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The ITER central solenoid (CS) components are currently being manufactured. This Nb3Sn superconducting magnet will provide the magnetic flux swing required to induce up to 15 MA as plasma current. It includes six identical coils, called modules, stacked on top of each other to form a solenoid, enclosed inside a structure split into nine subsets, to provide vertical precompression and mechanical support. High mechanical stresses in materials and high voltages call for the use of structural materials with high strength and toughness and high dielectric strength insulating materials, respectively. The pulsed operation imposes materials with high fatigue strength at cryogenic temperatures. Unlike for the structure, where large existing manufacturing tools were usable, the modules required the construction of a dedicated manufacturing line. A comprehensive qualification programme is performed at the manufacturers before applying procedures for the production of the CS components. The main characteristics of the CS components, their manufacturing routes and the different elements of the qualification programme are described. The overall plan for the manufacture is reported. The status of the first series production components manufactured is presented as well as the planned delivery schedule to the ITER site.

Within the framework for establishing standards of test methods for superconducting technical wires, various standards have been issued by the International Electrotechnical Commission (IEC) (standard documents IEC 61788-1 to -20). Following the successful round robin test (RRT) for tensile testing REBCO wires at room temperature (Osamura K et al 2014 Supercond. Sci. Technol. 27 085009), this effort is extended to tensile test HTS wires at cryogenic temperatures and is coordinated by the CryoMaK lab at Karlsruhe Institute of Technology. Five different commercially available REBCO wires from five different manufacturers and one BiSCCO wire from another supplier were provided by the Versailles Project on Advanced Materials and Standards (http://vamas.org) for testing. Samples were distributed between eight participating labs from five different countries for testing according to the specified guidelines. After the test results were delivered by all participants, the data were evaluated with statistical tools to investigate the main source of scatter and its magnitude in the test results. The final goal of the RRT is issuing an ISO/IEC standard for a cryogenic temperature tensile test for REBCO wires. In this report the results of the RRT for tensile testing REBCO wires at cryogenic temperatures are presented and discussed.

We performed a feasibility study on a high-strength Bi2-x Pb x Sr2Ca2Cu3O10-x (Bi-2223) tape conductor for high-field solenoid applications. The investigated conductor, DI-BSCCO Type HT-XX, is a pre-production version of Type HT-NX, which has recently become available from Sumitomo Electric Industries (SEI). It is based on their DI-BSCCO Type H tape, but laminated with a high-strength Ni-alloy. We used stress-strain characterizations, single- and double-bend tests, easy- and hard-way bent coil-turns at various radii, straight and helical samples in up to 31.2 T background field, and small 20-turn coils in up to 17 T background field to systematically determine the electro-mechanical limits in magnet-relevant conditions. In longitudinal tensile tests at 77 K, we found critical stress- and strain-levels of 516 MPa and 0.57%, respectively. In three decidedly different experiments we detected an amplification of the allowable strain with a combination of pure bending and Lorentz loading to ≥ 0.92% (calculated elastically at the outer tape edge). This significant strain level, and the fact that it is multi-filamentary conductor and available in the reacted and insulated state, makes DI-BSCCO HT-NX highly suitable for very high-field solenoids, for which high current densities and therefore high loads are required to retain manageable magnet dimensions.

The 32 T all superconducting magnet currently being developed at the NHMFL will contain two REBCO insert coils located within an outsert of several LTS coils. The REBCO coils will experience high stress levels, radial expansion, and axial compression as the coils are energized. Therefore, great consideration has been given to the method by which current enters and exits the coils. Terminals and terminal constraints have been designed to achieve low resistance and high strength, while allowing for necessary movement of the coil and reliable attachment practices. These designs have been fabricated and tested, both as stand-alone tests and on prototype coils, to demonstrate the desired performance characteristics. The general design features are discussed, fabrication procedures are described, and the results of testing at high stress are presented.

High-current superconducting CORC® wires, wound from RE-Ba2Cu3O7−δ (REBCO) coated conductors, are being developed for use in high-field magnets that would allow operation at magnetic fields exceeding 20 T. The combination of high engineering current densities and high magnetic fields results in large Lorentz forces acting on the CORC® wire that could cause irreversible degradation to its performance. The effect of axial tensile stress on the critical current of CORC® wires containing annealed solid copper formers has been measured in liquid nitrogen to determine the irreversible stress limit at which irreversible degradation occurs. The results show no significant change in critical current before the irreversible stress limit is reached, after which the critical current decreases irreversibly with applied stress. The irreversible stress limit as high as 177 MPa depends on the yield strength of the former, the number of superconducting tapes wound into the CORC® wire and the angle at which the tapes are wound. Although the irreversible stress limit of CORC® wires is lower than a rudimentary rule of mixtures estimation would suggest, the irreversible strain limit, as high as 0.85%, exceeds that of single REBCO tapes. Both effects are likely the result of the helical fashion in which the REBCO tapes are wound into CORC® wires. The performance of CORC® wires was also measured as a function of axial tensile stress fatigue cycling in liquid nitrogen. No significant performance degradation was measured up to 100 000 cycles as long as the peak stress remained below the irreversible stress limit. Only once the peak stress was increased significantly above the irreversible stress limit would the critical current suddenly decrease with stress cycles. The results indicate that CORC® wires have matured into extremely robust high-current magnet conductors capable of withstanding high levels of axial tensile stress and strain. The irreversible stress limit of CORC® wires could be increased further by using stronger formers and winding the REBCO tapes at comparable angles, while the irreversible strain limit could potentially be increased by tailoring the winding angle of the REBCO tapes, making CORC® wires one of the strongest and most elastic high-current superconducting magnet conductors available.

Superconducting CORC® cables and wires have become practical conductors for use in high-field magnets for fusion machines and particle accelerators by demonstrating their ability to carry very high currents in background magnetic fields of up to 20 T. The high mechanical stresses that develop on the CORC® conductor during such operation could result in permanent degradation of the conductor critical current. Transverse compressive stress is one of the predominant mechanical stresses when CORC® cables or wires are bundled into CICCs that allow fusion and detector magnets to operate at currents as high as 100 kA. The effect of transverse compressive load on the critical current of CORC® cables and wires has been investigated at 76 K to determine their irreversible load limit under monotonic loading and load cycling up to 100 000 cycles. The results show a clear effect of the CORC® conductor layout with respect to gap spacing between tapes, thickness of the copper plating surrounding the tapes and thickness of the former onto which the tapes are wound. CORC® cables and wires have demonstrated a remarkable resilience to transverse compressive load cycling where their critical current decreased by no more than a few percent after 100 000 load cycles at peak loads that resulted in an initial decrease in critical current of less than 5%. The results indicate that no significant degradation of CORC® cable and wire performance due to transverse compressive load is expected in large magnets after 100 000 load cycles, as long as the peak load on the conductor does not exceed the irreversible load limit defined at 95% retention in critical current. The irreversible load limit of CORC® conductors could be increased further by increasing the size or hardness of the former that makes up the conductor's core.

Large-grain Nb has become a viable alternative to fine-grain Nb for the fabrication of superconducting radio-frequency cavities. In this contribution we report the results from a heat treatment study of a large-grain 1.5 GHz single-cell cavity made of ``medium purity'' Nb. The baseline surface preparation prior to heat treatment consisted of standard buffered chemical polishing. The heat treatment in the range $800--1400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ was done in a newly designed vacuum induction furnace. ${Q}_{0}$ values of the order of $2\ifmmode\times\else\texttimes\fi{}{10}^{10}$ at 2.0 K and peak surface magnetic field (${B}_{p}$) of 90 mT were achieved reproducibly. A ${Q}_{0}$ value of $(5\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}{10}^{10}$ at 2.0 K and ${B}_{p}=90\text{ }\text{ }\mathrm{mT}$ was obtained after heat treatment at $1400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. This is the highest value ever reported at this temperature, frequency, and field. Samples heat treated with the cavity at $1400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ were analyzed by secondary ion mass spectrometry, x-ray photoelectron spectroscopy, energy dispersive x ray, point-contact tunneling, and x-ray diffraction, and revealed a complex surface composition which includes titanium oxide, increased carbon, and nitrogen content but reduced hydrogen concentration compared to a non-heat-treated sample.

Multifilamentary Ag-sheathed Bi2Sr2CaCu2Ox (Bi-2212) wire can carry sufficient critical current density Jc for the development of powerful superconducting magnets. However, the range of its applications is limited by the low mechanical strength of the Ag/Bi-2212 strand. A potential solution is to cable Ag/Bi-2212 wire with high-strength materials that are compatible with the Bi-2212 heat treatment in an oxygen atmosphere. Past attempts have not always been successful, because the high-strength materials reacted with Bi-2212 wires, significantly reducing their Jc. We examined the nature of reactions occurring when Ag/Bi-2212 wires are heat-treated in direct contact with several commonly used high-strength alloys and a new Fe-Cr-Al alloy. INCONEL X750 and INCONEL 600 resulted in significant Jc loss, whereas Ni80-Cr caused little or no Jc loss; however, all of them formed chromium oxide that subsequently reacted with silver, creating cracks in the silver sheath. We found that Fe-Cr-Al did not show significant reactions with Ag/Bi-2212 strands. Scanning electron microscopy (SEM) and energy dispersive x-ray (EDS) examinations revealed that the Fe-Cr-Al alloy benefits from the formation of a uniform, crack-free, continuous alumina layer on its surface that does not react with Ag and that helps minimize the Cu loss found with INCONEL X750 and INCONEL 600. We fabricated prototype 6-around-1 cables with six Bi-2212 strands twisted and transposed around an Fe-Cr-Al alloy core coated with TiO2. After standard 1 bar melt processing, the cable retained 100% of the total current-carrying capability of its strands, and, after a 10 bar overpressure processing, the cable reached a total current of 1025 A at 4.2 K and 10 T. Tensile tests showed that Fe-Cr-Al becomes brittle after being cooled to 4.2 K, whereas INCONEL X750 remains ductile and retains a modulus of 183 GPa. We proposed new cable designs that take advantage of the chemical compatibility of Fe-Cr-Al and high strength of INCONEL X750 for various high-field magnet applications.

The performance of Nb3Sn composite superconducting wire is highly dependent on its strain state, which is difficult to predict or measure accurately. There is limited data in the literature on Nb3Sn or conduit alloys for the thermal expansion/contraction that occurs during reaction heat treatments. Thermal expansion measurements of two contemporary Nb3Sn wires and two conduit alloys-316LN and JK2LB-are taken individually during reaction heat treatments in a wide temperature range (4-1200 K) dilatometer system at the NHMFL. The measurements observed here are compared with the existing data and predicted models. This work significantly increases the available data for Nb3Sn superconductors and conduit alloys that can be used in magnet design and predictive modeling.

In a tokamak, such as ITER, superconducting strands suffer from bending and uniaxial strain due to Lorentz force loading/unloading and thermal cool down which may de- grade the performance over time due to specific cable-in-conduit conductor (CICC) design choice. Under repeated uniaxial loading the Cu(Sn) matrix which surrounds the brittle filaments allows the possibility of some elastic-plastic deformation that can initiate filament cracking. Here we present a metallographic study of filament cracking under increasing uniaxial loading cycles (0, 1000, 10,000 and 30,000 cycles) for one ITER Toroidal field (TF) bronze-process strand (tested at 0.4%, 0.6% and 1% strain) and one ITER TF internal tin strand (tested at 0.4%, 0.6% and 0.7% strain). Significant cracking of filaments was found at close to the respective fracture limits (strain at which strand breaks under uniaxial tensile loading) for both strands. After 0.6% strain, filament cracking in the bronze-process strand tends to increase with increasing number of loading cycles up to 10,000 and then remains almost constant after increasing the loading cycles from 10,000 to 30,000. The internal tin strand on the other hand showed an increase in filament cracking with increasing loading cycles to 10,000 up to 0.6% strain. For both types of strand and in all conditions the cracks were most likely to be found adjacent to voids.

Insulation in a high field superconducting magnet plays a critical role. For high field magnets using coated conductor, insulation faces a few special challenges. In order to maximize the engineering current density, the conductor insulation has to be very thin (e.g.

The electromechanical performance of commercially available YBCO high-temperature superconductor (HTS) is characterized in support of the 32T all-superconducting magnet program at the National High Magnetic Field Laboratory. Accurate characterization of the HTS coated conductors mechanical properties are essential for the aggressive high-field magnet design. Here, we report the 77-K stress-strain behavior of a series of conductors with varying thickness coatings of copper. The electromechanical performance of the conductors is determined in tests to measure the critical current as a function of applied axial strain, at 77-K in self field. Available conductor lengths necessitate a magnet design with soldered lap joints and the subsequent evaluation the lap joint performance reported here. Reports of low transverse strength and conductor delamination due transverse ( c-axis) tensile stress prompted tests to measure the conductors' electrical performance in the presence of transverse stress that are also reported.