Nb3Sn Reaction Heat Treatment Research Articles

Evaluation of Various Nb-Rod-Method Cu-Nb/Nb3Sn Wires Designed for Practical React-and-Wind Coils

Bronze-processed Cu-Nb/Nb3Sn wires, composed of the Nb-rod-method Cu-Nb reinforcing stabilizer around the Nb3Sn filament bundle, have superior mechanical strength after Nb3Sn reaction heat-treatment at both low temperature and room temperature. The Cu-Nb/Nb3Sn wires are able to be applied to the react-and-wind (R&W) process. Moreover, the non-Cu- J c values of Cu-Nb/Nb3Sn wires are remarkably increased by using both the high-tin-bronze and appropriate pre-bending treatments. In this study, we developed Cu-Nb/Nb3Sn wires with various cross-sectional structures which were improved the I c values under tensile stress and transvers compression stress. Also, we confirmed I c improvements by pre-bending treatments of a Cu-Nb/Nb3Sn triplet cable and small size Rutherford cables. Cu-Nb/Nb3Sn wires and cables can be designed on requirements of Nb3Sn coils.

Secondary Phases Quantification and Fracture Toughness at Cryogenic Temperature of Austenitic Stainless Steel Welds for High-Field Superconducting Magnets

The ITER magnet system is based on the “cable-in-conduit” conductor concept, which consists of various types of stainless steel jackets filled with superconducting strands. The jackets provide high strength and fracture toughness to counteract the high stress imposed by, amongst others, electromagnetic loads at cryogenic temperature. Material properties of austenitic stainless steel at cryogenic temperature are known to some extent, but only partial information is available for their welds, particularly in combination with weld fillers envisaged for cryogenic service. When a full inspection of the welded components is not possible, it becomes of special interest an assessment of its fracture toughness under close-to-service conditions if a fracture mechanics' design approach is to be adopted. In absence of defects, brittle secondary phases are generally held responsible of the loss of ductility and toughness which is to be expected after postweld heat treatments. Their quantification becomes thus essential in order to explain the negative impact in fracture toughness after unavoidable thermal treatments. This paper investigates fracture toughness behavior at 7 K of AISI 316L and AISI 316LN tungsten inert gas welds using two fillers adapted to cryogenic service, EN 1.4453 and JK2LB. Additionally, the effect of such an aforementioned heat treatment, here the Nb 3Sn reaction heat treatment (650° for 200 h) on fracture toughness of the welds is evaluated. A correlation between the evolution of properties and the quantity of secondary phases as a result of the above treatment is provided.

Thermomechanical Behavior of the HL-LHC 11 Tesla Nb3Sn Magnet Coil Constituents During Reaction Heat Treatment

The knowledge of the temperature induced changes of the superconductor volume, and of the thermo-mechanical behaviour of the different coil and tooling materials is required for predicting the coil geometry and the stress distribution in the coil after the Nb3Sn reaction heat treatment. In the present study we have measured the Young's and shear moduli of the HL-LHC 11 T Nb3Sn dipole magnet coil and reaction tool constituents during in situ heat cycles with the dynamic resonance method. The thermal expansion behaviours of the coil components and of a free standing Nb3Sn wire were compared based on dilation experiments.

Validation of Helium Inlet Design for ITER Toroidal Field Coil

The ITER organization has performed design and its validation tests on a helium inlet structure for the ITER Toroidal Field (TF) coil under collaboration with CERN, KIT, and CEA-Cadarache. Detailed structural analysis was performed in order to optimize the weld shape. A fatigue resistant design on the fillet weld between the shell covers and the jacket is an important point on the helium inlet structure. A weld filler material was selected based on tensile test at liquid helium temperature after Nb3Sn reaction heat treatment. To validate the design of the weld joint, fatigue tests at 7 K were performed using heat-treated butt weld samples. A pressure drop measurement of a helium inlet mock-up was performed by using nitrogen gas at room temperature in order to confirm uniform flow distribution and pressure drop characteristic. These tests have validated the helium inlet design. Based on the validation, Japanese and European Union domestic agencies, which have responsibilities of the TF coil procurement, are preparing the helium inlet mock-up for a qualification test.

Direct observation of Nb3Sn lattice deformation by high-energy x-ray diffraction in internal-tin wires subject to mechanical loads at 4.2 K

With the aim of clarifying the relationship between lattice deformations and superconducting properties of Nb3Sn technological wires we have carried out high-energy x-ray diffraction experiments at the European Synchrotron Radiation Facility (ESRF) in Grenoble on individual samples of multi-filamentary internal-tin-type Nb3Sn wires. In particular, a test probe developed at the University of Geneva allowed us to perform these experiments at 4.2 K, while applying an axial tensile load to the specimen. In this way, the lattice parameter values of all the constituents (Nb3Sn, Nb, Cu) were determined, in both the parallel and orthogonal directions with respect to the applied load axis, as a function of the applied strain. The experiments were performed on industrial wires, which were reinforced by a stainless steel outer tube, applied before the Nb3Sn reaction heat treatment, in order to evaluate the effect of an additional pre-compression strain. The relation between the microscopically determined crystalline lattice deformations and the measured applied strain is discussed as a basis for the analysis of the superconducting performances of Nb3Sn wires subject to mechanical loads.

Characterization of High $J_{c}$${\rm Nb}_{3}{\rm Sn}$ Strands for the Series-Connected Hybrid Magnet

The series-connected hybrid magnet under construction at the National High Magnetic Field Laboratory uses cable-in-conduit-conductor with high Jc Nb3Sn strands. One of the candidate Nb3Sn strands is made by the restacked-rod process with a nominal Jc of ~ 2400A/mm2. We characterized the Jc as a function of axial and transverse strain, magnetic field and temperature. The Jc-strain measurements have been carried out by a straight pull device and two different Walters spring probes. In addition, we have investigated the effect of the Nb3Sn reaction heat treatment temperature on the irreversible strain. We found that the irreversible strain increases with the heat treatment temperature. Our results indicate that, in addition to its high J c, this type of strand has moderate strain sensitivity, therefore is suitable for the application of the series-connected hybrid magnet.

The inter-strand contact resistance ofNb3Sn cable-in-conduit conductor with hydrocarbon oil

The inter-strand contact resistance(RC)of Nb3Sn cable-in-conduit conductors (CICCs) is a very important parameter which strongly correlateswith ac losses and current redistribution behavior of the CICCs. One way to obtain the desiredRC is to apply a layer of hydrocarbon oil on theNb3Sn strands beforethe CICC Nb3Sn reaction heat treatment. In this paper, we measuredRC fora Nb3Sn CICC sample fabricated with hydrocarbon oil. The measurements were performed using anapparatus designed to apply transverse load and load cycling on CICCs at 4.2 K. ResistancesRC between strands of different cabling stages were measured with transverse load up to188 kN m−1 and load cycles up to 10 000. The results show thatRC of the sample increases almost linearly with the first loading. With load cycling,RC increases rapidly at first, but becomes saturated after∼100 cycles. After10 000 load cycles, RC decreases with increasing load at a decreasing rate. These behaviors are comparable toobservations for CICC with Cr plated strands reported in the literature. Therefore from theRC pointof view, Nb3Sn CICCs with hydrocarbon oil on strands may be used as a low cost alternative to CICCswith Cr plated strands.