For high current superconductors in high magnet fields with currents in the order of 50 kA, single ReBCO coated conductors must be assembled in a cable. The geometry of such a cable is mostly such that combined torsion, axial and transverse loading states are anticipated in the tapes and tape joints. The resulting strain distribution, caused by different thermal contraction and electromagnetic forces, will affect the critical current of the tapes. Tape performance when subjected to torsion, tensile and transverse loading is the key to understanding limitations for the composite cable performance. The individual tape material components can be deformed, not only elastically but also plastically under these loads. A set of experimental setups, as well as a convenient and accurate method of stress-strain state modeling based on the finite element method have been developed. Systematic measurements on single ReBCO tapes are carried out combining axial tension and torsion as well as transverse loading. Then the behavior of a single tape subjected to the various applied loads is simulated in the model. This paper presents the results of experimental tests and detailed FE modeling of the 3D stress-strain state in a single ReBCO tape under different loads, taking into account the temperature dependence and the elastic-plastic properties of the tape materials, starting from the initial tape processing conditions during its manufacture up to magnet operating conditions. Furthermore a comparison of the simulations with experiments is presented with special attention for the critical force, the threshold where the tape performance becomes irreversibly degraded. We verified the influence of tape surface profile non-uniformity and copper stabilizer thickness on the critical force. The FE models appear to describe the tape experiments adequately and can thus be used as a solid basis for optimization of various cabling concepts.

Differences in the thermal contraction of the composite materials in a cable in conduit conductor (CICC) for the International Thermonuclear Experimental Reactor (ITER) in combination with electromagnetic charging cause significant axial, transverse and bending strains in the Nb3Sn layer. These high strain loads degrade the superconducting properties of a CICC. Here we report on the influence of periodic bending load, using different bending wavelengths from 5 to 10 mm on a Nb3Sn powder-in-tube processed strand. The strand axial tensile stress–strain curve, the critical current versus applied axial strain results, the influence of cyclic loading on the RRR and assessment of the current transfer length from AC loss measurements, required for the analysis, are presented as well. For the strand under investigation, we find an influence of bending strain on the Ic that corresponds well to the predictions obtained from the applied classical relations, distinguishing ultimate boundaries of high and low interfilament electrical resistance. The reduction versus applied bending strain is similar for all wavelengths and equivalent to the low transverse resistance model, which is consistent with the estimated current transfer length. The cyclic behaviour in terms of critical current and n-value involves a component representing a permanent reduction as well as a factor expressing reversible (elastic) behaviour as a function of the applied load. The results from the set-up enable a discrimination in performance reduction per specific load type and per strand type. In this paper, we discuss the results of the pure bending tests.

The differences in thermal contraction of the composite materials in a cable in conduit conductor (CICC) for the International Thermonuclear Experimental Reactor (ITER), in combination with electromagnetic charging, cause axial, transverse contact and bending strains in the Nb 3 Sn filaments. These local loads cause distributed strain alterations, reducing the superconducting transport properties. The sensitivity of ITER strands to different strain loads is experimentally explored with dedicated probes. The starting point of the characterization is measurement of the critical current under axial compressive and tensile strain, determining the strain sensitivity and the irreversibility limit corresponding to the initiation of cracks in the Nb 3 Sn filaments for axial strain. The influence of spatial periodic bending and contact load is evaluated by using a wavelength of 5 mm. The strand axial tensile stress-strain characteristic is measured for comparison of the axial stiffness of the strands. Cyclic loading is applied for transverse loads following the evolution of the critical current, n-value and deformation. This involves a component representing a permanent (plastic) change and as well as a factor revealing reversible (elastic) behavior as a function of the applied load.The experimental results enable discrimination in performance reduction per specific load type and per strand type, which is in general different for each manufacturer involved. Metallographic filament fracture studies are compared to electromagnetic and mechanical load test results. A detailed multifilament strand model is applied to analyze the quantitative impact of strain sensitivity, intrastrand resistances and filament crack density on the performance reduction of strands and full-size ITER CICCs. Although a full-size conductor test is used for qualification of a strand manufacturer, the results presented here are part of the ITER strand verification program. In this paper, we present an overview of the results and comparisons.

Knowledge of the influence of bending on the critical current (Ic) of Nb3Sn strands is essential for the understanding of the reduction in performance due to transverse electromagnetic load. In particular, for the large cable-in-conduit conductors (CICCs) meant for the international thermonuclear experimental reactor (ITER), we expect that bending is the dominant mechanism for this degradation. We have measured the Ic of a bronze, a powder-in-tube and an internal tin processed Nb3Sn strand when subjected to spatial periodic bending using bending wavelengths from 5 to 10 mm. Two of these strands were applied in model coils for the ITER. We found that the tested strands behave according to the so-called low interfilament resistivity limit, confirming full current transfer between the filaments. This is supported by AC coupling loss measurements giving an indication of the interfilament current transfer length. The reduction of Ic due to bending strain can then be simply derived from the bending amplitude and the Ic versus axial applied strain (ε) relation. This Ic(ε) sensitivity can vary for different strand types but since the electromagnetic force is the driving parameter for strand bending in a CICC, the stiffness of the strands definitively plays a key role, which is confirmed by the results presented.

inanding magnet systems like high-field sdenoids and accelerator magnets. Whereas h e emphasis for the first application lies on LL high Bc2 and a high J, at about 20 T, the deiiianils for the later also include a sinall filatneiit diameter and a well-controlled copper €action. Powder-in-tube (PIT) processed Nb3Sn conductors have proven to be a good caodidate for both applications.The PlT method for manufaccturing superconducting NbjSn wire is characterized by niobim tuks containing a powder of the inter-tnetallic compound NhSn2. Usually after coil winding a rclatively short heat treatment of 40-100 hours at G75"C is n d d for the formation of the superconducting Nb$n phase.By difhsion, the tin from the powder COR reacts with the n i e bium and a Nb3Sn Iaycr grows uiitil aboiit 213 of the niobium wall has reacted. The exccSs of niobium tube matcrial outside the Nb3Sn layer acts as a ndtiral hairier to avoid contaanination of the copper inauix. Sincc the tin is confined in the t u b and the filaments are directly arranged in the copper matrix, this process should in principle enable a further rcduction of filament s i a with perfectly de-coupled filaments.Tlic preciirsor material for the powder cow, the niobium tubes and the matrix can be manipulated more or less indc-9?5 pendently within limits iniposed by mechanical rigidity and billet processitq. Therefor the PIT-process appears to be particulmly suited for incorporation of (artificial) pinning centers in the Nb3Sn layer by local additions [3],[4].A first step into this ,direction is the dcvelopmcnt of a 37 filameiit high-field conductor utilizing Nbn.Swt.%Ta tubes, which should particularly increase the Bc2 and the J, in the 20 T range.At the same time SMI is continuously improving the properties of the binary 1'IT-mSn conductors for application in accelerator magnets. Both program arc discussed in this papcr. BINARY PIT CONDUClOR FOR ACCEI.E~ 2200 Nmm2 10 T wit11 a copper fraction close to 50 %. With the matnhcturing of a

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.

We have developed a new probe for testing the influence of local contact load from crossing superconducting Nb 3 Sn strands. The probe is part of the TARSIS setup for strand stress-strain characterization. The results from the TARSIS setup with different probes developed for the characterization of axial tensile stress-strain, spatial periodic bending, contact from crossing strands and homogeneous transverse load enable discrimination in performance reduction per specific load type and per strand type.In particular, the electromagnetic charging of Nb 3 Sn cable in conduit conductors (CICC) causes transverse contact and bending strains in the wires and hence in the Nb 3 Sn filaments. More than ever, for high electromagnetic loads, such as in the conductors designed for the International Thermonuclear Experimental Reactor (ITER), the transverse load causes significant local strain concentrations in combination with strain differences due to the thermal contraction of the composite materials. These high local strains in the strands degrade the superconducting properties significantly. We report on the design of the probe and the first results demonstrating the influence of periodic transverse contact load from crossing strands, using a wavelength of 5 mm on an Nb 3 Sn powder-in-tube processed strand. The cyclic behaviour in terms of critical current and n-value involves a component representing a permanent reduction as well as a factor expressing reversible (elastic) behaviour as a function of the applied load. The results of the probe are used as input for the mechanical and electromagnetic modelling of a full-size ITER Nb 3 Sn conductor in order to optimize the final cable design.

The EuCARD-2 Enhanced European Coordination for Accelerator Research & Development project is co-funded by the partners and the European Commission under Capacities 7th Framework Programme, Grant Agreement 312453. This work is part of EuCARD-2 Work Package 10: Future Magnets (MAG). The electronic version of this EuCARD-2 Publication is available via the EuCARD-2 web site or on the CERN Document Server at the following URL: