Abstract
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.
Highlights
REBCO high-temperature superconductors (HTS) are a possible candidate for application in magnets for fusion, particle accelerators, and various power machines like electric motors, generators, and transmission power lines
The AC loss and inter-tape contact resistances from REBCO cables made by different cabling methods (CORC®, stacked tape conductors, Roebel cable, and HTS CICC) were measured and compared
As a consequence of the high aspect ratio of the HTS tapes, all non-twisted conductors and the Roebel cable have high hysteresis loss in an alternating magnetic field oriented perpendicular to the wide side of the REBCO layer
Summary
REBCO high-temperature superconductors (HTS) are a possible candidate for application in magnets for fusion, particle accelerators, and various power machines like electric motors, generators, and transmission power lines. The tape geometry affects the AC loss characteristics of HTS cables similar to low-temperature superconductors (LTS), mostly consisting of cables made from round strands. One such feature is the AC loss dependence on the magnetic field orientation to the wide side of the tape. That results in a significant hysteresis loss when the magnetic field orientation is mainly perpendicular to the wide side of the conductor. Multi-tape REBCO conductors will generate coupling currents between tapes that are in electrical contact, and the associated coupling loss adds to the hysteresis loss. The AC loss of eight REBCO conductor types, representing different cabling methods (CORC®, Roebel, stacked tapes, CICC) were measured under the same conditions. The contact resistance was measured on CORC®, CICC, and stacked tapes conductors at temperatures of 4.2 and 77 K
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