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-Ba 2 Cu 3 O 7−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.

Large-grain Nb has become a viable alternative to fine-grain Nb for the fabrication of superconducting radio-frequency cavities. In this contribution we report the results from a heat treatment study of a large-grain 1.5 GHz single-cell cavity made of "medium purity" Nb. The baseline surface preparation prior to heat treatment consisted of standard buffered chemical polishing. The heat treatment in the range 800 -1400 • C was done in a newly designed vacuum induction furnace. Q0 values of the order of 2 × 10 10 at 2.0 K and peak surface magnetic field (Bp) of 90 mT were achieved reproducibly. A Q0-value of (5 ± 1) × 10 10 at 2.0 K and Bp = 90 mT was obtained after heat treatment at 1400 • C. This is the highest value ever reported at this temperature, frequency and field. Samples heat treated with the cavity at 1400 • C were analyzed by secondary ion mass spectrometry, secondary electron microscopy, energy dispersive X-ray, point contact tunneling and X-ray diffraction and revealed a complex surface composition which includes titanium oxide, increased carbon and nitrogen content but reduced hydrogen concentration compared to a non heat-treated sample.

No-insulation (NI) superconducting REBCO magnets have advantages of self-quenchprotection, a very high engineering current density and high mechanical strength, and the potential to reach very high magnetic fields. However, NI REBCO magnets have drawbacks of a long magnet charging time and high field ramp losses. These can be mitigated by controlling the turn-to-turn contact resistivity (R c ). In an effort to control R c , we consider two approaches. One is coating a REBCO conductor with various resistive thin films, and the other is to use a stainless steel (SS) tape as an interlayer which is also coated with different metallic films. We present experimental results of R c of an as-received sample under cyclic contact pressure of 2.5-25 MPa up to 30 000 cycles. After an initial increase in R c for the first 10-20 cycles, R c decreases to about one tenth of its initial value after a few hundred cycles. A warm-up and cool-down thermal cycle does not significantly change the low R c resulting from a previously high number of load cycles. We also studied R c of REBCO tapes that are coated with different resistive layers and interlayers. In order to increase R c , we experimented with electro-and electroless plating of Ni, Cr, and Ni-P. We also measured R c with a thin metallic interlayer as a coil co-winding material which included Cu, SS, and SS plated with Ni and Cu. A SS interlayer increases R c by about three orders of magnitude; while the Cu plated SS interlayer only increases R c by one order of magnitude. Finally, we treated the as-received REBCO surface by oxidation using an Ebonol® C solution. This controlled oxidation allowed the R c to be controlled over a wide range.

High-current superconducting CORC ® wires, wound from RE-Ba 2 Cu 3 O 7−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.