnets such as over-920 MHz NMR magnet.

Development of high Sn content bronze processed Nb/sub 3/Sn superconducting wire for high field magnets

Development of Nb3Sn superconducting wires for high field magnets at Kobe Steel and JASTEC

We have developed a high-performance (high-J
 C
) Nb
 3
Sn wire via a distributed tin (DT) method. Non-Cu J
 C
 of 1100 A/mm
 2
 at 16 T, 4.2 K has been achieved by improving the Sn diffusion and optimizing the Ti content. With the future circular collider magnet planned by European Organization for Nuclear Research (CERN), the target of non-Cu J
 C
 is set to 1500 A/mm
 2
 at 4.2 K, 16 T. For this target, we have chosen the DT method, which is a type of internal Sn method, and because it has no limitation of Sn solubility, higher J
 C
 can be expected. This paper finds that further improvement of J
 C
 can be realized by controlling the Sn diffusion condition and the ternary additive elements. By setting the Sn diffusion distance to lower than 48 μm, the Nb
 3
Sn composition in multi-Nb modules becomes uniform and fine. In addition, by controlling the ternary element content (Ti) for improving the characteristics of the middle magnetic field, it is possible to achieve high J
 C
 at 16 T.

Development of a High Current Density Distributed Tin Method Nb3Sn Wire

A precursor for manufacturing a Nb 3 Sn superconducting wire by an internal diffusion process, includes: a multielement wire (12) including a bundle of a plurality of single-element wires (1) each including a Sn or Sn-based alloy core centrally disposed, and a Cu or Cu-based alloy matrix and a plurality of Nb or Nb-based alloy filaments surrounding the Sn or Sn-based alloy core; and a rod-like reinforcing member (9a,9b) disposed in and/or around the bundle of the single-element wires.

Nb3Sn superconducting wire manufactured by internal Sn process and precursor for manufacturing the same

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Hiroyuki Kato

Papers

Development of high Sn content bronze processed Nb/sub 3/Sn superconducting wire for high field magnets

Development of Nb3Sn superconducting wires for high field magnets at Kobe Steel and JASTEC

Development of a High Current Density Distributed Tin Method Nb3Sn Wire

Nb3Sn superconducting wire manufactured by internal Sn process and precursor for manufacturing the same

Development of Nb 3 Sn Superconducting Wires for High-field Magnets