[en] Highlights: • Investigation of the cyclic load degradation in the HTS cable prototype is performed. • Results of transverse load fatigue tests on the HTS strands at 77 K are presented. • Design of the new HTS cable prototype for the DEMO CS coils is discussed. - Abstract: Various DC tests performed recently with full-size 60 kA HTS cable prototypes in the EDIPO test facility demonstrated that proposed design of HTS strand at Swiss Plasma Center (SPC) – stack of HTS tapes twisted and soldered between two copper profiles – is applicable for high-current fusion cables, but improvement of the strand mechanical properties against the cyclic loading is still needed. Based on experimentally obtained correlation between the performances of cable prototypes at different operating conditions, further investigation of cyclic transverse load on the strand performance was performed at 77 K. Aiming to obtain a strand design able to withstand a continuous cyclic load operation of some thousand cycles, influence of the strand geometry and tape's manufacturer has been studied. Cyclic load has been applied up to 1000 cycles for straight and bent samples at the load amplitudes up to 4 MPa. Based on the obtained data, next design of HTS cable prototype will be discussed.

[en] Fe/MgB₂ tapes have been prepared starting with pre-reacted binary MgB₂ powders. As shown by resistive and inductive measurements, the reduction of particle size to a few microns by ball milling has little influence on B_c2, while the superconducting properties of the individual MgB₂ grains are essentially unchanged. Reducing the particle size causes an enhancement of B_irr from 14 to 16 T, while J_c has considerably increased at high fields, its slope J_c(B) being reduced. At 4.2 K, values of 5.3 x 10⁴ and 1.2 x 10³ A/cm² were measured at 3.5 and 10 T, respectively, suggesting a dominant role of the conditions at the grain interfaces. A systematic variation of these conditions at the interfaces is undertaken in order to determine the limit of transport properties for Fe/MgB₂ tapes. The addition of 5% Mg to MgB₂ powder was found to affect neither J_c nor B_c2. For the tapes with the highest J_c values, very high exponential n factors were measured: n=148, 89 and 17 at 3.5, 5 and 10 T, respectively and measurements of critical current versus applied strain have been performed. The mechanism leading to high transport critical current densities of filamentary Fe/MgB₂ tapes based on MgB₂ particles is discussed

[en] Roebel cables are one possible option for use in the manufacture of high current conductors for various technical applications. In this work the behaviour of critical current under transverse loads up to 90 MPa was studied in coated conductor Roebel cables. First the critical current evolution under transverse load was measured on 4 mm wide coated conductor tapes with Hastelloy substrate (copper surround stabilizer) and on punched tapes, which are used for the assembling of Roebel cables. It was found that they were able to withstand large transverse loads (>100 MPa) with less than 2% degradation of the critical current at 77 K. In contrast, with tapes, a significant reduction of the critical current (>20%) was observed in Roebel cable samples at pressure as low as 10 MPa. Marks of mechanical defects were observed in the strands extracted from the cable. Measurements of the local critical current on extracted strands confirmed that in the regions which were visually damaged the critical current was also reduced. The defects are attributed to pinching (stress concentration), which takes place in the cable. The results suggest that the integration of coated conductor Roebel cables into large systems should be carefully evaluated. (paper)

[en] A small coated conductor coil (central field 1 T at 200 A) was constructed and tested in order to evaluate the benefit and the possible disadvantages of non insulated, double pancake construction, in particular regarding quench protection. The insert coil consists of three double pancakes without turn to turn insulation, and it was impregnated with beeswax. The coil was equipped with voltage taps on each pancake, thermocouples and a quench heater. Quench tests were carried out because quench protection is one of the main issues in developing coil technology for coated conductors. Quenches were induced either by a heater or by exceeding the coil critical current; in all cases the quench protection system was able to detect the quench.

[en] Rare Earth based coated conductors are very promising conductors for the construction of high field solenoids. Development of coil winding technique was started at the Tsukuba Magnet Laboratory: coils of various sizes (from 18 mm to 220 mm of an inner diameter) have been fabricated by layer-winding without impregnation in order to test the mechanical and superconducting properties. In the thin 220 mm diameter coil the Hoop stress reached over 500 MPa at 130 A in 14 T background field, without degradation of the superconducting properties. In the small diameter coils (18 mm of inner diameter) the tape was insulated using enamel to minimize the cross section and to enhance the coil current density. The highest field generated was 2.1 T in a background field of 18 T (total field was 20.1 T) at 4.2 K and the coil current density was 240 A/mm².

[en] High temperature superconducting (HTS) materials have the potential to generate a magnetic field beyond the level obtainable with low temperature superconducting (LTS) materials. This review reports on past and present R and D on HTS cables and conductors for high field tokamaks, accelerator dipoles, and large solenoids. Among the HTS wires and tapes available commercially, coated conductor tapes are the most appealing because of their outstanding critical strength and future improvement margin. Limitations are the weakness against peeling and delamination and the short piece length. The prices of technical superconductors are reviewed because they play an important role in large projects; moreover HTS wires and tapes are discussed from the perspective of industrial production considering the historical development of the LTS wire market. Various designs have been proposed for HTS cables and conductors: some are better suited for soft materials, while others can exploit the anisotropy of coated conductors (by aligning the tape with the field), thus providing the highest current density. Recently, there has been an increase in the size and complexity of the prototypes; however some peculiar features of HTS, such as high stability margins and high mechanical limits, have not yet been fully incorporated into the designs: for example the transposition requirements for HTS have not yet been studied in detail. There are elements indicating that rectangular wires and tapes (even if anisotropic) can be used for manufacturing cables and magnets of any size and have advantages with respect to round wires. (topical review)

[en] Multifilamentary superconducting Nb₃Sn wires are widely used for industrial applications. Wires processed by the Bronze Route technique are characterized by a large number of filaments Nb_1-xSn_x, with 0.18≤x≤0.25, corresponding to a distribution of T_c between<10 and 18 K. This distribution is a critical property of the wires and is important for the optimization of the conductors. However, it is not accessible to conventional techniques, due to percolation and/or magnetic shielding effects. In order to determine the T_c distribution in the sample, we have carried out specific heat measurements of various Nb₃Sn multifilamentary wires (including the bronze matrix) in zero field and at 14 T. A deconvolution of the calorimetric data at the superconducting transition by means of a thermodynamical model was used for obtaining the distribution of T_c in the whole wire volume. The measurements were extended to Nb₃Sn wires containing Ti additions, and the results were compared. The present calorimetric method is of primary importance for the complete characterization of Nb₃Sn wires

[en] The effect of uniaxial strain on the critical current of 0.8 m long Nb₃Sn wires up to 21 T is studied by the modified Walters spring (WASP). For Nb₃Sn wires, prepared by both the bronze route and the internal Sn diffusion process, the critical current density as a function of the uniaxial strain ε is found to exhibit an asymmetric behaviour on both sides of the strain ε_m, where J_c reaches its maximum. Revisiting earlier x-ray and neutron diffraction measurements on bronze route processed wires between 10 and 600 K, it is shown that the asymmetric behaviour of J_c(ε) on both sides of the strain value ε_m is connected to individual variations of the stress-induced tetragonal lattice parameters a and c. The present measurements of J_c versus strain for Nb₃Sn wires show stronger strain dependence for wires prepared by the internal Sn diffusion method with respect to those obtained by the bronze route. The reasons for this difference are attributed to the individual details of the filament configuration in both types of wire, for example the different Sn distributions inside the filaments and the very different filament sizes, 4 and 80 μm, respectively

[en] Full text: High temperature superconductors (HTS) may become in future an option for the superconducting magnets of commercial fusion plants. The general requirements for HTS conductors and coils are presented together with a tentative roadmap for the related R&D activity. The issue of the material cost and its evolution is also discussed in comparison to the low temperature superconductor (LTS) option. At the Swiss Plasma Center (SPC) the R&D activity toward HTS high-current, high-field cable suitable for fusion magnets started in 2012 and led in 2015 to the assembly of the first 60 kA, 12 T prototype conductor. The basic component is a thin ceramic tape (0.1 mm thick, 4 mm wide) of coated conductor, generically named ReBCO and commercially available since about a decade. The main challenge for the design of high current cable is the assembly of a large number of brittle tapes in a mechanically stable configuration, able to withstand the large operating loads. The electrical requirements, tape transposition for balanced current distribution and moderate AC loss, set additional restrictions to the cable layout. The cable concept developed at SPC is based on the principle of “soldered, twisted stacks” of ReBCO tapes. The required number of stacks is then assembled in a cored flat cable, cooled by forced flow of supercritical helium. The EDIPO facility at SPC, with 12.4 T background field, 100 kA test current and 1 m long high field section, is the ideal tool for qualification of high current HTS cables, where long cable pitches, in the range of 1 m, are mandatory to control the bending and torsion strain of the ceramic tapes. To test HTS cable samples at elevated temperature, the EDIPO test environment is upgraded with a low heat conduction “adapter”, inserted between the HTS sample and the NbTi based transformer. A counter-flow heat exchanger between inlet and outlet coolant allows cold gas return from the HTS sample to the cryoplant. The first test of a HTS high current sample was carried out in 2015. The initial performance fulfilled the design target, but some degradation upon electromagnetic load cycles was observed and investigated after dismantling the cable. A new prototype HTS conductor is being now assembled at SPC with improved layout, based on the tentative spec for the high grade conductor of the DEMO central solenoid. (author)

[en] 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. (paper)