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[IEEE-TAS_4PoA04_DEMO-Nb3Sn_v7_Final-version.pdf](https://mdr.nims.go.jp/filesets/d803a271-2087-4c03-bad8-9bbbddb7f1a0/download)

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[Nobuya Banno](https://orcid.org/0000-0002-7141-541X), Toshihisa Asano, [Tsuyoshi Yagai](https://orcid.org/0000-0003-1842-7881), [Shinya Kawashima](https://orcid.org/0000-0001-5282-4720), [Masahiro Sugimoto](https://orcid.org/0000-0002-7752-4617), [Satoshi Awaji](https://orcid.org/0000-0003-2043-1628), [Hiroyasu Utoh](https://orcid.org/0000-0002-0936-210X), [Yoshiteru Sakamoto](https://orcid.org/0000-0002-5396-9654)

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[Characterization of Japan's DEMO Candidate Reinforced Nb<sub>3</sub>Sn Wires Under Crossover Contact Stress](https://mdr.nims.go.jp/datasets/5ce6ca26-2d0c-4430-8ab8-b5c94df5fda2)

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1 4PoA04   IEEE Trans. Appl. Supercond. Characterization of Japan’s DEMO Candidate Reinforced Nb3Sn Wires under Crossover Contact Stress  Nobuya Banno, Toshihisa Asano, Tsuyoshi Yagai, Shinya Kawashima, Masahiro Sugimoto, Satoshi Awaji,  Hiroyasu Utoh, and Yoshiteru Sakamoto    Abstract— The size of Japan’s demonstration power plant (JA DEMO) reactor is planned to be approximately 1.4 times larger than that of ITER magnet. Mechanical behavior and the reinforcement of the Nb3Sn strands under a complex distribution of electromagnetic force in the cable-in-conduit conductor (CICC) are of great concern. In this study, an experimental setup for Ic measurement under crossover contact stress in external magnetic fields, which is regarded as more realistic situation in CICC, was constructed. Then, we studied superconducting properties and microstructures of JA DEMO candidate reinforced Nb3Sn wires under crossover contact stress. CuNb-reinforced bronze-route Nb3Sn wires made by Furukawa Electric and distributed-tin (DT) Nb3Sn wires reinforced with brass matrix made by Kobe Steel were tested. The reinforced wires have shown better Ic properties against contact stress, compared with conventional Cu matrix wires. Under contact stress, the outer copper sheath was predominantly deformed, which is believed to contribute to relaxation of the stress concentration. The brass matrix seems to be significantly effective in suppressing distortion in the filament region. The use of reinforced Nb3Sn wires increases the reliability of the TF magnet operation.  Index Terms— Crossover contact stress, DEMO, Nb3Sn, Reinforcement  I. INTRODUCTION HE Joint Special Design Team for Fusion DEMO [1] is conducting the conceptual designing of Japan’s fusion demonstration (JA DEMO) reactor, whose size  Submitted for review September 19, 2023 This work was supported by QST Research Collaboration for Fusion DEMO (04K067). (Corresponding author: Nobuya Banno.) N. Banno is with the Research Center for Energy and Environmental Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan (e-mail: banno.nobuya@nims.go.jp).  T. Asano is with National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan. T. Yagai is with Sophia University, Tokyo 102-8554, Japan. Shinya Kawashima is with Kobe Steel Ltd., Kobe 651-2271, Japan.  M. Sugimoto is with Furukawa Electric Co., Ltd., Tochigi 321-1493, Japan. S. Awaji is with the High Field Laboratory for Superconducting Materials, Institute for Material Research, Tohoku University, Sendai 980-8577, Japan. H. Utoh, and Y. Sakamoto are with the Rokkasho Fusion Institute, National Institutes for Quantum Science and Technology, Rokkasho, Aomori 039-3212, Japan. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier will be inserted here upon acceptance.  is planned to be approximately 1.4 times larger than the ITER magnet [2], [3]. Increase in the toroidal magnetic field (TF) and the operation current forces the conductor in the TF coil to undergo approximately 1.5 times higher electromagnetic force than that of the ITER TF [4], [5]. Short twist pitch designs for CICC are expected to suppress severe damage in the brittle Nb3Sn strands due to the buckling of the strands [6]–[13]. Therefore, the JA DEMO design team is currently considering the adoption of a short twist pitch design for every cabling stage of the CICC. However, the huge transverse electromagnetic force in TF coil still remains even in the short twist pitch design. The hoop stress by the electromagnetic force releases slightly the axial compressive stress of the cable, that is caused by a thermal contraction of the stainless-steel conduit, while the majority of the electromagnetic force is expected to be applied to the cable in the conduit in the radial direction of the TF coil. Using the designed cable diameter of 50.4 mm for DEMO TF CICC, the average electromagnetic force is calculated to be approximately 23 MPa. This value itself is not so high. However, considering the fact that twisted strands make point contact with each other, very high stresses are concerned to be locally applied to the strands. Furthermore, there are also manufacturing problems with short-pitch cabling such as strand buckling and breakage [14]. This may demand a longer twist pitch in the cable, resulting in increase in the effective strain of the cable in operations. Hence, strengthening the Nb3Sn strands are highly required to ensure the high performance of CICC. Against this background, we have previously investigated the microstructure and superconducting properties under bending strain for JA DEMO candidate reinforced Nb3Sn strands [15]: the distributed-tin (DT) strand reinforced with a brass matrix (made by Kobe Steel, Ltd.) [16] and the CuNb reinforced bronze-route Nb3Sn strand (made by Furukawa Electric Co., Ltd.) [17], [18]. The previous results obtained through scanning electron microscopy (SEM)-electron backscatter diffraction (EBSD) analysis and cryogenic current test have revealed that the reinforcement for the matrix and outer sheath is effective for improving stress tolerance against bending. In this study, the superconducting properties and microstructure of the JA DEMO candidate reinforced Nb3Sn wires were evaluated under crossover contact stress as more realistic situation in CICC. An experimental setup for critical T 2 4PoA04   IEEE Trans. Appl. Supercond. current (Ic) measurement was newly developed. The SEM-EBSD strain analysis for residual strain after loading indicated that the outer Cu sheath is predominantly deformed, resulting in relaxation of the stress concentration to the filamentary region. The reinforced strands exhibited better Ic characteristics under contact stress than that of the normal Cu matrix wire. Notably, stress sensitivity of the brass-matrix DT wires was substantially small, compared with conventional Cu-matrix DT wires. II. EXPERIMENTAL A. Setup for Ic measurement Fig. 1 illustrates schematically crossover contact between strands in CICC under the electromagnetic force, and the developed setup for Ic measurement simulating crossover contact stress state. During the compaction of the conduit with the twisted cable inside, the strands are optimally rearranged, and some strands should ride up on top of others.  Although many measurement systems under transverse load have been demonstrated [19]–[25], the Ic measurement system under crossover contact stress was chosen here because it is believed to be more realistic situation for strands subjected to a transverse electromagnetic force in the CICC. The design and configuration of the setup was determined through lots of trial and error. The wire sample with a length of 37 mm is soldered at both ends to the electrodes over a distance of 11 mm. The voltage tap distance was 10 mm. Then, the dummy same wire is placed on the top of the sample orthogonally to the sample. The electrodes are screwed to the current leads. Great care was taken in mounting the sample on the probe not to damage the sample. A transverse load is applied by a stepping motor and the load value is measured by a load cell that is available in a 50 kg capacity. The fixed anvil is made of stainless-steel, which is placed on the GFRP table. The load is transmitted to the wire through the GFRP anvil with a diameter of 5 mm. A displacement gauge is set to measure the total displacement including the sample deformation, an elongation of the center rod etc. The contact stress applied to the wire is calculated by dividing the load value by the projected area where the sample and dummy wire overlap (that is calculated by the square of the wire diameter). In a practical situation, the adjacent wires contact each other at some angle θ instead of 90° under the same load. In this situation, the projected area increases by a factor of 1/sin θ, i.e., the contact stress decreases by a factor of sin θ. Furthermore, a GFRP support plate against the Lorentz force is placed. The plate height is slightly larger than the wire radius. To eliminate the generation of electromagnetic forces due to magnetism during the measurement, the setup is constructed with non-magnetic materials such as brass and GFRP. Measurement procedure is as follows. First, a magnetic field is applied to the sample. Then a load is applied to a given value. Fig. 2 shows a typical example of the applied stress as a function of total displacement. The actual stress value applied to the wire is obtained by subtracting a nearly constant stress offset mainly due to friction during motor drive from the applied stress. During the measurement, the load is constantly     Fig. 1.  Sketch of crossover contact in CICC and the setup for Ic measurement simulating crossover contact stress state.  Stepping motorLoad cellSample wireDummy wireCurrent leadLHeStainless-steel anvilSample wireDummy wireGFRP anvilStainless-steel anvilGFRP support plateagainst Lorentz forceLorentzforceDisplacement gaugeSketch of crossover contact in CICCContact stress by electromagnetic forceStrand   Fig. 2. A typical example of the applied stress as a function of total displacement (Sample: KOBE Cu sheath DT Nb3Sn strand (C-DT2017)).  0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2050100150200250300350400KOBE C-DT20174.2 KApplied stress (Pa)Total displacement (mm)Stress offset: 100 MPa3 4PoA04   IEEE Trans. Appl. Supercond. controlled to an indicated value by program control within ±1% error. Subsequently, Ic is measured at given magnetic fields. Then, the load is controlled to a next value and Ic is measured again in magnetic fields. These Ic measurement steps are repeated, when the Ic value degrades drastically. B. Samples Fig. 3 shows transverse cross-sections of the tested reinforced Nb3Sn wires: a normal Cu sheath and a CuNb-reinforced bronze-processed Nb3Sn wires (developed by Furukawa Electric (LK316, LK288)) [26], and a distributed-tin (DT) configuration Nb3Sn wires with Cu and a brass (Cu-15at%Zn) matrix (developed by Kobe Steel (C-DT2017, Z-DT2017)) [16]. Table 1 summarized the specifications of the wires. C. Metallography and EBSD strain analysis SEM and EBSD were observed by a normal metallographic technique [15]. In EBSD strain analysis, we measured the grain reference orientation deviation (GROD = θi –θAVG, where θi and θAVG are the orientation of the ith pixel and the average orientation within a grain, respectively).  GROD is an index expressing the grain orientation deviation at a given pixel from the average orientation, and a useful index of the qualitative internal strain (residual stress) distribution in the grain due to plastic or elastic deformation [15], [27]. III. RESULTS AND DISCUSSION A. Ic characteristics under crossover contact stress Fig. 4 plots the voltage (V) vs. current (I) curve for F-Cu-BZ (LK316) at 4.2 K and 14 T. The V–I curves exhibited a small resistive component before a steep voltage increase by a superconducting-normal phase transition. That is presumably due to the effect of current transferring from the electrode to the sample and/or heat generation at the electrodes in the short sample measurement. Here, Ic was defined for Furukawa wires with a criterion of 3 μV/cm and for Kobe wires with 2 μV/cm. The n-index in the equation V=aIn was determined in the range of 4 to 10 μV. The Ic values of LK316 and LK288 at almost no stress were 166 and 170 A at 14 T (4.2 K) and 142 and 139 A at 15 T (4.2 K), respectively, which were similar to the reported values [26]. Since the KOBE wires showed quench phenomenon at 14 T, Ic measurement was performed at 17 T for KOBE wires. The Ic values of C-DT2017 and Z-DT2017 at zero stress were 59.4 and 66 A at 17 T and 4.2 K, similar to the reported value [16]. Fig. 5 shows normalized Ic vs. contact stress characteristics. As shown in Fig. 5(a) and (b), the reinforced bronze wire exhibited a better stress tolerance. Fig. 6(a) and (b) exhibit the GROD maps on the longitudinal cross-section near the contact point for F-Cu-BZ (LK316) and F-RF2-BZ (LK288), respectively, after loading test. The residual strain spread across the whole region of outer Cu sheath in LK316, whereas in LK288, the high residual strain concentrated predominantly  TABLE I SPECIFICATIONS OF THE TESTED WIRES Furukawa bronze-route wire F-Cu-BZ (LK316) F-RF2-BZ (LK288) Wire diameter (mm) 0.825 0.827 Nb filament diameter (μm) 2.3 2.3 Sn diffusion barrier Ta Ta Matrix Cu–15.7wt%Sn–0.3wt%Ti Cu–15.7wt%Sn–0.3wt%Ti Reinforcement – Nb-rod-method Cu-20vol%Nb Cu / CuNb / non-Cu ratio 50 / 0 / 50 27 / 23/ 50  Kobe DT wires C-DT2017 Z-DT2017 Wire diameter (mm) 0.6 0.6 Nb ratio within barrier (%) 38.6 38.6 Nb filament diameter (μm) 3.4 3.4 Matrix of Nb module Cu Cu–15wt%Zn Matrix of Sn core Cu Cu Nb module diameter (μm) 45 45 Ti ratio within barrier (wt%) 0.7 0.7 Zn ratio within barrier (wt%) 0 5.6 Nb / Sn atomic ratio 2.24 2.24 Cu / non-Cu ratio 1.12  1.12       Fig. 4. A typical example of voltage vs. current curves at 14 T and 4.2 K (Sample: Furukawa Cu sheath bronze-processed Nb3Sn strand (LK316).  0 20 40 60 80 100 120 140 160 180 2000123456789F-Cu-BZ (LK316)14 T, 4.2 KVoltage (V)Current (A) 20 MPa 40 MPa 60 MPa 80 MPa 100 MPa 140 MPa 180 MPa 220 MPa 260 MPa 280 MPa   Fig. 3. Cross-sectional optical microscope images of precursor wires (before heat-treatment) for tested Nb3Sn wires: (a) Furukawa Cu sheath bronze-processed Nb3Sn strand (LK316), (b) Furukawa CuNb-reinforced bronze-processed Nb3Sn strand (LK288), (c) Kobe DT Nb3Sn strand (Cu–15at%Zn matrix, Z-DT2017), and (d) a magnified view of Z-DT2017.  4 4PoA04   IEEE Trans. Appl. Supercond. at the outermost Cu region but not at the CuNb region. The CuNb reinforcement is expected to be effective in reducing distortion of the internal filament region against contact stress. Notably, as shown in Fig. 5(c), Ic degradation with respect to contact stress in Z-DT2017 was very small up to approximately 200 MPa. Ic property of the conventional Cu matrix DT strand under contact stress was comparable to that under uniform transverse stress [28]. The excellent transverse stress tolerance of the brass matrix DT strand can be attributed to the advantage of matrix reinforcement to inhibit the distortion of the filamentary region. B. Crack observation A few microcracks were recognized at the opposite side to the contact point in both Furukawa bronze-processed Nb3Sn wires as shown in Fig. 7(a) and (b). This is similar to the situation where bending strain is applied to the strand. Reportedly, when the wire is subjected to transverse load, cracks tend to form inside the Nb3Sn filament parallel to the applied transverse load along the longitudinal direction [29], [30]. In such a case, cracks parallel to the longitudinal direction might be difficult to see in the longitudinal cross-section. On the contrast, in Kobe Z-DT2017, large transverse cracks were visible at the opposite side to the contact point. This is presumably due to the mechanical properties of the brass matrix. Generally, in hard matrix multifilamentary wires, once a small crack is initiated, cracks tend to propagate significantly [15], [31]. IV. CONCLUSION The stress sensitivity of Ic under crossover contact stress seems to be small, compared with that under uniform transverse stress [19], [22], [23], [26], [32]. The deformation of the Cu sheath by crossover contact is thought to relieve stress concentration to the filamentary region. The reinforcement for the wires was effective to improve the tolerance against transverse stress. Particularly, the DT wire with reinforced matrix exhibited superior tolerance against transverse stress up to approximately 200 MPa. It can be concluded that the use of reinforced Nb3Sn wires increases the reliability of the TF magnet operation.   Fig. 6.  GROD maps on the longitudinal cross-section of outer sheath for (a) F-Cu-BZ (LK316), (b) F-RF2-BZ (LK288) near the contact point.  Press500 μm500 μmPressCu CuCuNbCuNbCuNbBarrierBarrier266 MPa286 MPaCuCuCu25 μm 25 μmFR-RF2-BZ (LK288)FR-Cu-BZ (LK316)  Fig. 7.  GROD maps on the longitudinal cross-section near the contact point for (a) F-Cu-BZ (LK316), (b) F-RF2-BZ (LK288), and (c) Z-DT2017 after loading test.  10 μmCrack10 μmPressPress(a)(b)500 μm500 μm500 μmPress(c) 100 μmCrackCrack  Fig. 5. (a) and (b) Normalized Ic and n-value vs. contact stress for Furukawa bronze-processed Nb3Sn wires (LK316 and LK288) at 14 and 15 T, 4.2 K, and (c) normalized Ic and n-value vs. contact stress for Kobe C-DT2017 and Z-DT2017 at 17 T, 4.2 K.  0 100 200 300 4000.00.20.40.60.81.01.21.4  F-RF2-BZ (LK288) F-Cu-BZ (LK316)14 Tshort: 3 V/cmNormalized IcContact stress (MPa)4.2 KIcn-value-30-20-1001020n-value0 100 200 300 4000.00.20.40.60.81.01.21.4Icn-value  Z-DT2017 C-DT201717 Tshort: 2 V/cmNormalized IcContact stress (MPa)4.2 K-30-20-1001020n-value0 100 200 300 4000.00.20.40.60.81.01.21.4  F-RF2-BZ (LK288) F-Cu-BZ (LK316)15 Tshort: 3 V/cmNormalized IcContact stress (MPa)4.2 KIcn-value-30-20-1001020n-value(a)(b)(c)5 4PoA04   IEEE Trans. Appl. Supercond. REFERENCES [1] “Joint Special Design Team for Fusion DEMO.” [Online]. Available: https://www.fusion.qst.go.jp/rokkasyo/ddjst/l [2] K. 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