IEEE TRANSACTIONS ONAPPLIED SUPERCONDUCTIVITY, VOL. 29, NO. 5, AUGUST 2019 4802404 Electromechanical Characterization of the MgB 2 Wire for Transmission Line Magnet System Mukesh Dhakarwal , Toru Ogitsu, Michnaka Sugano, Hideki Tanaka , and Hiroyuki Watanabe Abstract—With a high transition temperature (39 K) and as a possible cheaper alternative of Niobium-based superconductors in terms of manufacturing, operation temperature, and cost, MgB 2 seems to be a promising candidate in the accelerator and electric power industry. Looking for the feasibility and future scopes of the MgB 2 conductor, for the transmission line magnet and as a candi- date in upgrading accelerator facility at J-PARC, several critical parameters were analyzed. Here we are presenting two studies that were carried out at KEK on the MgB 2 wire developed by HITACHI Ltd. 1) Electromechanical characterization of the MgB 2 wire was performed, to determine the effects on the critical current (I c ) of MgB 2 wire when subjected to tensile stress and magnetic field. 2) For the quench protection of the MgB 2 cable to handle the large current, it needs appropriate material composition. Millions of amp squared seconds (MIITs) value was determined to estimate the cop- per to superconductor (Cu:SC) ratio, for the safer operation of the magnet system which gives the final cable composition for the trans- mission line. Index Terms—MgB 2 , critical current (I c ), tensile stress, millions of amp squared seconds (MIITs), transmission line magnet. I. INTRODUCTION I N THE FRAMEWORK of the Transmission Line Magnet System, MgB 2 seems a promising candidate for the magnet transmission line and a possible candidate for the upgrade of J-PARC main ring [3]. In the current design, a single aperture warm iron superferric magnet is to be built around 80 kA su- perconducting transmission line due to space constraint in the existing J-PARC tunnel. The transmission line is subjected to a large amount of forces during fabrication and operation, which can degrade the performance and current carrying capacity of the superconductor (SC). Electromechanical characterization is a vital parameter for the SC Transmission Line design. It is crucial to study the mechanical properties (tensile stress) of the wire at room temperature (RT), 77 K and 4.2 K to estimate the thermal expansion and maximum stress-strain limit of the Manuscript received October 30, 2018; accepted March 1, 2019. Date of publication March 7, 2019; date of current version April 5, 2019. This work was supported by JSPS KAKENHI under Grant 16H04512. (Corresponding author: Mukesh Dhakarwal.) M. Dhakarwal is with SOKENDAI (The Graduate University for Advanced Studies), Tsukuba 305-0801, Japan (e-mail:, dhakarwalmukesh@gmail.com). T. Ogitsu and M. Sugano are with the High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan. H. Tanaka is with the Research and Development Group, Hitachi Ltd., Hitachi 319-1292, Japan. H. Watanabe is with the Healthcare Business Unit, Hitachi Ltd., Hitachi 317- 8511, Japan. 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.2019.2903649 Fig. 1. OPERA 2D FEA results showing flux density and flux lines. superconductor during the fabrication and operation process, without degradation. In this study, we are considering in-situ PIT (powder-in-tube) processed MgB 2 wire. The degradation of critical current as a function of applied strain at 4.2 K was characterized using a customized test probe [1], [2]. Additionally, we had calculated the millions of amp squared seconds (MIITs) limit for the quench protection and required Cu: SC ratio to carry high current (up to 80 kA) with necessary safety margin [9]. II. MAGNET DESIGN The FEA analysis of C-shape combined function magnet with target fields (1.5 T and -3.14 T/m) was performed using Opera 2D as shown in Fig. 1 [10]. In the initial design study of the transmission line with 4 sets of cables, each carrying a current up to 20 kA with a total capacity of up to 80 kA, we had ob- served that around 20–25 kN force is acting on the transmission line. Stable performance of the transmission line with the acting forces needs a strong support structure with a necessary safety margin. The optimum distance between the support structures over the entire magnet length, requires the stress and strain limit that can be handled by the superconductor, without degrading its performance. The electromechanical behavior under varied tem- perature (RT, 77 K, and 4.2 K) is a necessary study for complete design stability. III. EXPERIMENTAL DETAILS A. Sample Wire MgB 2 Sample wire fabricated by Hitachi Ltd uses the powder-in-tube (PIT) method and in situ process, consisting of magnesium and boron powder with a purity of 99.8% and 98.5% respectively, carbon as a dopant material was added with coronene (C 24 H 12 ) [1]. The sample wire composition used in this study as listed in Table I. and cross-sectional view as shown 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.