[en] 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

[en] We have developed a new probe for testing the influence of local contact load from crossing superconducting Nb₃Sn 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 Nb₃Sn cable in conduit conductors (CICC) causes transverse contact and bending strains in the wires and hence in the Nb₃Sn 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 Nb₃Sn 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 Nb₃Sn conductor in order to optimize the final cable design

[en] Measurements have been made of the critical current on an Nb₃Sn superconducting strand destined for the ITER (International Thermonuclear Experimental Reactor) prototype cable-in-conduit conductors. Characterization of the strand was performed on a recently developed spring device, named Pacman, allowing measurements of the voltage-current characteristic of an Nb₃Sn strand over a wide range of applied axial strain, magnetic field, temperature and currents up to at least 700 A. The strand was measured in a magnetic field between 4 and 11 T, temperatures of 4.2-10 K and applied axial strain ranged from -0.9% (compressive) to +0.3% (tensile). The critical currents were then used to derive the superconducting and the deformation-related parameters for the scaling of measured results, based on the so-called 'improved' deviatoric strain model. We also demonstrate that the same values can be derived from a partial critical-current data set without spoiling the overall scaling accuracy. This indicates that the proposed scaling relation can be used not only as a fitting tool, but is promising for reliable extrapolation as well, providing substantial savings in cost and time for the experimental routine

[en] Knowledge of the influence of bending on the critical current (I_c) 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 I_c 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 I_c due to bending strain can then be simply derived from the bending amplitude and the I_c versus axial applied strain (ε) relation. This I_c(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

[en] We have measured the critical current (I_c) of a high current density Nb₃Sn strand subjected to spatial periodic bending, periodic contact stress and uniaxial strain. The strand is destined for the cable-in-conduit conductors (CICC) of the European dipole (EDIPO) 12.5 T superconducting magnet test facility. The spatial periodic bending was applied on the strand, using the bending wavelengths from 5 to 10 mm with a peak bending strain of 1.5%, a periodic contact stress with a periodicity of 4.7 mm and a stress level exceeding 250 MPa. For the uniaxial strain characterization, the voltage-current characteristics were measured with an applied axial strain from -0.9% to +0.3%, with a magnetic field from 6 to 14 T, temperature from 4.2 to 10 K and currents up to almost 900 A. In addition the axial stiffness was determined by a tensile axial stress-strain test. The characterization of the strand is essential for understanding the behaviour of the strand under mainly axial thermal stress variation during cool down and transverse electromagnetic forces during charging, which is essential for the design of the CICC for the dipole magnet. The strand appears to be fully reversible in the compressive regime during the axial strain testing, while in the tensile regime, the behaviour is already irreversibly degraded when reaching the maximum in the critical current versus strain characteristic. The degradation is accentuated by an immediate decrease of the n value by a factor of 2. The parameters for the improved deviatoric strain description are derived from the I_c data, giving the accuracy of the scaling with a standard deviation of 4 A, which is by far within the expected deviation for the large scale strand production of such a high J_c strand. The I_c versus the applied bending strain follows the low resistivity limit, indicative of full interfilament current transfer, while a strong decrease is observed at a peak bending strain of ∼0.5%. For the periodic contact stress it appears that beyond 60 MPa, the I_c becomes noticeably irreversible. The n value, being a better indicator for the irreversible behaviour than the I_c, seems to indicate a somewhat lower level of the transverse stress irreversibility. Since the strand appears to be quite sensitive to the tensile strain, for the EDIPO CICC design, the tensile intrinsic axial strain in the filamentary region should stay well below zero (only compressive) and the contact stress sufficiently below 50 MPa. These requirements are feasible, in particular for a CICC design with steel conduit which would provide sufficient thermal compression, a sufficiently low void fraction and a cabling pattern that limits the peak bending strain during transverse load to about 0.3%

[en] We performed simulations with the numerical CUDI-CICC code on a typical short ITER (International Thermonuclear Experimental Reactor) conductor test sample of dual leg configuration, as usually tested in the SULTAN test facility, and made a comparison with the new EFDA-Dipole test facility offering a larger applied DC field region. The new EFDA-Dipole test facility, designed for short sample testing of conductors for ITER, has a homogeneous high field region of 1.2 m, while in the SULTAN facility this region is three times shorter. The inevitable non-uniformity of the current distribution in the cable, introduced by the joints at both ends, has a degrading effect on voltage-current (VI) and voltage-temperature (VT) characteristics, particularly for these short samples. This can easily result in an underestimation or overestimation of the actual conductor performance. A longer applied DC high field region along a conductor suppresses the current non-uniformity by increasing the overall longitudinal cable electric field when reaching the current sharing mode. The numerical interpretation study presented here gives a quantitative analysis for a relevant practical case of a test of a short sample poloidal field coil insert (PFCI) conductor in SULTAN. The simulation includes the results of current distribution analysis from self-field measurements with Hall sensor arrays, current sharing measurements and inter-petal resistance measurements. The outcome of the simulations confirms that the current uniformity improves with a longer high field region but the 'measured' VI transition is barely affected, though the local peak voltages become somewhat suppressed. It appears that the location of the high field region and voltage taps has practically no influence on the VI curve as long as the transverse voltage components are adequately cancelled. In particular, for a thin conduit wall, the voltage taps should be connected to the conduit in the form of an (open) azimuthally soldered wire, averaging the transverse conduit surface potentials initiated in the joints

[en] 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

[en] The flexible nature of the cable bundles in the sizeable cable-in-conduit-conductors for ITER containing more than a thousand strands, in combination with a void fraction of around 30%, gives scope for significant cable compression and strand deflection. In particular, the transverse stiffness of the Nb₃Sn type of cabled superconductors, being subjected to large electromagnetic forces, is critical for their long-term performance considering the impact of the strain variation on the transport properties. What is more, the compression of the cable bundle under load and the permanent deformation and relaxation in time or that associated with quenches, have an effect on the cooling and pressure drop along the turns of the windings and are valuable to account for in large magnets such as for ITER. The electromagnetic AC losses of ITER Nb₃Sn and NbTi CICCs, related to changing magnetic field and in this manner important for their stability, were broadly studied and reported but the associated mechanical losses have received less attention so far. The lifetime characteristics in terms of cable compression, changes in transverse stiffness and mechanical losses are experimentally determined on several prototype ITER NbTi and Nb₃Sn conductors in the Twente press and a summary of the results is given. The nonlinear stress-strain characteristics of the cable bundle and its moderate time-dependent nature can be considered as a viscoelastic-plastic phenomenon. The evolution of the stiffness and the mechanical loss depends on the peak load, void fraction, strand type and strand coating and changes with the number of load cycles. The dissipated heat from mechanical energy is not a critical issue for ITER magnet operation but is not negligible, in particular in the case of NbTi conductors.

[en] We have developed and validated a straightforward and fast method to investigate the response of technological superconducting strain sensitive wires (e.g., Nb₃Sn) to a spatial periodic bending strain. In the present concept of cabled superconductors for application in nuclear fusion reactors the wires are twisted and cabled in several stages. When subjected to transverse electromagnetic forces after charging the magnet, the individual strands are subjected to spatial periodic bending with wavelengths in the order of 5-10 mm. Several apparatuses are presently under development to study the effect of bending on the transport properties, i.e., the voltage-current transition in terms of critical current (I_c) and n value. We propose a supplementary simple method to investigate the influence of bending strain by using a spatial periodic wire support on a broadly used standard I_c measurement barrel in combination with a Lorentz force. The bending force (BI) is varied by changing the applied field B. The peak bending strain in the Nb₃Sn filaments is determined by the amplitude of the bending deflection, which is deduced from the mechanical axial tensile stress-strain properties of the wire. Three different spatial periodic wavelengths are applied and the results are in good agreement with the predictions. In addition we found a good agreement with results obtained by a more advanced experiment, named TARSIS, which is described briefly. The 'barrel-with-slots' method can be applied easily and straightforward with minor effort and cost in laboratories having a standard I_c measurement facility for superconducting wire

[en] We have developed and validated a straightforward and fast method to investigate the response of technological superconducting strain sensitive wires (e.g., Nb₃Sn) to a spatial periodic bending strain. In the present concept of cabled superconductors for application in nuclear fusion reactors the wires are twisted and cabled in several stages. When subjected to transverse electromagnetic forces after charging the magnet, the individual strands are subjected to spatial periodic bending with wavelengths in the order of 5-10 mm. Several apparatuses are presently under development to study the effect of bending on the transport properties, i.e., the voltage-current transition in terms of critical current (I_c) and n value. We propose a supplementary simple method to investigate the influence of bending strain by using a spatial periodic wire support on a broadly used standard I_c measurement barrel in combination with a Lorentz force. The bending force (BI) is varied by changing the applied field B. The peak bending strain in the Nb₃Sn filaments is determined by the amplitude of the bending deflection, which is deduced from the mechanical axial tensile stress-strain properties of the wire. Three different spatial periodic wavelengths are applied and the results are in good agreement with the predictions. In addition we found a good agreement with results obtained by a more advanced experiment, named TARSIS, which is described briefly. The 'barrel-with-slots' method can be applied easily and straightforward with minor effort and cost in laboratories having a standard I_c measurement facility for superconducting wire