Focus on selected work from MEM16

The second-generation high temperature superconductor (2G HTS) wire is the most promising conductor for high-field magnets such as accelerator dipoles and compact fusion devices. The key element of the wire is a thin Y₁Ba₂Cu₃O₇ (YBCO) layer deposited on a flexible metal substrate. The substrate, which becomes incorporated in the 2G conductor, reduces the electrical and mechanical performance of the wire. This is a process that exfoliates the YBCO layer from the substrate while retaining the critical current density of the superconductor. Ten-centimeter long coupons of exfoliated YBCO layers were manufactured, and detailed structural, electrical, and mechanical characterization were reported. After exfoliation, the YBCO layer was supported by a 75 μm thick stainless steel foil, which makes for a compact, mechanically stronger, and inexpensive conductor. The critical current density of the filaments was measured at both 77 K and 4.2 K. The exfoliated YBCO retained 90% of the original critical current. Similarly, tests in an external magnetic field at 4.2 K confirmed that the pinning strength of the YBCO layer was also retained following exfoliation.

Conductor on Round Core (CORC^®) technology has achieved a long sought-after benchmark by enabling the production of round, multifilament, (RE)Ba₂Ca₃O_7−x coated conductors with practical current densities for use in magnets and power applications. Recent progress, including the demonstration of engineering current density beyond 300 Amm⁻² at 4.2 K and 20 T, indicates that CORC^® cables are a viable conductor for next generation high field magnets. Tapes with 30 μm substrate thickness and tape widths down to 2 mm have improved the capabilities of CORC^® technology by allowing the production of CORC^® wires as thin as 3 mm in diameter with the potential to enhance the engineering current density further. An important benefit of the thin CORC^® wires is their improved flexibility compared to thicker (7–8 mm diameter) CORC^® cables. Critical current measurements were carried out on tapes extracted from CORC^® wires made using 2 and 3 mm wide tape after bending the wires to various diameters from 10 to 3.5 cm. These thin wires are highly flexible and retain close to 90% of their original critical current even after bending to a diameter of 3.5 cm. A small 5-turn solenoid was constructed and measured as a function of applied magnetic field, exhibiting an engineering current density of 233 Amm⁻² at 4.2 K and 10 T. CORC^® wires thus form an attractive solution for applications between 4.2 and 77 K, including high-field magnets that require high current densities with small bending diameters, benefiting from a ready-to-use form (similar to NbTi and contrary to Nb₃Sn wires) that does not require additional processing following coil construction.

REBCO (RE = rare earth) based high temperature superconducting (HTS) wires are now being utilized for the development of electric and electromagnetic devices for various industrial, scientific and medical applications. In the last several years, the increasing efforts in using the so-called second generation (2G) HTS wires for some of the applications require a further increase in their engineering current density (J_e). The applications are those typically related to high magnetic fields where the higher J_e of a REBCO wire, in addition to its higher irreversibility fields and higher mechanical strength, is already a major advantage over other superconducting wires. An effective way to increase the J_e is to decrease the total thickness of a wire, for which using a thinner substrate becomes an obvious and attractive approach. By using our IBAD-MOCVD (ion beam assisted deposition-metal organic chemical vapor deposition) technology we have successfully made 2G HTS wires using a Hastelloy^® C276 substrate that is only 30 μm in thickness. By using this thinner substrate instead of the typical 50 μm thick substrate and with a same critical current (I_c), the J_e of a wire can be increased by 30% to 45% depending on the copper stabilizer thickness. In this paper, we report the fabrication and characterization of the 2G HTS wires made on the 30 μm thick Hastelloy^® C276 substrate. It was shown that with the optimization in the processing protocol, the surface of the thinner Hastelloy^® C276 substrate can be readily electropolished to the quality needed for the deposition of the buffer stack. Same in the architecture as that on the standard 50 μm thick substrate, the buffer stack made on the 30 μm thick substrate showed an in-plane texture with a Δϕ of around 6.7° in the LaMnO₃ cap layer. Low-temperature in-field transport measurement results suggest that the wires on the thinner substrate had achieved equivalent superconducting performance, most importantly the I_c, as those on the 50 μm thick substrate. It is expected the 2G HTS wires made on the 30 μm thick Hastelloy^® C276 substrate, the thinnest and with the highest J_e to date, will greatly benefit such applications as high field magnets and high current cables.

Coated conductor (CC) tapes utilized in high-current-density superconducting cables are commonly subjected to different loading modes, primarily torsion and tension especially in the case of twisted stacked-tape cable. Torsion load can occur due to twisting along the length or when winding the CC tapes around a former, while tension load can occur due to pre-tension when coiled and as a hoop stress when the coil is energized. In this study, electromechanical properties of single CC tapes under torsion load were investigated using a new test apparatus. The results could provide basic information for cable designers to fully characterize stacked cables. Copper-electroplated and brass-laminated CC tapes fabricated with different deposition techniques were subjected to pure torsion and combined tension-torsion loading. The critical current, I_c degradation behaviours of CC tapes under torsional deformation were examined. Also, the effect of further external lamination on the I_c degradation behaviour of the CC tapes under such loading conditions was investigated. In the case of the combined tension-torsion test, short samples were subjected to twist pitches of 200 mm and 100 mm. Critical parameters including reversible axial stress and strain in such twist pitch conditions were also investigated.

Practical REBCO and BSCCO-2223 tape-shaped wires are now manufactured on an industrial scale. They are a typical composite material consisting of superconducting layer/filaments together with functional components. These functional components affect directly the stress and strain dependences of the critical current. When applying an external stress R, the critical current I_c was measured. Then the external stress was reduced to R = 0 and the recovered critical current I_cr was again measured. The tensile stress and strain dependences of both normalized critical currents divided by the original value, I_c/I_c0 and I_cr/I_c0 were investigated. In general I_cr/I_c0 recovered close to unity when the applied stress was low, but its recovering level decreased gradually with increasing applied stress. The definition of the reversible stress and strain limits was investigated and its validity was proved using the cyclic loading test. The original definition of reversible stress and strain limits of critical current relates to: (1) when releasing the applied stress and strain, the I_c shall recover to the original value, and (2) when applying the cyclic stresses, the I_c shall keep the original value. Here, as a practical definition for the reversible stress and strain limits, the tensile stress and strain at 99% recovery of I_c have been proposed. On the other hand, it was made clear that the stress and strain at I_c 95% retention are not valid for use commonly as a criterion of reversible stress and strain limits for both practical REBCO and BSCCO-2223 wires.

PIT and RRP^® Nb₃Sn strands are being developed for high field accelerator magnet upgrades for the high luminosity LHC. Here we report a quantitative study of the shape and position of PIT filaments and RRP^® sub-elements after rolling lengths of unreacted PIT and RRP^® round wires to simulate cabling deformation. In the as-drawn condition, filament shape distortion occurs preferentially in the outer ring filaments. By contrast, rolling induces non-uniform shear bands that generate greater distortion of inner ring filaments. By making a full digitization of the shapes of all filaments, we find that a critical distortion occurs for thickness reductions between 10% and 20% when filament shapes in inner filament rings heavily degrade, especially in the vicinity of the strong 45^° shear bands imposed by the rolling. It is well known that maintaining diffusion barrier integrity is vital to retaining adequate RRR in the stabilizing copper needed for magnet stability. Diffusion barrier breaks occur preferentially in these distorted inner filaments and drive local Sn leakage during reaction, increasing RRR degradation.

In previous years, the R & D program between CERN and Columbus Superconductors SpA led to the development of several configurations of MgB₂ wires. The aim was to achieve excellent superconducting properties in high-current MgB₂ cables for the HL-LHC upgrade. In addition to good electrical performance, the superconductor shall have good mechanical strength in view of the stresses during operation (Lorenz forces and thermal contraction) and handling (tension and bending) during cabling and installation at room temperature. Thus, the study of the mechanical properties of MgB₂ wires is crucial for the cable design and its functional use. In the present work we report on the electro-mechanical characterization of ex situ processed composite MgB₂ wires. Tensile tests (critical current versus strain) were carried out at 4.2 K and in a 3 T external field by means of a purpose-built bespoke device to determine the irreversible strain limit of the wire. The minimum bending radius of the wire was calculated taking into account the dependence of the critical current with the strain and it was then used to obtain the minimum twist pitch of MgB₂ wires in the cable. Strands extracted from cables having different configurations were tested to quantify the critical current degradation. The Young's modulus of the composite wire was measured at room temperature. Finally, all measured mechanical parameters will be used to optimize an 18-strand MgB₂ cable configuration.

A large-bore 14-T CuNb/Nb₃Sn Rutherford coil was developed for a 25 T cryogen-free superconducting magnet. The magnet consisted of a low-temperature superconducting (LTS) magnet of NbTi and Nb₃Sn Rutherford coils, and a high-temperature superconducting magnet. The Nb₃Sn Rutherford coil was fabricated by the react-and-wind method for the first time. The LTS magnet reached the designed operation current of 854 A without a training quench at a 1 h ramp rate. The central magnetic field generated by the LTS magnet was measured by a Hall sensor to be 14.0 T at 854 A in a 300 mm cold bore.

During the first test campaign of the 60 kA HTS cable prototypes in the EDIPO test facility, the feasibility of a novel HTS fusion cable concept proposed at the EPFL Swiss Plasma Center (SPC) was successfully demonstrated. While the measured DC performance of the prototypes at magnetic fields from 8 T to 12 T and for currents from 30 kA to 70 kA was close to the expected one, an initial electromagnetic cycling test (1000 cycles) revealed progressive degradation of the performance in both the SuperPower and SuperOx conductors. Aiming to understand the reasons for the degradation, additional cycling (1000 cycles) and warm up-cool down tests were performed during the second test campaign. I_c performance degradation of the SuperOx conductor reached ∼20% after about 2000 cycles, which was reason to continue with a visual inspection of the conductor and further tests at 77 K. AC tests were carried out at 0 and 2 T background fields without transport current and at 10 T/50 kA operating conditions. Results obtained in DC and AC tests of the second test campaign are presented and compared with appropriate data published recently. Concluding the first iteration of the HTS cable development program at SPC, a summary and recommendations for the next activity within the HTS fusion cable project are also reported.

As part of the ITER conductor qualification process, 3 m long cable-in-conduit conductors (CICCs) were tested at the SULTAN facility under conditions simulating ITER operation so as to establish the current-sharing temperature, T_cs, as a function of multiple full Lorentz force loading cycles. After a comprehensive evaluation of both the toroidal field (TF) and the central solenoid (CS) conductors, it was found that T_cs degradation was common in long twist pitch TF conductors while short twist pitch CS conductors showed some T_cs increase. However, one kind of TF conductors containing superconducting strand fabricated by the Bochvar Institute of Inorganic Materials (VNIINM) avoided T_cs degradation despite having long twist pitch. In our earlier metallographic autopsies of long and short twist pitch CS conductors, we observed a substantially greater transverse strand movement under Lorentz force loading for long twist pitch conductors, while short twist pitch conductors had negligible transverse movement. With help from the literature, we concluded that the transverse movement was not the source of T_cs degradation but rather an increase of the compressive strain in the Nb₃Sn filaments possibly induced by longitudinal movement of the wires. Like all TF conductors this TF VNIINM conductor showed large transverse motions under Lorentz force loading, but T_cs actually increased, as in all short twist pitch CS conductors. We here propose that the high surface roughness of the VNIINM strand may be responsible for the suppression of the compressive strain enhancement (characteristic of long twist pitch conductors). It appears that increasing strand surface roughness could improve the performance of long twist pitch CICCs.

We have studied the influence of transverse compressive stress on the resistance and critical current (I_c) of soldered REBCO tape lap splices. Internal contact resistances dominate the overall REBCO lap splice resistances. Application of transverse compressive stress up to 250 MPa during the resistance measurements does not alter the resistance and I_c of the soldered REBCO splices that were studied. The resistance of unsoldered REBCO tape lap splices depends strongly on the contact pressure. At a transverse compressive stress of 100 MPa, to which Roebel cables are typically exposed in high field magnets, the crossover splice contact resistance is comparable to the internal tape resistances.