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

We have developed a model that describes the transverse load degradation in Nb3Sn CICCs, based on strand and cable properties, and that is capable of predicting how such degradation can be prevented. The Nb3Sn cable in conduit conductors (CICCs) for the International Thermonuclear Experimental Reactor (ITER) show a significant degradation in their performance with increasing electromagnetic load. Not only do the differences in the thermal contraction of the composite materials affect the critical current and temperature margin, but mostly electromagnetic forces cause significant transverse strand contact and bending strain in the Nb3Sn layers. Here, we present the model for transverse electro-magnetic load optimization (TEMLOP) and report the first results of computations for the ITER type of conductors, based on the measured properties of the internal tin strand used for the toroidal field model coil (TFMC). As input, the model uses data describing the behaviour of single strands under periodic bending and contact loads, measured with the TARSIS set-up, enabling a discrimination in performance reduction per specific load and strand type. The most important conclusion of the model computations is that the problem of the severe degradation of large CICCs can be drastically and straightforwardly improved by increasing the pitch length of subsequent cabling stages. It is the first time that an increase of the pitches has been proposed and no experimental data are available yet to confirm this beneficial outcome of the TEMLOP model. Larger pitch lengths will result in a more homogeneous distribution of the stresses and strains in the cable by significantly moderating the local peak stresses associated with the intermediate-length twist pitches. The twist pitch scheme of the present conductor layout turns out to be unfortunately close to a worst-case scenario. The model also makes clear that strand bending is the dominant mechanism causing degradation. The transverse load on strand crossings and line contacts, abbreviated as contact load, can locally reach 90 MPa but this occurs in the low field area of the conductor and does not play a significant role in the observed critical current degradation. The model gives an accurate description for the mechanical response of the strands to a transverse load, from layer to layer in the cable, in agreement with mechanical experiments performed on cables. It is possible to improve the ITER conductor design or the operation margin, mainly by a change in the cabling scheme. We also find that a lower cable void fraction and larger strand stiffness add to a further improvement of the conductor performance.

https://iopscience.iop.org/article/10.1088/0953-2048/19/9/011/pdf

Transverse load optimization in Nb3Sn CICC design; influence of cabling, void fraction and strand stiffness

Nb3Sn superconductors show a dependence of the critical current and temperature on the strain state of the superconducting material. The basic causes of Nb3Sn strain effects, primarily differential thermal contraction between elements of the strand, have been known for 30 years, but have received more attention lately as part of a drive to achieve much higher operating current densities and make use of them in practical multistrand cables. The use of the cable-in-conduit (CICC) type of conductors to achieve high current capacity has proved popular, as the conductors offer good local cooling of the strands and distributed electrical contact between strands that is essential to provide stability against the inevitable current non-uniformity that arises with parallel connection of the strands. However, the essential openness of the cable means that the strands have to support local magnetic loads as well as being exposed to the overall magnet strain displacements. Simple structural models are developed based on mechanical measurements on cable-in-conduit conductors which are able to successfully simulate the measured superconducting performance. These suggest that degradation observed in large cables is due to a combination of the repeated bending strain experienced by the strands and filament fracture, which is starting to occur to a significant extent in some large cables. Superconducting performance improvements in strands can only be properly utilized with improved support of the strands in the cable, implying a more ordered structure than in a multistage.

https://iopscience.iop.org/article/10.1088/0953-2048/18/12/029/pdf

Operating strain effects in Nb3Sn cable-in-conduit conductors

We review progress in the design of high field superconducting cable-in-conduit conductors (CICCs) for fusion applications, with special attention to the results of recent key experiments, leading to the state-of-the-art CICC technology: the ITER Toroidal Field and Central Solenoid programs, the EFDA Dipole conductor development program, the NHFML Hybrid Magnet project, the EU-TF Alt conductor demonstration, and the CRPP React & Wind flat cable test. For these projects, the main CICC design driver was the mitigation of Nb3Sn conductor performance degradation with electro-magnetic loading cycles. This was achieved by proper choice of cable layout and of conductor geometry, depending on the specific operating conditions and project requirements. In all cases, the necessity to limit cable movements inside the conductor jacket was identified to be of crucial importance. The main aspects of CICC manufacture are also discussed here, at least for what is the experience gained by the authors in both CICC jacketing and cabling processes. Finally, the state of the art of high-temperature superconducting (HTS) cables is discussed: at present, this technology is still in its infancy, but it is highly likely that major technological improvements could eventually lead to a widespread use of HTS CICCs.

https://iopscience.iop.org/article/10.1088/0953-2048/28/5/053001/pdf

Cable-in-conduit conductors: lessons from the recent past for future developments with low and high temperature superconductors

For a few years there has been an increasing effort to study the impact of (bending) strain on the transport properties of superconducting wires. As the stress distribution, originated by differences in the thermal expansion and electromagnetic load, is the driving factor for the final strains, the axial and transverse stiffness of the strand play a crucial role in the final performance. Since the strain state of the Nb3Sn filaments in strands determines the transport properties, basic experimental stress?strain data are required at the strand level for accurate modelling and analysis and eventually for optimizing cable and magnet design. We performed axial tensile stress?strain measurements on several types of Nb3Sn strands used for the manufacture of the International Experimental Thermonuclear Reactor (ITER) central solenoid and toroidal field model coils and a powder-in-tube processed wire. In total 48 wire samples were tested at boiling helium, boiling nitrogen and at room temperature. We present the computation of the stress?strain characteristic with a straightforward 1D model using an independent materials database, obtaining a good agreement with the experimental results. The details from the take-off origin of the measured stress?strain curves are discussed and the data are evaluated with respect to some commonly used functions for fitting stress?strain curves. The measurements are performed in the new setup TARSIS (test arrangement for strain influence on strands). A double extensometer connected to the sample enables us to determine the strain level whereas a load cell is used to monitor the stress level. For higher levels of applied stress (100?MPa), we found typically a higher strain for bronze route wires compared to a powder-in-tube and internal tin type of strand. Stress?strain results are essential to assess more accurately the impact of thermal and electromagnetic induced stress on the strain state of the Nb3Sn filaments for wires from various manufacturing processes.

https://iopscience.iop.org/article/10.1088/0953-2048/18/11/020/pdf

N. Mitchell

Papers

Finite element simulations of elasto-plastic processes in Nb3Sn strands