2470 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005 Characteristics of Critical Current and Index of Bi-2223/Ag Tape up to 30 T at 4.2 K Jung Ho Kim, Jinho Joo, Seyong Choi, Wansoo Nah, Dong-Woo Ha, Hong-Soo Ha, Akiyoshi Matsumoto, Hiroaki Kumakura, Tsutomu Koizumi, Michinaka Sugano, and Tsukasa Kiyoshi Abstract—We fabricated 55-filament Bi-2223/Ag tape and eval- uated the dependence of critical current and index in an external magnetic field up to 30 T at 4.2 K, in order to explore the possible application for a magnet in persistent current mode. and index were characterized as a function of temperature and the magnitude and orientation of the magnetic field by increasing and decreasing field. It was observed that was dependent on the magnitude and orientation of the magnetic field. The decreased exponentially as the field increased; in a parallel and increasing field, was 128 A at 30 T, which is approximately 40% of the in self-field. On the other hand, index was not significantly dependent on the field up to 30 T, nor did it vary significantly on increasing or decreasing field; the value in the range of was 23.0 5.2 and 27.8 8.0 on increasing and decreasing field, respec- tively, in the range of 0 30 T. In addition, we observed that local and index of a long length of tape (4.2 m) were dispersed over a wide range and that the variation of each of these two values with the position of the tape was similar, suggesting that these proper- ties depend on the microstructural uniformity. Index Terms— characteristics, characteristic, index , NMR insert magnet, persistent current mode. I. INTRODUCTION F OR THE development of a high field nuclear magnetic res- onance (NMR) magnet of higher than 900 MHz (21.14 T), it is necessary to use a high temperature superconductor (HTS) magnet as an insert coil, because of its high upper critical mag- netic field, which is higher than 21.14 T at 4.2 K. It is well known that the field limit of existing magnets made of NbTi and wire is approximately 21 T at 4.2 K. Thus, for 1 GHz NMR, which requires the generation of a field as high as 23.49 T, the HTS insert coil should be designed so as to generate an addi- tional 2.35 T and operate in persistent current mode, in order to maintain the field stability. To improve the field stability in persistent mode, the insert coil must minimize dissipation, which mainly occurs due to Manuscript received October 3, 2004. This work was supported by Korea Research Foundation Grant (KRF-2003-041-D00268). J. H. Kim and J. Joo are with the School of Metallurgical and Materials Engi- neering, Sungkyunkwan University, Suwon, Korea (e-mail: jinho@skku.ac.kr). S. Choi and W. Nah are with the School of Information and Communication Engineering, Sungkyunkwan University, Suwon, Korea. D.-W. Ha and H.-S. Ha are with the Applied Superconductivity Laboratory, Korea Electrotechnology Research Institute, Changwon, Korea. A. Matsumoto and H. Kumakura are with the Superconducting Materials Center, National Institute for Materials Science, Tsukuba, Japan. T. Koizumi is with the Showa Electric Wire & Cable Co. Ltd., Sagamihara, Japan. M. Sugano and T. Kiyoshi are with the Tsukuba Magnet Laboratory, National Institute for Materials Science, Tsukuba, Japan. Digital Object Identifier 10.1109/TASC.2005.847482 small index and the joint resistance [1], [2]. Index is conven- tionally used to characterize the sharpness of transition, which is affected by intrinsic and extrinsic properties of superconductor. In general, BSCCO tape shows a smooth transition from a su- perconducting state to a normal state, due to thermally activated flux motion, resulting in lower value than that of low temper- ature superconductors. Even if BSCCO tapes are joined by the superconducting-joint method, the low value (10 20) of the tape has a detrimental effect on the field stability of the insert coil [3]. For this reason, it is important to investigate the varia- tion of index to use the tape in the persistent current operation in practical applications [4]. In addition, the magnetic anisotropy of BSCCO tape should be considered for analysing the critical properties of HTS mag- nets and for designing a related system [5]. Because BSCCO has a highly anisotropic lattice structure, and since the grains are aligned along the tape due to mechanical deformation, the crit- ical current strongly depends on the orientation and magni- tude of the applied field. Substantial research has been carried out, in order to understand the nature of the characteristics, and the variation of as a function of the external magnetic field and temperature have been widely reported [6], [7]. How- ever, the behavior of index with respect to temperature and the magnitude and orientation of the external field has not been systematically studied. In this study, therefore, we fabricated Bi-2223/Ag tape by the powder-in-tube (PIT) method and evaluated both the and index characteristics. Specifically, these two parameters were investigated as a function of temperature (4.2 K and 77.3 K) and the external magnetic field, up to 30 T, in increasing and decreasing fields. To estimate the effect of uniformity, and index were obtained at different positions along a 4.2 m long tape and their values compared with one another. II. EXPERIMENTAL PROCEDURE We fabricated 55-filament Bi-2223/Ag (BSCCO) tape by the conventional PIT method. Commercial powder with a nominal composition of (Merck Co. Ltd.) was loaded into an Ag tube (34 mm outer diameter (O.D.), 29 mm inner diameter (I.D.)) and the assemblies were com- pacted by swaging and drawing. Subsequently, the wires were re-stacked in an Ag-0.3 wt%Mn tube (20 mm O.D., 17.2 mm I.D.), and then drawn and rolled to fabricate 55 multifilament BSCCO tape. Each drawing and rolling step reduced the diam- eter and thickness of the wire/tape by less than 15% to avoid the sausaging of the core. The tape was sintered at 831 for 50 hr in an ambient atmosphere and then re-rolled and sintered again 1051-8223/$20.00 © 2005 IEEE