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[en] High-current superconducting CORC® wires, wound from RE-Ba2Cu3O7−δ (REBCO) coated conductors, are being developed for use in high-field magnets that would allow operation at magnetic fields exceeding 20 T. The combination of high engineering current densities and high magnetic fields results in large Lorentz forces acting on the CORC® wire that could cause irreversible degradation to its performance. The effect of axial tensile stress on the critical current of CORC® wires containing annealed solid copper formers has been measured in liquid nitrogen to determine the irreversible stress limit at which irreversible degradation occurs. The results show no significant change in critical current before the irreversible stress limit is reached, after which the critical current decreases irreversibly with applied stress. The irreversible stress limit as high as 177 MPa depends on the yield strength of the former, the number of superconducting tapes wound into the CORC® wire and the angle at which the tapes are wound. Although the irreversible stress limit of CORC® wires is lower than a rudimentary rule of mixtures estimation would suggest, the irreversible strain limit, as high as 0.85%, exceeds that of single REBCO tapes. Both effects are likely the result of the helical fashion in which the REBCO tapes are wound into CORC® wires. The performance of CORC® wires was also measured as a function of axial tensile stress fatigue cycling in liquid nitrogen. No significant performance degradation was measured up to 100 000 cycles as long as the peak stress remained below the irreversible stress limit. Only once the peak stress was increased significantly above the irreversible stress limit would the critical current suddenly decrease with stress cycles. The results indicate that CORC® wires have matured into extremely robust high-current magnet conductors capable of withstanding high levels of axial tensile stress and strain. The irreversible stress limit of CORC® wires could be increased further by using stronger formers and winding the REBCO tapes at comparable angles, while the irreversible strain limit could potentially be increased by tailoring the winding angle of the REBCO tapes, making CORC® wires one of the strongest and most elastic high-current superconducting magnet conductors available. (paper)
[en] High-field, low-inductance superconducting magnets in particle accelerators and fusion machines require high operating currents, often in combination with high current densities and for some applications conductor bending radii of less than 50 mm. These requirements form a major challenge for magnet conductors consisting of high-temperature superconductors, which are required for reaching magnetic fields exceeding 20 T, or allowing for operating temperatures above 20 K. The high tolerance of RE-Ba2Cu3O7−δ coated conductors to axial tensile and compressive strain has led to the concept of CORC® cables in an attempt to develop a round and mechanically as well as electrically isotropic, high-performance conductor that would meet these challenging requirements. This review article will outline how CORC® cables evolved from a concept into a practical and robust conductor for high-field magnets and compact superconducting power cables. This review article provides an extensive overview of how CORC® cable technology has overcome most of the challenges associated with its use in large magnets for fusion, particle accelerators and in helium gas cooled power and fault current limiting cables, while further development is ongoing that will push the CORC® cable technology to even higher performance levels. (topical review)
[en] Superconducting CORC® cables and wires have become practical conductors for use in high-field magnets for fusion machines and particle accelerators by demonstrating their ability to carry very high currents in background magnetic fields of up to 20 T. The high mechanical stresses that develop on the CORC® conductor during such operation could result in permanent degradation of the conductor critical current. Transverse compressive stress is one of the predominant mechanical stresses when CORC® cables or wires are bundled into CICCs that allow fusion and detector magnets to operate at currents as high as 100 kA. The effect of transverse compressive load on the critical current of CORC® cables and wires has been investigated at 76 K to determine their irreversible load limit under monotonic loading and load cycling up to 100 000 cycles. The results show a clear effect of the CORC® conductor layout with respect to gap spacing between tapes, thickness of the copper plating surrounding the tapes and thickness of the former onto which the tapes are wound. CORC® cables and wires have demonstrated a remarkable resilience to transverse compressive load cycling where their critical current decreased by no more than a few percent after 100 000 load cycles at peak loads that resulted in an initial decrease in critical current of less than 5%. The results indicate that no significant degradation of CORC® cable and wire performance due to transverse compressive load is expected in large magnets after 100 000 load cycles, as long as the peak load on the conductor does not exceed the irreversible load limit defined at 95% retention in critical current. The irreversible load limit of CORC® conductors could be increased further by increasing the size or hardness of the former that makes up the conductor’s core. (paper)
[en] BaFe2As2 (Ba-122) and Ba0.6K0.4Fe2As2 (K-doped Ba-122) powders were successfully synthesized from the elements using a reaction method that incorporates a mechanochemical reaction using high-impact ball milling. Mechanically activated, self-sustaining reactions (MSRs) were observed while milling the elements together to form these compounds. After the MSR, the Ba-122 phase had formed, the powder had an average grain size <1 μm, and the material was effectively mixed. X-ray diffraction confirmed Ba-122 was the primary phase present after milling. Heat treatment of the K-doped MSR powder at high temperature (1120 ° C) and pressure yielded dense samples with high phase purity, but only granular current flow could be visualized by magneto-optical imaging. In contrast, a short, low temperature (600 ° C) heat treatment at ambient pressure resulted in global current flow throughout the bulk sample even though the density was lower and impurity phases were more prevalent. An optimized heat treatment involving a two-step, low temperature (600 ° C) heat treatment of the MSR powder produced bulk material with very high critical current density above 0.1 MA cm−2 at 4.2 K and self-field (SF). (paper)
[en] Single crystals of superconducting BaFe1.8Co0.2As2 were exposed to neutron irradiation in a fission reactor. The defects introduced decrease the superconducting transition temperature (by about 0.3 K) and the upper critical field anisotropy (e.g. from 2.8 to 2.5 at 22 K) and enhance the critical current densities by a factor of up to about 3. These changes are discussed in the context of similar experiments on other superconducting materials.
[en] SmFeAsO1-xFx was irradiated in a fission reactor by a fast (E>0.1 MeV) neutron fluence of 4 x 1021 m-2. The introduced defects increased the normal state resistivity due to a reduction in the mean free path of the charge carriers. This leads to an enhancement of the upper critical field at low temperatures. The critical current density within the grains, Jc, increases upon irradiation. The second maximum in the field dependence of Jc disappears and the critical current density becomes a monotonically decreasing function of the applied magnetic field.
[en] One of the biggest challenges in developing conductor on round core (CORC"®) magnet cables for use in the next generation of accelerator magnets is raising their engineering current density J _E to approach 600 A mm"−"2 at 20 T, while maintaining their flexibility. One route to increase J _E could be to add more RE-Ba_2Cu_3O_7_−_δ coated conductors to the cable, but this would increase the cable size and reduce its flexibility. The preferred route to higher J _E is a reduction in diameter of the CORC"® cable, while maintaining the number of tapes wound into the cable. The availability of very thin tapes containing substrates of 30 μm thickness enabled us to wind a 5.1 mm diameter CORC"® cable from 50 coated conductors, while maintaining a tape critical current I _c of about 97% after cabling. The cable I _c was 7030 A at 4.2 K in a background field of 17 T, corresponding to a J _E of 344 A mm"−"2, which is the highest performance of any CORC"® cable so far. The magnetic field dependence allowed us to extrapolate the cable performance to 20 T to predict an I _c of 5654 A and a J _E of 309 A mm"−"2. The results clearly show that rapid progress is being made on overcoming the J _E hurdle for use of CORC"® cables in the next generation of accelerator magnets. Further optimization of the cable layout will likely increase J _E towards 600 A mm"−"2 at 20 T in the near future, while further reduction in cable size will also make them even more flexible. (paper)
[en] High-temperature superconducting (HTS) direct current (dc) power cables allow high levels of power transmission and distribution at low loss and can be tailored to effectively limit fault currents. HTS Conductor on Round Core (CORC®) power transmission cables offer additional benefits over other HTS cable designs, including a much higher current density and a higher degree of flexibility. These benefits make CORC® cables most suitable for applications in confined spaces where tight bends are required, such as onboard naval ships and in data centers. The development of CORC® power transmission cables for operation in pressurized helium gas is described, including their ability to act as fault current limiting cables. The 10 m long bipolar dc CORC® power transmission cable system is designed to operate at a current of 4000 A per pole at 50 K in pressurized helium gas. The test results at temperatures between 60 K–74 K in helium gas at a pressure of 1.7 MPa are described both during normal operation and during an overcurrent event. The results demonstrate the potential of CORC® cables to operate at currents exceeding 10 000 A per pole at 50 K at current densities of more than 200 A mm−2, resulting in the most energy dense superconducting power transmission cable to date. The successful operation during an overcurrent event also shows the added benefits of the high level of current sharing between tapes in CORC® cables that allow them to be operated as FCL cables without the need to incorporate a substantial amount of stabilizer. The successful test is a major milestone towards reliable high energy density power transmission in helium gas cooled superconducting power systems based on CORC® cables. (paper)
[en] We report the influence of atomic disorder introduced by sequential neutron irradiation on the basic superconducting properties, flux pinning and grain connectivity. Two different polycrystalline Sm-1111 samples (SmFeAsO1-xFx) and two Ba-122 single crystals (BaFe1.8Co0.2As2) were investigated. The monotonic decrease of the transition temperature with neutron fluence degrades the upper critical field, at least in the investigated temperature region. Pinning, on the other hand, is largely improved, with a different optimal defect concentration (fluence) in the two materials. The analysis of the current flow in the polycrystalline samples reveals weak link behavior in the majority of grain connections and the existence of stronger grain connections. The density of the latter seems to be close to the percolation threshold (i.e. the minimum fraction for a continuous current path). Both types of connections are sensitive to disorder and degrade upon neutron irradiation.
[en] A trapped field of over 1 T at 5 K and 0.5 T at 20 K has been measured between a stack of magnetized cylinders of bulk polycrystalline Ba_0_._6K_0_._4Fe_2As_2 superconductors 10 mm in diameter and 18 mm in combined thickness. The trapped field showed a low magnetic creep rate (∼3% after 24 h at 5 K), while magneto-optical imaging revealed a trapped field distribution corresponding to uniform macroscopic current loops circulating through the sample. The superconductors were manufactured by hot isostatic pressing of pre-reacted powders using the scalable powder-in-tube technique. A high Vickers hardness of ∼3.5 GPa and a reasonable fracture toughness of ∼2.35 MPa m"0"."5 were measured. Given the untextured polycrystalline nature of the cylinders and their large irreversibility field (>90 T), it is expected that larger bulks could trap fields in excess of 10 T. (fast track communication)