IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009 2615 Characterization of High Strands for the Series-Connected Hybrid Magnet Jun Lu, Ke Han, Robert P. Walsh, Iain Dixon, Alexandre Ferrera, and Bernd Seeber Abstract—The series-connected hybrid magnet under construc- tion at the National High Magnetic Field Laboratory uses cable-in- conduit-conductor with high strands. One of the can- didate strands is made by the restacked-rod process with a nominal of . We characterized the as a function of axial and transverse strain, magnetic field and tem- perature. The -strain measurements have been carried out by a straight pull device and two different Walters spring probes. In ad- dition, we have investigated the effect of the reaction heat treatment temperature on the irreversible strain. We found that the irreversible strain increases with the heat treatment tempera- ture. Our results indicate that, in addition to its high , this type of strand has moderate strain sensitivity, therefore is suitable for the application of the series-connected hybrid magnet. Index Terms—Critical current, , strain. I. INTRODUCTION T HE series-connected hybrid magnet (SCH) uses cable-in-conduit-conductor (CICC) for its supercon- ducting coil [1]. For the design of the SCH superconducting coil, it is convenient to take advantage of the recent develop- ment of high wires made by restacked-rod process (RRP) [2]. However, there is a concern that the high wire may be more strain sensitive. The strain sensitivity is especially important in the application of CICC where signifi- cant degradation has been associated with localized strains from large transverse Lorentz forces [3]–[5]. In order to predict the performance of the CICC and to identify and analyze the possible degradation based on the CICC tests results, the characterization of strand for versus axial and transverse strains is crucial. The irreversible strain, defined as the tensile axial strain beyond which degrades permanently, is an important param- eter which is related to the CICC degradation under cyclic load. For a similar high RRP strand, it has been observed that the becomes irreversible even before the zero intrinsic strain is reached [6]. This is not very well understood. Manuscript received August 25, 2008. First published June 30, 2009; current version published July 15, 2009. This work was supported in part by the U.S. National Science Foundation Grant DMR-0603042 and Core, Helmholtz Center Berlin, and Spallation Neutron Source. J. Lu, K. Han, R. P. Walsh, and I. Dixon are with the National High Magnetic Field Laboratory, Tallahassee, FL 32310 USA (e-mail: junlu@magnet.fsu.edu). A. Ferreira and B. Seeber are with the Institute of Applied Physics, University of Geneva, CH-1211 Geneva 4, Switzerland (e-mail: Bernd.Seeber@physics. unige.ch). 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.2009.2018365 TABLE I STRAND PARAMETERS Since low wires are less strain sensitive, it may be possible that the under reacted wire have larger irre- versible strain. Therefore by lowering the heat treatment temper- ature, one may reach a compromise between high and high irreversible strain. In addition, an investigation on heat treatment temperature effect is relevant to applications where a tempera- ture excursion during the heat treatment may happen. Therefore from the risk management point of view, the investigation of the heat treatment temperature effect on the -strain properties is also very important. II. EXPERIMENTAL The strand under investigation is Ti doped made by restacked-rod process (RRP) by Oxford Instrument, Supercon- ducting Technology (OIST) (billet no. 10019 and 10091). The strand is similar to what is used for EDIPO [6]. The main strand parameters are shown in Table I; and its cross-sectional picture is shown in Fig. 1. The OIST recommended heat treatment schedule for achieving optimum and RRR is . With this schedule, an ITER barrel and a few Wal- ters Spring coil samples are reacted. In order to investigate the effect of the heat treatment temperature, five groups of straight samples were heat treated with the final stage temperatures of 610, 620, 630, 640, and 650 C respectively. The rate for all the temperature ramps is 50 C/h. Three experimental devices are used for -strain measure- ments. A Walters spring probe (WASP) [7] for versus axial strain measurement; a device for versus transverse strain measurement [8]; and a straight pull device for investigation of the irreversible strain versus heat treatment temperature [9]. III. RESULTS AND DISCUSSIONS A. ITER Barrel Measurements The non-Cu of the OIST-10019 wire was first measured using a Ti-6Al-4V ITER barrel at 4.2 K in fields from 8 to 18 T. 1051-8223/$25.00 © 2009 IEEE