The Influence of Wire Bending and Wire Diameter on Transport Critical Current Density in Small MgB 2 Superconducting Coi
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ORIGINAL PAPER
The Influence of Wire Bending and Wire Diameter on Transport Critical Current Density in Small MgB2 Superconducting Coils for Applications in Multi-Section Coils D. Gajda 1 & A. J. Zaleski 1 & A. Morawski 2 & T. Cetner 2 & M. Rindfleisch 3 Received: 15 June 2020 / Accepted: 21 July 2020 # The Author(s) 2020
Abstract This article presents the impact of MgB2 wire bending and diameter on transport critical current density and irreversible magnetic field of a resultant coil. Unreacted MgB2 wires 500 mm in length and 0.63 or 0.83 mm in diameter have been used in the fabrication of small diameter (14 mm) superconducting coils. The coils were subsequently annealed under isostatic pressure of 1 GPa for 15 min at 700 °C and 725 °C. Our results indicate that larger wire diameter, higher annealing temperature, and bending lead to slight reduction of critical current density and irreversible magnetic field in the coil. Keywords Coils . Critical current density . MgB2 wires
1 Introduction The first research on superconducting coils was conducted by Heike Kamerlingh Onnes [1, 2]. These studies indicated that the winding of superconducting wires reduced the critical current density [1]. Degradation of performance was due to defects introduced to the structure of the superconducting material during the winding process, e.g., damage in the connections between the grains. A few of these defects can be removed by annealing the unreacted MgB2 wire post winding [3–6]. Our previous research showed that annealing under high isostatic pressure of coils made with 10% SiC-doped MgB2 wire with large bending diameters leads to similar Jc as that in a straight wire [7]. The small difference in Jc is due to two factors. Firstly, thermal treatment under high isostatic pressure improves the uniformity and density of the MgB2
* D. Gajda [email protected] 1
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 Wrocław, Poland
2
Institute of High Pressure Physics, Polish Academy of Sciences, Sokołowska 29/37, 01-142 Warszawa, Poland
3
Hyper Tech Research, Inc., 539 Industrial Mile Rd, Columbus, OH 43228, USA
material, leading to an increase in the number of connections between grains and the density of pinning centers [8, 9]. The second factor is the large amount of admixture [7], which also improves the homogeneity and density of the MgB2 material [7]. Moreover, the large amount of admixture effectively increases Jc at low temperatures (4.2 K) [10]. Uchiyama et al. [11], Wang et al. [12], and Gajda et al. [13] showed that wire diameter reduction as the result of cold drawing and constant annealing temperature increases critical current density, irreversible magnetic field, pinning force, and n value and does not decrease the critical temperature and dominant pinning mechanism. However, Xu et al. indicates that a significant reduction of wire diameter causes a decrease of Jc [14]. A multi-section coil comprises of several coils, namely, a main coil and compensating coils. The main
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