Degradation mechanism of the superconducting performance of a Bi2212 cable under magnetic fields

Abstract In magnetic confinement fusion reactors, superconducting magnet systems are essential for generating and controlling high magnetic fields. To increase the magnetic field, new superconducting materials such as Bi2212 (Bi 2 Sr 2 CaCu 2 O 8+ x ) have been selected in the design of magnet systems. However, the stability of the Bi2212 superconductor under magnetic fields must be studied for the routine and safe operation of magnet systems. In this work, the stability and degradation mechanism of a Bi2212 cable under magnetic fields were investigated. With a magnetic field of 5.8 T, the cable carrying 29 kA was exerted with a force of ∼168.2 kN m −1 . In the core area of the cable, moved wires were detected by computed tomography. The macroscopic movement of the wires would vary with the axial position, which could be related to the twist structure. Then, the cable was decomposed, and the acquired wires were tested under 12 T at 4.2 K by four-probe method. The results indicated that the inner wires had relatively lower critical currents, which should be the reason for the degradation of cable performance. Scanning electron microscope images of the superconducting phase within the wires confirmed that cracks existed in the superconducting phase of the inner wires, while intact crystals were found in that of the outer wires. The variation in microstructures gave rise to changes in the wire performance.