First performance test of FeSe0.5Te0.5-coated conductor coil under high magnetic fields

Iron-based superconductors have attracted intense interest from scientists around the world. The first FeSe0.5Te0.5 hybrid coil, including one FeSe0.5Te0.5 single pancake coil (SPC) and eight YBCO double pancake coils (DPCs), was successfully fabricated and tested in this study. The FeSe0.5Te0.5 tape used in the hybrid coil is encapsulated to strengthen the tape, and has a thickness of 0.15 mm and a width of 10.5 mm to withstand the high stress during the cold test. The FeSe0.5Te0.5 encapsulated tape exhibits good performance in the 4.2 K cold test. YBCO DPCs were divided into two groups connected to both sides of the FeSe0.5Te0.5 SPC. The critical current of the FeSe0.5Te0.5 coil was measured as the background field increased from 0 T to 10 T. The transport critical current of the FeSe0.5Te0.5 SPC was 108.1 A at self-field and 17.4 A at 10 T, which had almost the same performance as the FeSe0.5Te0.5 tape. The results of this study showed that the FeSe0.5Te0.5-coated conductor coil has good performance in high magnetic fields.

Iron-based superconductors have attracted intense interest from scientists around the world. The first FeSe 0.5 Te 0.5 hybrid coil, including one FeSe 0.5 Te 0.5 single pancake coil (SPC) and eight YBCO double pancake coils (DPCs), was successfully fabricated and tested in this study. The FeSe 0.5 Te 0.5 tape used in the hybrid coil is encapsulated to strengthen the tape, and has a thickness of 0.15 mm and a width of 10.5 mm to withstand the high stress during the cold test. The FeSe 0.5 Te 0.5 encapsulated tape exhibits good performance in the 4.2 K cold test. YBCO DPCs were divided into two groups connected to both sides of the FeSe 0.5 Te 0.5 SPC. The critical current of the FeSe 0.5 Te 0.5 coil was measured as the background field increased from 0 T to 10 T. The transport critical current of the FeSe 0.5 Te 0.5 SPC was 108.1 A at self-field and 17.4 A at 10 T, which had almost the same performance as the FeSe 0.5 Te 0.5 tape. The results of this study showed that the FeSe 0.5 Te 0.5 -coated conductor coil has good performance in high magnetic fields. 9 Authors contributed equally to this work. * Authors to whom any correspondence should be addressed.
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Introduction
Since 2008, when Dr Hosono ′ s group at the Tokyo Institute of Technology discovered superconductivity at 26 K in the iron-based layered material LaFeAsO 1−x F x [1], ironbased superconductors (IBSs), which are expected to be the next-generation high temperature superconductors, have attracted intense interest from scientists around the world [2][3][4][5][6][7][8][9]. The IBS has a high upper critical field (>100 T), high critical temperature (∼55 K), low anisotropy (1.5-2) and strong current carrying capacity. In the current research, there are four main structures of IBSs: 1111 phase (for example, LnOFeAs), 122 phase (for example, Ba(Sr,K)Fe 2 As 2 , BaxK 1−x Fe 2 As 2 ), 111 phase (for example, LiFeAs) and 11 phase (for example, FeSe, FeSe x Te 1−x ) [10]. The 122 phase and the 11 phase are mainly studied among the four structures. The iron-based superconducting coils of 122 phase have been successfully fabricated and test under high fields. As it has a simple structure, low anisotropy, nontoxicity, and promising prospects in high-field applications [11], the 11 phase represented by FeSe x Te 1−x arouses great interest in researchers.
Most of the substrates to grow FeSe 0.5 Te 0.5 films are CaF 2 , MgO and SrTiO 3 , and high-quality FeSe 0.5 Te 0.5 films are typically deposited by pulsed laser deposition (PLD) [12]. Southeast University has investigated the factors affecting the transport properties of superconducting FeSe 0.5 Te 0.5 thin films deposited by PLD, such as substrate type, superconducting film thickness, and the proportions of each element [2][3][4][5][6]. In 2013, Braccini studied the superconducting properties of a Fe(Se,Te)-coated conductor, and showed that the superconducting property of a thin film grown on CaF 2 substrate with a T c of 20 K exhibited a critical current density, J c , as high as 1 × 10 6 A cm −2 at 4.2 K in self-field [13], and a J c for FeSe 0.5 Te 0.5 as high as 10 5 A cm −2 at 9 T was measured in Kawale's study [14]. In addition, Bhatt has successfully synthesized large-size FeSe single crystals doped with Fe and Co and studied their transition temperatures and upper critical fields [15]. The FeSe x Te 1−x -coated conductor shows good high-field performance in the above studies. However, Yuan pointed out that the thermal expansion coefficient of the substrate is much larger than that of the FeSe 0.7 Te 0.3 film [5]. Also, when cooling from room temperature to a low temperature of 4.2 K, the larger contraction of the substrate leads to compressive stress on FeSe 0.5 Te 0.5 films [16], which will damage the properties of the superconducting layer. When coils are fabricated using the FeSe 0.5 Te 0.5coated conductor, compressive stresses inevitably occur on the film of the FeSe 0.5 Te 0.5 -coated conductor. Therefore, there is still great difficulty and challenge to develop FeSe 0.5 Te 0.5coated conductor coil. At present, almost all research is mainly focused on improving the performance of FeSe 0.5 Te 0.5 -coated conductor tape preparation. ReBCO (ReBa 2 Cu 3 Ox), which is also a coated conductorstructure, has been successfully used in the fabrication of magnets and cables [17][18][19], but fabrication of a coil of FeSe 0.5 Te 0.5 -coated conductor and a study of its high-field performance have not been attempted.
In this paper, a hybrid coil with a FeSe 0.5 Te 0.5 -coated conductor was first successfully fabricated and then tested at liquid helium temperature in background fields from 0 T to 10 T. The FeSe 0.5 Te 0.5 tape was provided by Shanghai Jiao Tong University in China. The tape is 10 mm in width and 0.1 mm in thickness. Because the IBAD (ion beam assisted deposition)-MgO layers showed pure (001) orientation and excellent inplane texture [20], the FeSe 0.5 Te 0.5 film was deposited on IBAD-MgO layers using the PLD method. Besides, to protect the thin FeSe 0.5 Te 0.5 films, a 2 µm silver layer was deposited on top of the FeSe 0.5 Te 0.5 thin film after the PLD process. However, the binding force between the silver layer and the superconducting layer is not sufficient to fully resist the thermal strain at low temperature. The silver layer easily partially separates from the superconducting layer, damaging the superconductivity of the tape. In order to strengthen the mechanical properties of the FeSe 0.5 Te 0.5 tape, the FeSe 0.5 Te 0.5 tape was encapsulated at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP) [21]. In this study, the strengthened FeSe 0.5 Te 0.5 encapsulated tape is 10.5 mm in width and 0.15 mm in thickness. Specifications of the FeSe 0.5 Te 0.5 tape are shown in table 1.

Encapsulated FeSe 0.5 Te 0.5 tape characteristics
A short sample cut from the end of the strengthened, encapsulated tape was tested at 4.2 K using liquid helium. The critical current I c of the FeSe 0.5 Te 0.5 encapsulated tape was measured as the background magnetic field parallel to the tape was increased from 0 T to 10 T. The values of I c were measured by the standard four-probe method with a criterion of 1 µV cm −1 . Figure 1 shows the magnetic field dependence of I c at 4.2 K for the FeSe 0.5 Te 0.5 tape. Under a 10 T background field, the I c of the tape is 17 A at 4.2 K, and the corresponding transport current density J c about 1.42 × 10 5 A cm −2 . The critical current of the FeSe 0.5 Te 0.5 tape decreases quickly when adding a small background field less than 0.3 T, and then the I c of the tape decreases slowly as the background field increases, as shown in figure 1. To improve the greatly reduced I c when adding a small background field, the tape production and fabrication process should be further optimized. However, the transport current I c is less dependent on the high background field, and I c decreases by only 12 A when the background field is increased from 4 T to 10 T.

Insert coil design
The hybrid coil consists of eight YBCO DPCs and one FeSe 0.5 Te 0.5 -coated conductor SPC. The parameters of the hybrid coil are listed in table 2. Considering the aperture of the background magnet, the inner diameter of the magnet is set as 45 mm, and the outer diameter is 52.23 mm. The dimensions of the SPC are 51.04 mm in inner diameter and 52.23 mm in outer diameter. Figure 2 shows the magnetic field distribution of the hybrid coil using COMSOL finite element method software. The center field of the hybrid coil is 0.26 T with a current of 100 A. Figure 3 shows the maximum stress on the FeSe 0.5 Te 0.5 coil and the YBCO coils with the critical current of FeSe 0.5 Te 0.5 tapes with 0-10 T background fields. The coil stress under each background field is less than 1 MPa which is within the allowable pressure range of the wire. Therefore, it  is not necessary to apply additional preload on the surface of the hybrid coil.

Insert coil fabrication
The FeSe 0.5 Te 0.5 encapsulated tape was used to fabricate the insert hybrid FeSe 0.5 Te 0.5 coil. The FeSe 0.5 Te 0.5 encapsulated tape was wound on a stainless-steel frame. Eight YBCO DPCs were divided into two groups connected to both sides of the FeSe 0.5 Te 0.5 SPC; a small piece of YBCO tape was used to connect the YBCO DPC and the FeSe 0.5 Te 0.5 SPC. The voltage signal lines were placed at the 1st turn and the 3.5th turn. The voltage signal line distance of the FeSe 0.5 Te 0.5 coil was set to 560 mm. Figure 4 shows the fabrication process of the hybrid FeSe 0.5 Te 0.5 coil. Figure 4(a) shows the FeSe 0.5 Te 0.5 encapsulated tape and Figure 4(b) the outer view of the FeSe 0.5 Te 0.5 SPC. Figure 4(c) shows the outer view of the hybrid FeSe 0.5 Te 0.5 insert coil.

Cold test
The hybrid coil was tested at a liquid helium temperature of 4.2 K. The hybrid coil was inserted into a uniform field magnet with a 70 mm aperture and the field was increased from 0 to 10 T. The uniform field was perpendicular to the hybrid FeSe 0.5 Te 0.5 coil. A Hall-effect Gauss meter was placed in the center of the FeSe 0.5 Te 0.5 SPC to measure the magnetic field. Figure 4(d) is the part of the cold test device. The transport critical current I c of the FeSe 0.5 Te 0.5 coil at 4.2 K was also measured by the standard four-probe method with a criterion of 1 µV cm −1 . As the voltage signal line distance of the FeSe 0.5 Te 0.5 coil was 560 mm in this study, the quench criterion of the coil was 56 µV. The voltages of the FeSe 0.5 Te 0.5 coil during the cold test with 0-10 T background fields are shown in figure 5. At selffield, the value of I c reached 108.1 A and the n value was 24.
Under the 10 T field, the I c reached 17.4 A and the n value was 17. Figure 6 shows the cold test result of the hybrid coil. The critical current values are slightly higher than the short FeSe 0.5 Te 0.5 encapsulated tape. The most likely reason is that it is not uniform at the end of the FeSe 0.5 Te 0.5 tape. The critical current at the tape end will be slightly lower than that of in the entire tape. A Hall-effect Gauss meter in the center of the FeSe 0.5 Te 0.5 SPC measured the magnetic field intensity. After the cold test, the hybrid FeSe 0.5 Te 0.5 coil was cooled to room temperature and placed the coil for 48 h. Then the cold test of the hybrid coil was repeated. The second coil test result is also shown in figure 6. The transport critical current I c of the FeSe 0.5 Te 0.5 SPC was 106.8 A at self-field and 17.4 A at 10 T field, which was almost the same result as the first test. The test result proves that there is no decrease of transport I c when the inner diameter of the FeSe 0.5 Te 0.5 coil is 51 mm. Besides, the performance of the FeSe 0.5 Te 0.5 hybrid coil is stable and  repeatable in the two cold tests. This study also proves that the use of a copper protective layer can effectively withstand the high stress on the FeSe 0.5 Te 0.5 coil during the cold test, avoiding damage to the superconducting layer due to the larger contraction of the substrate.

Conclusion
In this study, we successfully fabricated and tested a FeSe 0.5 Te 0.5 hybrid coil including one FeSe 0.5 Te 0.5 SPC coil with a 51 mm inner diameter and eight DPC YBCO coils with a 45 mm inner diameter. The bare FeSe 0.5 Te 0.5 tape was soldered into encapsulated tape, 10.5 mm in width and 0.15 mm in thickness, to withstand the high stress during the cold test of the hybrid coil in this study. The critical current of the FeSe 0.5 Te 0.5 coil was measured at self-field and as the background field increased from 0 to 10 T. The transport I c value is 17.4 A at 4.2 K with the 10 T field, and the corresponding transport J c was about 1.42 × 10 5 A cm −2 , which was almost the same performance as the short FeSe 0.5 Te 0.5 tape. The results of this study showed that the FeSe 0.5 Te 0.5 -coated conductor coil can be an option for developing strong magnets. It has good performance in high-field magnet applications.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).