1051-8223 (c) 2018 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. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TASC.2018.2829778, IEEE Transactions on Applied Superconductivity IEEE TRANSACTION ON APPLIED SUPERCONDUCTIVITY TAS-2017-0125 1 Abstract— We fabricated four superconducting racetrack coils wound by bare in-situ MgB2/Fe mono and multi-filamentary wires produced in our laboratory by using the wind and react method. Transport measurements in self-field were performed in a liquid helium dewar. The magnetic field flux density B = 25 mT for I = 92 A was measured to verify how the current flowed inside the coil for one of the coils by means of a cryogenic axial Hall sensor placed into central bore region of the coil. The coil with 36 turns wound by mono MgB2/Fe wire of l = 24 m has a highest current carrying capacity of more than 150 A at T = 4.2 K and self-field among the coils. The coils fabricated by multi-filamentary wires have relatively low engineering critical current values of Ice = 67 A for 18+1 and Ice = 57 A for 4+5 superconducting/copper filament wires at T = 4.2 K in self-field. Our results form essential contribution towards determination of possible use of low cost MgB2/Fe wires in low field superconductor coil applications. Index Terms— MgB2 wires, powder-in-tube, racetrack coil, wind&react. I. INTRODUCTION HE main advantages of superconducting MgB2 materials are their simple structure, light weight, high critical temperature (which allows cryogen-free cooling), low cost, and high availability of their constituents. MgB2 based superconductors can be effectively exploited at nuclear and fusion power plants as current leads, for creation of magnetic fields in tokamaks, in flywheel energy storages as magnetic bearings, and MRI magnets [1, 2]. However, the weak intergrain connectivity, porosity, and low MgB2 core density are the main factors which affect Jc performance of MgB2 samples [3-6]. The heat treatment of in-situ MgB2 wires creates a lot of voids which reduce active superconducting cross- sectional area for the current transport, i.e. percolation problem [5, 7-10]. Such a detrimental effect on the critical current density due to the porosity is also enhanced by the formation of MgO or other oxide layers deposited at the grain boundaries. These insulating oxide layers act as an effective insulating This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) (Grant number: 113F080). It was also partially supported by the International Laboratory of High Magnetic Fields and Low Temperatures, and partly by the Ministry of Development of the Republic of Turkey under the Grant Number 2010K120520. We thank to Başoğlu Kablo Inc. for their valuable support in providing braiding machine. Fırat Karaboğa is with the Abant Izzet Baysal University, Mehmet Tanrikulu School of Health Services, Bolu, Turkey (e-mail: karabogafirat@gmail.com) Hakan Yetiş is with the Abant Izzet Baysal University, Physics Department, 14030, Bolu, Turkey (e-mail: yetis_h@ibu.edu.tr) barrier that confines electric current paths and prevents current flow between the superconducting grains. [3, 5, 6, 11-13]. The feasible usage of MgB2-based superconductors in most applications necessitates the achievement of high current carrying capacity. Several methods have been reported to obtain a high critical current for the MgB2 wires [14-19]. Currently, a progressive enhancement on the critical current density at 4.2 K and 20 K has been reached for short MgB2 wire samples [20- 26]. The realization of high performance superconducting MgB2 wires at long scales [27-30] will also lead to a substantial reduction in electric power losses at high power currents, increasing efficiency, larger output power, reduced size and lower weight in energy related systems. Following its discovery, development of MgB2 superconducting wires have progressed very quickly, allowing many coils to be produced [31], some in the form of racetrack coils [32-34]. Dou et al. made solenoids using in-situ prepared MgB2/Cu single core wire with 1 mm outer diameter. The prepared wire has a total length of 3 m and reached transport critical value of 72 A (Jc = 1.3 ×10 5 A/cm 2 ) at T = 4 K in self- field [35]. Bhatia et al. measured 6 × 10 4 A/cm 2 for Cu/Cu sheath strands and 2.2 × 10 5 A/cm 2 for Fe/monel sheath strands in coils under 1 T at 4.2 K [36]. A small solenoidal coil made by using ex-situ 10 m long MgB2/Ni tape reached a critical current value of 105 A [37]. In 2004, a squat solenoid coil wound with in-situ MgB2/Fe monocore wire and the critical current was 121 A at 15 K and 1 T, and 114 A at 4.2 K and 2 T [38]. Serquis et. al. produced a roll coil using stainless steel sheathed ex-situ MgB2 strands having an outer diameter of 1.00 mm and a length of 25 m. The critical current value of 350 A for a HIPed coil was achieved under an applied field of 1.25 T at 4 K [39]. Musenich et. al reported that a pancake coil wounded by ex-situ MgB2/Ni tape reached a maximum current of 347 A and generated a field of 0.75 T at the conductor [40]. In 2005, Hyper Tech Research measured a critical current value of 120 A on racetrack type coils produced by using 42 m MgB2/Cu wire [41]. Sumption and colleagues reported in 2006 that they produced five racetrack type coils, three of them were Mustafa Akdoğan is with the Abant Izzet Baysal University, Physics Department, 14030, Bolu, Turkey (e-mail: akdogan_m@ibu.edu.tr) Daniel Gajda is with the International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka 95, Wroclaw, 53-421, Poland (e-mail: dangajda@op.pl) İbrahim Belenli is with the Abant Izzet Baysal University, Physics Department, 14030, Bolu, Turkey (e-mail: belenli_i@ibu.edu.tr) Design, Fabrication, and Testing of MgB 2 /Fe Racetrack Coil Firat Karaboga * , Hakan Yetis, Mustafa Akdogan, Daniel Gajda and Ibrahim Belenli T