Hoop Stress Modification, Stress Hysteresis and Degradation of a REBCO Coil Due to the Screening Current Under External Magnetic Field Cycling

• Published in: IEEE transactions on applied superconductivity Vol. 30; no. 4; pp. 1 - 7
• Main Authors: Takahashi, Shunji, Suetomi, Yu, Takao, Tomoaki, Yanagisawa, Yoshinori, Maeda, Hideaki, Takeda, Yasuaki, Shimoyama, Jun-ichi
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.06.2020; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Coils; Computer simulation; Conductors; Current measurement; Degradation; Hoop stress; Hysteresis; inhomogeneous hoop stress; Magnetic fields; Magnetic hysteresis; REBCO coil; Screening; screening current; Strain; Strain measurement; Stress; Stress concentration; Stress distribution; stress modification
• ISSN: 1051-8223; 1558-2515
• DOI: 10.1109/TASC.2020.2974837

Degradation of a REBCO coil under external magnetic fields is one of the major technical problems in the field of HTS magnet technology. A possible cause of such degradation is an inhomogeneous hoop stress distribution, or hoop stress modification (both increase and decrease), induced by the screening current. In this work, we investigate such a hoop stress modification by a small coil experiment with a strain measurement and a numerical simulation. An experimental result shows a very high stress increase factor of >4.1, defined by the maximum circumferential stress over B z JR stress, and the simulated result is in qualitative agreements. The strain (stress) shows a hysteresis effect corresponding to the screening current behavior. A large hoop stress modification causes not only a hoop stress increase, but also buckling of the conductor, which induces delamination and micro-clacks of the superconducting layer. We also show the stress modification can be reduced by bonding turns with epoxy.