Many high-temperature superconductor (HTS) applications require superconducting cables with high currents while operating in an alternating magnetic field. HTS cables should be composed of numerous superconducting tapes to achieve the required current capacity. Alternating current and magnetic fields cause AC losses in such cables and can provoke conductor instability. AC losses and contact resistances were measured of several cable designs based on commercially available REBCO tapes at the University of Twente. The AC loss was measured under identical conditions for eight REBCO conductors manufactured according to three types of cabling methods—CORC® (Conductor on Round Core), Roebel, and stacked tape, including a full-size REBCO CICC (cable in conduit conductor). The measurements were done at T = 4.2 K without transport current in a sinusoidal AC magnetic field of 0.4 T amplitude and frequencies from 5 to 55 mHz. The AC loss was measured simultaneously by calibrated gas flow calorimeter utilizing the helium boil-off method and by the magnetization method using pick-up coils. Also, the AC loss of two CORC® conductors and a Roebel cable was measured at 77 K. Each conductor was measured with and without background field of 1 T. The measured AC coupling loss in the CORC® and Roebel conductors is negligible at 4.2 K for the applied conditions while at 77 K coupling loss was observed for all conductors. The absence of coupling loss at 4.2 K can be explained by shielding of the conductor interior; this is confirmed with measurement and calculation of the penetration field of CORC® and Roebel cables. The inter-tape contact resistance was measured for CORC® and stacked tape samples at 4.2 and 77 K. It was demonstrated that a short heat treatment of CORC® conductor with solder-coated tapes activates tape-to-tape soldering and decreases the contact resistance. The reduction of contact resistance by two orders in magnitude to tens of nΩm is comparable with the interstrand contact resistance in ITER Nb3Sn type conductors.

A future circular collider (FCC) with a center-ofmass energy of 100 TeV and a circumference of around 100 km, or an energy upgrade of the LHC (HE-LHC) to 27 TeV require bending magnets providing 16 T in a 50-mm aperture. Several development programs for these magnets, based on Nb 3 Sn technology, are being pursued in Europe and in the U.S. In these programs, cos-theta, block-type, common-coil, and canted-costheta magnets are ex-plored; first model magnets are under manufacture; limits on con-ductor stress levels are studied; and a conductor with enhanced characteristics is developed. This paper summarizes and discusses the status, plans, and preliminary results of these programs.

No-insulation coils are in general self-protecting and can therefore generally be operated at higher current densities. However, the electrical turn-to-turn connections may cause additional AC loss when charging the coil or when it is exposed to a time-dependent magnetic field. In this work, we study the case of a no-insulation ReBCO tape racetrack coil exposed to a uniform AC field applied parallel to the tape surface. We show that an anisotropic continuum model allows to formulate efficient analytical approximations for coupling loss in the low- and high-frequency limits. For intermediate frequencies, the continuum model needs to be evaluated numerically. The model was validated with representative measurements of AC loss in the coils, measured calorimetrically as well as magnetically using pick-up coils. The validation experiment nicely confirms the predicted frequency dependence of the coupling loss, which is P ∝ f 2 at low frequencies and P ∝ √f at high frequencies, due to the skin effect. The transition between low- and high-frequency regimes occurs around a characteristic frequency f c that is directly related to the characteristic time constant τ = 1/2πf c associated with the current decay in straightforward (dis)charge experiments.

ReBCO racetrack coils may be used in high-dynamic superconducting linear motor systems, typically replacing either permanent- or electromagnets in the DC stator. Even so, in order to achieve a significant increase in force density, the superconductor needs to carry a high transport current while simultaneously experiencing the time-varying magnetic field from the copper mover coils. To aid with the design of such devices, a 2D numerical model has been developed that predicts the AC loss under motor-relevant conditions, i.e. under the combined influence of a stationary transport current and an alternating external magnetic field. The main aim of the experiments described in this paper is to validate this model with dedicated AC loss measurements. To this end, we constructed a set-up that simultaneously measures magnetization-, transport current- and overall AC loss. Two identical insulated sub-scale ReBCO racetrack coils were tested at 4.2 K while carrying a stationary transport current of up to 700 A in a sinusoidal, alternating magnetic field up to 1.5 T, applied perpendicular to the broad face of the windings. Just like with metallic superconductors, the transport current significantly increases the AC loss level and lowers the penetration field. The inductive, electric and calorimetric data were found to be consistent with each other, validating the experimental calibration methods involved. Furthermore, the numerical model accurately predicted all AC loss components in the coils without any fitting to the data and can thus reliably be used in the design of superconducting machines.

The European Organization for Nuclear Research (CERN) is developing a six-around-one conductor on round core (CORC)-strand-based cable-in-conduit conductor (CICC) for use in detector and other large magnet systems. The CICC comprises six ReBCO-CORC strands helically wound around a central tube or rod and inserted in a square aluminum jacket. A major design challenge is finding a simple yet low-resistive method of injecting current homogeneously into the CORC strands of the CICC. In the production of joints for single-CORC cables, we are currently pursuing a method in which the different ReBCO layers at both ends of the CORC cable are trimmed into a staircase-like geometry. A similar trimming method is developed for joint terminals for the ReBCO-CORC-based CICC. A demonstration joint terminal is made to test the various steps of the trimming and manufacturing process before fabricating a joint terminal with real CORC strands. This paper presents an overview of CIC-joint terminal design, simulation results, and the different steps in the manufacturing process.