AC loss and contact resistance in highly flexible rebco cable for fusion applications

Abstract

High Temperature Superconductor (HTS) materials can operate at higher magnetic fields up to 20 T with high critical current and higher operating temperature, compared to low temperature superconductors (LTS). A Highly Flexible REBCO Cable (HFRC) is introduced at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP); a cabling method that is suitable for REBCO HTS tape having anisotropic material properties in its thin REBCO layer. This type of HTS superconducting cable shows high potential for applications in nuclear fusion. The alternating currents and magnetic fields in tokamak type of fusion magnets, cause AC power losses in such cables, which can provoke instability of the conductor by induced currents and increase the temperature. As a first step in characterizing the electromagnetic (EM) performance of an HFRC cable, the AC loss and contact resistance of the HFRC prototype cable were measured at the University of Twente. The measurements were done in liquid helium (4.2 K) with AC magnetic fields, applied perpendicular to the cable's long axis. The AC loss was measured simultaneously by a calibrated gas flow calorimeter utilizing the helium boil-off method, and by the magnetization method using pick-up coils. For the applied test conditions, no coupling loss could be distinguished as a part of the overall AC loss. It is suggested that this might be explained by the shielding of the conductor interior from the applied magnetic field by the outer tape layer due to the high critical current density of the REBCO tape, leading to a high penetration field.