IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 29, NO. 5, AUGUST 2019 9001505 Characterization of I c Degradation in Bent YBCO Tapes Lingfeng Lai , Chen Gu , Yubin Yue , Siwei Chen , Meng Song, Nan N. Hu , Timing Qu , and Shengnan Zou Abstract—In this paper, the I c degradation of bent YBCO tapes has been experimentally studied. The I c characteristics in length direction was measured through a non-contact, continuous I c mea- surement equipment (Mcorder), which is suitable for all kinds of high temperature superconducting (HTS) tapes. The detailed J c mapping of bent tapes was investigated by a Hall microscopy equipment (Mcorder 2-D), which scans the fields distribution on the surface of the films or bulks. A 4.1-m-long YBCO tape was wound on a 4.5-mm-diameter core with YBCO face outside or in- side. The critical current of the YBCO tape before and after wind- ing was measured lengthwise. And the 2-D J c map of the samples of degraded sections was measured. The result shows that the small- est safe bending diameter is larger than 14 mm and smaller than 4.5 mm, for YBCO face outside and inside, respectively, which in- dicates that the YBCO face is recommended to be inside in bending cases. The relation between critical current of YBCO face outside bent tapes and diameters was fitted to a sigmoid-like function, to help further development where YBCO face outside bending is unavoidable. Index Terms—Bending, critical current, continuous measure- ment, HTS tape. I. INTRODUCTION W ITH the rapid development in high temperature super- conducting technology, various applications of HTS are undergoing or have been finished, including HTS motor [1], current limiter [2], SMES [3], fusion magnet [4], MRI [5] and so on. In all these HTS applications, the HTS tape is bent into a coil as the basic unit of the devices. In addition, bending is used to eliminate anisotropy and reduce AC loss, such as CORC Manuscript received October 30, 2018; accepted February 5, 2019. Date of publication February 19, 2019; date of current version March 26, 2019. This work was partially supported by the National Natural Science Foundation of China under Contract No. 51477088. (Corresponding author: Chen Gu.) L. Lai and Y. Yue are with the Eastforce Superconducting Technology Com- pany Ltd., Beijing 100085, China. C. Gu is with the Teaching Center of Experimental Physics, Tsinghua Uni- versity, Beijing 100084, China (e-mail:, guchen@tsinghua.edu.cn). T. Qu is with the Key Laboratory for Advanced Materials Processing Tech- nology, Department of Mechanical Engineering,Tsinghua University, Beijing 100084, China. S. Chen was with the Eastforce Superconducting Technology Company, Ltd., Beijing 100085, China. He is now with the Department of Mechanical Engi- neering, University of Houston, Houston, TX 77004 USA. M. Song and N. N. Hu are with the Power Research Institute, Guangdong Power Grid Company, Guangzhou 10620, China. S. Zou is with the Quanzhou Institute of Equipment Manufacturing, Haixi Institutes, Chinese Academy of Sciences, Jinjiang 362200, Quanzhou, China. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2019.2900354 TABLE I SAMPLE TAPE [6] and TSTC [7]. Bending could also provide mechanical flex- ibility to endure load variation [8]. Thus bending is common in HTS applications, and the degradation caused by bending should be studied for designing, manufacturing and predicting the behavior of HTS applications. P. Skov-Hansen and Z. Han discussed the definitions and concepts with regards to the stresses and strains in HTS tapes under deformation [9]. Bending characteristics in HTS mate- rial have been widely studied, including BSCCO tape [10], YBCO tape and YBCO strand [11], CICC [12], CORC [13], out-of-plane bending [14] and ellipse bending [15]. Verebelyi et al. proposed a composite laminated architecture to optimize the mechanical properties in bending [16]. Bending effect has been considered in the optimization of HTS devices including Bi-2223/Ag SMES magnet [17] and 500 kV saturated iron core superconductive fault current limiter [2]. In the previous studies, the HTS tape was bent to mandrels with different curvatures, then discrete results were measured. In this paper, the dependence of the critical current on the bending diameter of superconducting tape was lengthwise continuously investigated at 77 K for the curvature diameter varied from 4.5 mm to 14 mm with YBCO face outside or inside. The 2D critical current density distribution in sections of the YBCO face outside bent tape was measured, which shows the damage pat- tern in the tape. The relation between critical current of YBCO face outside bent tapes and bending diameters was fitted to a sigmoid-like function. II. EXPERIMENTAL DETAILS A. Sample Preparing The sample YBCO tape was manufactured by SSTC (Shang- hai Superconductor Technology Co., Ltd). The tape details are in Table I. The tape was cut at the midpoint, creating 2 sepa- rated tapes with almost the same initial state (Fig. 1(a)). The new created 2 tapes were wound on a stainless cylinder core whose 1051-8223 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.