Evaluation of a 426 kJ Cryocooled Magnet and a Model Magnet With REBCO Coated Conductors

Abstract

REBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (REBCO, RE = rare earth) coated conductors are expected to show high performance for superconducting applications because of their high current density and high mechanical strength. Fujikura has developed REBCO coated conductor over 570 A/cm width at 77 K, self field with over 800 m length. In addition, we have succeeded in developing a 5 T cryocooled REBCO magnet with a stored energy of 426 kJ. The magnet is composed of 24 pancake coils with an inner diameter of 260 mm, and the total length of the REBCO coated conductors is approximately 7.2 km. We confirmed that the REBCO magnet could be excited up to 5 T for 720 min 9 months after the fabrication of the magnet. It is important to validate the long-term operation test results of the REBCO magnets with larger diameters in order to use these conductors in practical superconducting applications. On the other hand, we have fabricated and evaluated a model magnet before we fabricated a 426 kJ cryocooled magnet. The model magnet is composed of 6 REBCO pancake coils and 18 “dummy coils” with an inner diameter of 260 mm and an outer diameter of 515 mm. The model magnet is about the same size and shape as the 426 kJ cryocooled magnet. We have confirmed that the calculated critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) of the top pancake coil of the model magnet was in good agreement with the measured coil I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> from 30 to 77 K under the conduction-cooled conditions. In addition, we have evaluated the thermal runaway behavior of the model magnet under the conduction-cooled conditions. As a result, we have confirmed that the model magnet with a 0.3-mm-thick copper stabilizer had sufficient time to ramp down without degradation at 361 A transport current after the detected normal transition.