# Fileset [ASC2024-KIMver0.pdf](https://mdr.nims.go.jp/filesets/0de55653-f4ba-4865-a122-15d7c4b8d0e7/download) ## Creator [SeokBeom Kim](https://orcid.org/0000-0002-2946-5763), Tomoya Sakamoto, Riku Onoue, Satoshi Nishikori, [Ryota Inoue](https://orcid.org/0000-0001-5486-2288), [Hiroshi Ueda](https://orcid.org/0000-0003-4976-8638), [Akihiro Kikuchi](https://orcid.org/0000-0002-5044-7156), [Yasuo Iijima](https://orcid.org/0000-0002-9008-9429) ## Rights © 2025 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/) ## Other metadata [Critical Current and AC Loss Characteristics of Ultrafine Nb3Al and Nb3Sn Superconducting Wires Under Conduction Cooling](https://mdr.nims.go.jp/datasets/bae1497d-06a9-4d73-8bcd-802cc613b849) ## Fulltext Microsoft Word - 2024ASC Ver 01 ASC2024-1MPo1C-06 Critical Current and AC Loss Characteristics of Ultrafine Nb3Al and Nb3Sn Superconducting Wires Under Conduction Cooling SeokBeom Kim, Tomoya Sakamoto, Riku Onoue, Satoshi Nishikori, Ryota Inoue, Hiroshi Ueda, Akihiro Kikuchi, and Yasuo Iijima  Abstract— Wind and React (W&R) technology is required to produce superconducting magnets using A15 superconducting wire. Recently, the Jelly-roll processed Nb3Al and the bronze processed Nb3Sn ultrafine superconducting wires with a wire diameter of less than 50 m were fabricated by the National Institute for Materials Science (NIMS) in Japan. The ultrafine Nb3Al and Nb3Sn superconducting wires will enable the fabrication of superconducting magnets using the React and Wind (R&W) method. In order to apply Nb3Al and Nb3Sn ultrafine wires to DC and AC applications, it is necessary to clarify the temperature dependence of the critical properties and AC loss characteristics of these wires. In this study, the critical properties and AC transport loss characteristics of the developed Nb3Al and Nb3Sn ultrafine wires were experimentally determined using a cryocooler system. The critical temperature (Tc) and critical current density (Jc) of Nb3Sn wire were higher than those of Nb3Al wire, but the Nb3Al wire showed a higher engineering critical current density (Je) than the Nb3Sn wire. The magnitude of AC transport current loss of the Nb3Al wire was smaller than that of the Nb3Sn wire with the same wire diameter. Index Terms—ultrafine Nb3Al & Nb3Sn wires, critical properties, AC transport current losses, cryocooler system. I. INTRODUCTION 15 superconducting wires such as Nb3Sn and Nb3Al are suitable for high magnetic field magnets, but their strain sensitivity requires the Wind & React (W&R) method. Therefore, there is a strong demand for the development of A15 superconducting wire that enables the fabrication of superconducting magnets by the React & Wind (R&W) method. The bending strain of A15 superconducting wire is expected to be minimized by reducing the wire diameter, and National Institute for Materials Science (NIMS) has been developing ultrafine A15 superconducting wire. Recently, the flexible ultrafine Nb3Al mono-core (monofilament or mono-block) wires with a wire diameter of less than 50 m were fabricated by the Jelly-roll process [1], [2]. In the Jelly-roll process, Ta, Nb, and Cu were tested as core materials to improve flexibility and drawability, and the amount relationship between the Cu core and the outermost layer of Cu Submitted for review September 25, 2024 S.B. Kim, T. Sakamoto, R. Onoue, S. Nishikori, R. Inoue, H. Ueda are with the Graduate School of Environmental, Life, Natural Science and Technology, Okayama University, Okayama 700-8530, Japan. (email: kim@ec.okayama-u.ac.jp) stabilizer was investigated to increase the critical current density. The results of these studies, especially the critical current characteristics, have already been reported [3]. The bronze processed multifilamentary Nb3Sn ultrafine superconducting wires were fabricated [4], and their Jc-B properties in liquid helium (bundle cable) and mechanical properties at room temperature (single wire) are measured and reported [5], [6]. The developed flexible Nb3Al and Nb3Sn wires are scheduled for application to DC (miniature high field coils) and AC (wireless power transmission) coils with cryogen-free conduction cooling, and the R&W method is expected to be applicable to these coil fabrications. To achieve this research goal, it is necessary to evaluate the temperature dependence of the critical and AC loss properties of the developed ultrafine wires. In this study, the both properties of Nb3Sn and Nb3Al single wires with different wire diameters were experimentally evaluated using a conduction cooling system. II. NB3AL AND NB3SN ULTRAFINE WIRES Fig. 1 shows the cross-sectional view of Nb3Al and Nb3Sn wires with wire diameter of 50 m, and specifications of both wires are listed in Table I. Two samples of Nb3Al wire were prepared with wire diameters of 33 m and 50 m, and the area ratio of the Cu core to the outermost Cu layer is 1:1. The Nb3Sn wires have diameters of 30 m and 50 m and consist of 19 filaments. The diameter of the filament in the Nb3Sn wire with A. Kikuchi and Y. Iijima are with National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan. A Fig. 1. Cross-sectional view of the (a) Jelly-roll processed mono-block Nb3Al wire and (b) Bronze processed the 19-filaments Nb3Sn wire. 2 ASC2024-1MPo1C-06 a wire diameter of 50 m is 3 m. The superconducting region of the Nb3Al wire has a cylindrical structure, while the Nb3Sn wire has a filament structure. The area of the superconducting region of the Nb3Al wire is larger than that of Nb3Sn wire, and the proportion of the superconducting region of the Nb3Al wire is 33% (Nb3Sn wire is 6.8%). Fig. 2 shows the critical temperatures (Tc) of both wires measured by the DC four probe method with 1 mA in the cryocooler system. The Tc of the Nb3Al wire is approximately 15.5 K and ΔT is 0.5 K, and Nb3Sn wire is approximately 17 K and ΔT is 0.4 K. The Tc of the Nb3Sn wire shows almost the same value measured by the magnetic moment method with 1 mT [5]. III. CRITICAL CURRENT PROPERTIES The sample wire placed on the sample stage in the cryocooler system is adiabatic state except for the current leads. Therefore, wire breakage can easily occur when measuring the critical current of ultrafine wires. In a previous study, a sample wire was soldered to a copper wire to prevent wire breakage during critical current measurement [3]. However, in this study, since the critical current and AC transport loss of ultrafine wires are measured at the same time, a copper wire cannot be attached. So, in this study, the critical current of ultrafine wires was measured by carefully combining the pulse current and current sweep methods. Although we measured while being careful, many of the sample wires broke, especially those with diameters of 33 m or less. Since it is very difficult to directly measure the temperature of the ultrafine wires, the temperatures TABLE I SPECIFICATIONS OF NB3AL AND NB3SN WIRES Nb3Al wire Nb3Sn wireProcess Jelly-roll method Bronze methodWire diameter (m) 33 50 30 50Cu/Non-Cu ratio 1.0 0.89Barrier material Nb NbSuperconducting area 2.8 6.5 0.48 1.34(10-10 m2)Superconducting portion (%) 33 6.8 Fig. 2. Measured the critical temperature of Nb3Al and Nb3Sn wires by 4-probe method with 1 mA in cryocooler system. -2024681014.5 15 15.5 16 16.5 17 17.5Voltage (µV)Temperature (K)15.3K15.8K16.8K17.2KNb3AlNb3Sn Fig. 3. Temperature dependence of the measured critical current of (a) Nb3Al and (b) Nb3Sn wires in self-magnetic field. 00.511.522.533.544.5512 13 14 15 16Critical current (A)Temperature (K)A (50φ)B (33φ)Nb3Al, 50φNb3Al, 33φ(a)00.511.522.533.544.5512 13 14 15 16 17Critical current (A)Temperature (K)C (50φ)D (30φ)Nb3Sn, 50φNb3Sn, 30φ(b) Fig. 4. (a) Critical current density of Nb3Al and Nb3Sn wires calculated by area of superconducting region and (b) engineering current density calculated by total cross-sectional area of wire. 0246810121412 13 14 15 16 17Critical current density(×109A/m2 )Temperature (K)A (Nb3Al, 50φ)B (Nb3Al, 33φ)C (Nb3Sn, 50φ)D (Nb3Sn, 30φ)Nb3Al, 50φNb3Sn, 50φNb3Sn, 30φNb3Al, 33φ(a)00.511.522.512 13 14 15 16 17Engineering current density(×109A/m2 )Temperature (K)A (Nb3Al, 50φ)B (Nb3Al, 33φ)C (Nb3Sn, 50φ)D (Nb3Sn, 30φ)Nb3Al, 50φNb3Sn, 50φNb3Sn, 30φNb3Al, 33φ(b)3 ASC2024-1MPo1C-06 of the aluminum nitride sample stage and the copper current leads were used as the reference temperatures. Details of the measurement system using a conduction cooling system are described in previous studies [3], [7]. Fig. 3 shows the measured the critical current (Ic) of four sample wires, and critical current density (Jc) and engineering current density (Je) are shown in Fig. 4. The Jc of the sample wires are calculated by dividing the Ic value by the area of the superconducting portion, so it is different from the Non-Cu Jc. The value of Je is the Ic value divided by the total cross-sectional area of the wire. The Jc of the Nb3Sn wire at 12 K is almost the same with the value of the REBCO wire at 77 K (31010 A/m2). It is clear that the Ic of Nb3Al wire is higher than that of Nb3Sn wire, but the Jc of Nb3Sn wire is higher than that of Nb3Al wire. From the measured Tc and Jc values, we can say that the superconducting properties of the Nb3Sn wire are better than Nb3Al wire, but Nb3Al wire has an advantage in terms of Je. The temperature dependence of the n-values of the sample wires were obtained from the I-V curves during Ic measurement by current sweep and pulse current methods. The n-values of the four sample wires from 8 K to 15 K are 2 to 8. These values are very small and affect the AC transport current loss characteristics. Incidentally, the n-values of 19-strands Nb3Sn wire in liquid helium are 13 to 80 (@1-18T) [5]. IV. AC TRANSPORT CURRENT LOSS CHARACTERISTICS Several institutions have tried to measure the AC transport current losses of the developed Nb3Al and Nb3Sn ultrafine wires in a conduction-cooled system. However, no one has measured it yet, because heat generation in the wire under adiabatic conditions is the cause. After measuring Ic of the sample wires by the 4-probe method, the AC transport current losses were measured. The AC transport current losses in the ultrafine sample wires were measured by the standard technique (electrical method) with a lock-in amplifier [8]. The output AC current from the power supply and reference signal for the lock-in amplifier were controlled and applied by multifunction generator. The phase of the reference was adjusted using a Rogowski coil. Figs. 5 and 6 show the frequency dependence of AC transport current losses of Nb3Al and Nb3Sn wires as a function of amplitude of AC current. In Fig. 5, the AC transport current losses slightly increase with the frequency at large current values, and this might be due to the temperature increasing in the wire. However, the measured frequency and current dependence of the AC losses indicates hysteresis loss due to the superconductivity of the sample wires. Although the AC transport current losses were small because the magnitude of the applied current was small, it is clear that the AC losses could be measured under conductive cooling from the results in Figs. 5 and 6. Figs. 7 and 8 show the dependence of AC transport current losses on wire diameter in Nb3Al and Nb3Sn wires at 400 Hz. The peak value of applied AC current (Ip) is normalized with the Ic of the wire at each temperature. The AC losses were divided by the square of Ic to allow comparisons between wires with different critical currents. In Figs. 7 and 8, the dash-dotted line represent theoretical value in the round wire based on the Bean model [9], [10]. In the region of low values of Ip/Ic, the experimental values are larger than the theoretical values, and in the region of high values of Ip/Ic, the experimental values are smaller than the theoretical values. Such results are rarely observed, the discrepancy between the measured values and the Fig. 5. Frequency dependence of AC transport current losses of Nb3Al wire with wire diameter of 50 m as a function of amplitude of AC current at 7.3 K. 1.E-111.E-101.E-091.E-081.E-071.E-06100 1000AC Loss (J/m/cycle)Frequency (Hz)100mA 150mA 200mA 300mA 400mA600mA 1A 1.5A 2A 3A4A 5A 5.5A 6A 7A Fig. 6. Frequency dependence of AC transport current losses of Nb3Sn wire with wire diameter of 30 m as a function of amplitude of AC current at 7 K. 1.E-101.E-091.E-081.E-0740 400AC Loss (J/m/cycle)Frequency (Hz)100mA 200mA 300mA 400mA 500mA 600mA Fig. 7. Dependence of AC transport current losses on wire diameter in Nb3Al wire at 400 Hz. The dash-dotted line represent theoretical value in the round wire based on the Bean model. Nb3Al wirey = 2E-08x2.1131y = 2E-08x2.12361.E-141.E-121.E-101.E-081.E-060.001 0.01 0.1 1AC Loss (J/m/cycle/A2 )Ip/Ic33φ,6.8K50φ,7.3KNorris (elliipse)4 ASC2024-1MPo1C-06 theoretical values based on the Bean model might be due to the low n value of the sample wire. It may be unreasonable to apply the Bean model to superconductors with small n value. However, the dependence of AC transport current losses on wire diameter was not observed. Figs 9 and 10 show the compare the AC losses of Nb3Al and Nb3Sn wires at approximately the same wire diameter and approximately the same temperature. The AC transport current losses of Nb3Sn wires with filamentary structure are larger than that of Nb3Al wires with cylindrical structure (mono-block) when the wire diameter, Ip and Ip/Ic are the same. The filaments in the Nb3Sn wire are arranged along concentric circles, and the inductance of the outer filaments is lower than that of the inner filaments. Therefore, it is expected that the current is biased to the outermost filaments, the Ip/Ic value of the outermost layer increases, and the AC transport loss increases. V. CONCLUSION Even though many sample wires were broken during the experiment due to mechanical and thermal reasons, the Ic and AC loss properties of the developed ultrafine Nb3Al and Nb3Sn wires were successfully measured under conduction cooling. The superconducting properties (Tc and Jc) of Nb3Sn wire are better than that of Nb3Al wire, but Nb3Al wire has an advantage in terms of engineering current density (Je). The magnitude of AC transport current loss of the mono-cored Nb3Al wire is smaller than that of the 19-filaments Nb3Sn wire. REFERENCES [1] A. Kikuchi, et. al., “Trial manufacturing of Jelly-Rolled Nb/Al monofilamentary wire with very small diameter below 50 microns,” IOP Conf. Ser., Mater. Sci. Eng. 756 art no. 012016, 2020. [2] A. Kikuchi, et. al., “Ultra-fine Nb3Al mono-core wires and cables,” IEEE Trans. Appl. Supercond., vol. 31, no. 5, 6000105, Aug. 2021. [3] H. Fukuda, et. al., “Experimental study on the critical current properties of flexible Nb3Al superconducting wires in cryocooler system,” IEEE Trans. Appl. Supercond., vol. 33, no. 5, 6000704, Aug. 2023. [4] A. Kikuchi, et. al., “The bronze processed Nb3Sn ultra-thin superconducting wires,” IEEE Trans. Appl. Supercond., vol. 32, no. 4, 6000104, Jun. 2022. [5] X. Wang, et. al., “Superconducting properties of a Nb3Sn bundle cable made of ultra-fine wires,” IEEE Trans. Appl. Supercond., vol. 33, no. 5, 6000105, Aug. 2023. [6] M. Sugano, et. al., “Mechanical properties of ultra-thin Nb3Sn composite wires,” IEEE Trans. Appl. Supercond., vol. 33, no. 5, 8400905, Jun. 2023. [7] S.B. Kim et al., “Critical characteristics of ultrafine Nb3Al superconducting wires under conduction cooling conditions.” IEEE Trans. Appl. Supercond., vol. 32, no. 6, 6001005, 2022. [8] K. Kajikawa, et. al., “Dependence of transport-current losses in MgB2 superconducting wire on temperature and frequency,” IEEE Trans. Appl. Supercond., vol. 20, no. 3, pp.2111-2114, 2010. [9] C. P. Bean, “Magnetization of hard superconductors,” Phys. Rev. Lett., vol. 8, no.6, pp.250-253, 1962. [10] H. London, “Alternating current losses in superconductors of the second kind,” Phys. Lett., vol. 6, no. 2, pp.162-165, 1963. Fig. 8. Dependence of AC transport current losses on wire diameter in Nb3Sn wire at 400 Hz. The dash-dotted linerepresent theoretical value in the round wire based on the Bean model. Nb3Sn wirey = 5E-08x2.1534y = 5E-08x2.21471.E-121.E-101.E-081.E-060.01 0.1 1AC Loss (J/m/cycle/A2 )Ip/Ic30φ,7K50φ,7KNorris (elliipse) Fig. 9. Comparison of AC transport current losses between Nb3Al and Nb3Sn wires with the same wire diameter (30 and 33 m). y = 2E-08x2.1131y = 4E-08x2.15341.E-111.E-091.E-070.1 1AC Loss (J/m/cycle)Peak transport current, Ip (A)Nb3Al,33φ,6.8KNb3Sn,30φ,7KNb3Al & Nb3Sn wiresNb3Sn, 30φ,7KNb3Al, 33φ,6.8K Fig. 10. Comparison of AC transport current losses between Nb3Al and Nb3Sn wires with the same wire diameter (50 m). y = 1E-08x2.1236y = 3E-08x2.21471.E-111.E-091.E-070.1 1AC Loss (J/m/cycle)Peak transport current, Ip (A)Nb3Al,50φ,7.3KNb3Sn,50φ,7KNb3Al & Nb3Sn wiresNb3Al, 50φ,7.3KNb3Sn, 50φ,7K