The World’s Largest 5 T Yttrium-based High Temperature Superconducting Magnet with a 20-cm-diameter Room Temperature Bore Masanori Daibo,1 Shinji Fujita,1 Masashi Haraguchi,1 Yasuhiro Iijima,1 Masahiko Itoh,1 and Takashi Saitoh1 Y-based superconducting wires are expected to show high performance in superconducting applications because of their high mechanical strength and high current density in magnetic fields. We have succeeded in developing the world's largest Yttrium-based high-temperature superconducting (HTS) magnet, which stores energy of 426 kJ. The magnet is composed of 24 pancake coils with an inner diameter of 260 mm. The total length of the Y-based superconducting wires is approximately 7.2 km. These superconducting wires were fabricated using ion-beam-assisted deposition (IBAD) and pulsed laser deposition (PLD) methods. In 2012, we excited the magnet up to 5 T around 25 K, successfully. We have demonstrated that the magnet can be excited up to 5 T in 720 min 9 months after the fabrication of the magnet. We have also confirmed that the developed magnet can be used as a background magnet for measuring critical current in magnetic fields of the Y-based superconducting magnet. 1. Introduction Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials which are cooled below a specific temperature. It was discovered by a Dutch physicist H. K. Onnes in 1911. Conventional superconductors, which are called low temperature superconductors (LTS), show superconductivities just above the boiling point of liquid helium (4 K (- 296 °C)). On the other hand, several kind of cuprate-perovskite ceramic materials were discovered in 1987, which showed superconductivities at unusually high critical temperatures far above the boiling point of liquid nitrogen (77 K(- 196 °C)). The material is called high temperature superconductors (HTS). An LTS wire has already used in devices such as MRI (Magnetic Resonance Imaging) scanners for medical application and NMR (Nuclear Magnetic Resonance) for analysis of life science and material research. That is because superconducting coils realize small magnets and generate a high magnetic field in a large space using only a small amount of electric power. Yttrium (Y) -based superconducting wires are expected to show high performances as a second-generation HTS in a wide range of applications because of their high mechanical strength and high current density in the magnetic field. Recently, high performance Y-based superconducting wires, which show over 500 A/cm width at 77 K in self field(s.f.) with a few hun 1 : Superconductor Business Development Division Fujikura Technical Review, 2014 dred meters length, have been commercialized by Fujikura Ltd. 1)-3) Fujikura has also developed over 570 A/cm at 77 K, s.f. with over 800 m length 2)3). We have developed several Y-based superconducting coils 4)-6). However, various characteristic data of Y-based superconducting magnets with larger diameters are required in order to utilize the conductors in practical superconducting applications. In particular, a cryocooled system is suitable for use in superconducting applications because it does not require cryogenic fluids such as liquid helium. In 2012, we have succeeded in developing a 5 T Y-based superconducting magnet with a 20-cm-diameter bore at room temperature, which stored energy is 426 kJ 7)8). This report describes the development of the new cryocooled magnet. 2. S pecifications of Y-based superconducting wires A photograph of Y-based superconducting wire is shown in Fig. 1. Y-based superconducting wires are manufactured using the ion-beam-assisted deposition (IBAD) and pulsed laser deposition (PLD) methods. The specifications of the Y-based superconducting wires for the magnet are listed in Table 1. These superconducting wires are 10 mm wide with a 0.1-mmthick Hastelloy® substrate and a 0.3 mm thick copper stabilizer, which is soldered to a silver layer on a superconducting layer. In Table 1, it was specified the critical current (Ic) with the criterion of 10-7 V/cm, and the n-value ranged from 10-8 to 10-7 V/cm. In particular, a thickness of a stabilizer is important 25 Abbreviations, Acronyms, and Terms. stabilizer (Cu) protection layer (Ag) superconducting layer buffer layers substrate Superconducting Coil L Power Supply insulation (polyimide tapes) Normal zone Resistance r external magnetic fields Quench–Transition from superconductivity to normal conductivity, a rapid irreversible process Cryocooled–Cooled by a low-temperature refrigerator Gifford-McMhon cryocooler–A cryocooler developed by W. Gifford and H. McMahon in the early 1960s. Cryostat–An enclosing container to maintain a cryogenic environment. Dumping Resistor R Yttrium-based superconducting wire–High temperature superconductor includes Yttrium(Y) or Gadolinium(Gd) etc. It is also called Rare Earth-based superconducting wire. It features high current density and high performance at a temperature near liquid nitrogen. Critical current–A specific current in a superconductive material above which the material is normal and below which the material is superconducting, at a specified temperature and in the absence of Fig. 2. Protection circuit for a superconducting magnet. Fig. 1. Photograph of a Y-based superconducting wire. Table 2. Specifications of a 5T Y-based superconducting magnet with a 20-cm-diameter room temperature bore. Table 1. Specifications of Y-based superconducting wires for the 5 T Y-based superconducting magnet. Parameters Values Width 10 mm Thickness of substrate 0.1 mm Thickness of copper stabilizer 0.3 mm (laminated) Critical current (Ic) at 77 K, s. f. >467 A n-values at 77 K, s. f. 24-38 for coil protection in order to safely operate a magnet. In this work, we determined the required stabilizer thickness of Y-based superconducting wires for coil protection by means of evaluating the heat generation under adiabatic conditions using the following heat balance equation 9)10) : ∂T Ct ( T ) = Qj (T ) (1) ∂t where C t is the volumetric heat capacity of the conductor, and Q j is the Joule heating density. Assuming that heat generation occurs only in the copper stabilizer layer, we obtain the following relationship by the integration Eq. (2) from the temperature when dumping begins (T0) to the maximum temperature after quenching (Tm) 9)10) : 26 Parameters Values Inner diameter 260 mm Outer diameter 535 mm Coil height 271 mm Number of pancake coils 24 Total length 7.2 km (300 m ¥ 24) Total number of turns 5775 Operating temperature 25 K Operating current 333 A Central magnetic field 5.0 T Inductance 7.68 H Stored energy 426 kJ Ú Tm Ct ( T ) S dT = stab St r stab ( T ) E Vm I 0 J 2stab,0 (2) where St is the cross-sectional area of the conductor, Sstab is the cross-sectional area of the stabilizer, rstab is the electrical resistivity of the stabilizer, Jstab is the current density of the stabilizer, td is the delay time from the beginning of quench until the opening of the current breaker, Jstab,0 is the current density of the stabilizer when dumping begins, E is the stored energy of the magnet, Vm(=RI0) is the maximum voltage applied to the coil, I0 is the current when energy dumping begins and R is the dumping resistance, as shown in Fig. T0 td + GM cryocooler Table 3. Specifications of Y-based superconducting wires for the model magnet. Parameters HTS pancake coils Values Width 10 mm Thickness of substrate 0.1 mm Thickness of copper stabilizer 0.3 mm (laminated) Critical current (Ic) at 77 K, s. f. 350-426 A dummy coils (using copper tape) HTS pancake coil heater Table 4. Specifications of the model magnet. Values 260 mm Outer diameter of coil 515 mm Coil height 271 mm Total number of pancake coils 24 - Number of superconducting coils 6 - Number of "dummy coils" 18 Total number of turns of superconducting coils 1350 Total length of superconducting wires 1646 m 2. Eventually, we employed a 0.3-mm-thick copper stabilizer in order to suppress the maximum temperature after quenching below 300 K (573 °C) for the magnet specified in Table 2. 7) 3. Evaluation of the model magnet 11) 3.1 Fabrication of the model magnet We fabricated and evaluated a model magnet of the 5 T Y-based superconducting magnet for design and technology validation before the fabrication of the 5 T magnet. The specifications of the Y-based superconducting wires used in the model magnet are listed in Table 3. The specifications of the model magnet are listed in Table 4. The model magnet is composed of 6 superconducting pancake coils and 18 “dummy coils”, which are epoxy-impregnated pancake coils using the same dimension copper tapes of the Y-based superconducting wires, with an inner diameter of 260 mm and an outer diameter of 515 mm. The height of the double pancake coil (DPCC) is approximately 20.5 mm. The Ic of each superconducting coil is measured in liquid nitrogen after epoxy-impregnation. The n-values of all the superconducting coils are greater than 21 in the electric field ranging from 10-8 to 10-7 V/cm. This indicates that none of the superconducting coils is damaged during coil fabrication. 12) These 6 superconducting coils and 18 “dummy coils” are stacked with copper cooling plates, and all the superconducting coils are electrically jointed. In addition, the model magnet is thermally connected with the 2nd stage of a Gifford-McMahon (GM) cryocooler through heat conduction bands made of copper as shown in Fig. 3. Four thermo sensors are attached on the 2nd stage of the GM cryocooler and on the cooling plates of the coil. Fujikura Technical Review, 2014 Thermo sensors bobbin splice dummy coils cryostat Fig. 3. Schematic view of the model magnet. Coil IC (A) (0.1µV/cm criterion) Parameters Inner diameter of coil 600 Calculation Measured 500 400 300 200 100 0 20 30 40 50 60 Temperature (K) 70 80 Fig. 4. Comparison between the measured and the calculated coil I c of the top pancake coil of the model magnet. 3.2 Evaluation of the model magnet We have compared the calculated and measured Ic of the top pancake coil, which is the lowest coil Ic of the model magnet. For the calculation of the coil Ic, we have used the following equation 5) : n ( B,,) Tq I (3) V = 2 pr ¥ 10 -6 Ic ( B,,) T q where r is the radius, Ic is the critical current of the Ybased superconducting wires of the short sample with a criterion of 10-7 V/cm, B is the magnetic field, T is the temperature, q is the magnetic field angle, and n is the n-value of the Y-based superconducting wires. In the calculation, the Ic -B-q characteristics of the Y-based superconducting wires are obtained from the fit of the experimental results from 30 K to 77 K under the conduction-cooled conditions. Fig. 4 shows the calculated and measured coil Ic of the top coil from 30 K to 77 K under the conductioncooled conditions. The coil Ic is defined with a criteri- Â 27 on of 10-7 V/cm at each temperature. The calculated coil Ic of the top coil is in good agreement with the measured one. We have confirmed that the difference between the calculated approximate expression and the measured coil Ic from 30 K to 77 K is within 5%. 4. Fabrication and evaluation of the 5 T Y-based superconducting magnet 7)8)11) 4.1 F abrication of the 5 T Y-based superconducting magnet After the evaluation of the model magnet, 24 single pancake coils were fabricated for the 5 T Y-based superconducting magnet with a unit length of approximately 300 m. The specifications of the pancake coils and the magnet are listed in Table 2. All pancake coils were of the same size. Ic of each DPCC was measured in liquid nitrogen after each DPCC was impregnated by epoxy-based resin. The splice between two single pancake coils were carefully soldered with a copper plate. A photograph of the DPCC is shown in Fig. 5. The inner diameter of each pancake coil was 260 mm, and the outer diameter was approximately 535 mm; the height of the DPCC was approximately 20.5 mm. 535 mm Fig. 6 shows a comparison between the V-I characteristics of a pancake coil measured before and after impregnation. The n-values of the pancake coil in an electric field ranging from 10-8 to 10-7 V/cm before and after impregnation were unchanged 27 and 27, respectively. Fig. 6 also shows good agreement between the V-I characteristics measured before and after impregnation. Fig. 7 shows the measured Ic and n-value of all pancake coils in liquid nitrogen. Ic of the pancake coils were specified with the criterion of 10-7 V/cm, and the n-values of pancake coils were measured in an electric field ranging from 10-8 to 10-7 V/cm, respectively. From Fig. 7, the n-values of all pancake coils were greater than 24 in the electric field ranging from 10-8 to 10-7 V/cm. This indicates that none of the pancake coils was damaged during fabrication. Fig. 8 shows a schematic view of the 5 T Y-based superconducting magnet. The magnet is composed of cooling plates and superconducting double pancake coils which are electrically jointed in series. The magnet is also thermally connected with the 2nd stage of a Gifford-McMahon (GM) cryocooler. A photograph of the magnet with a 20-cm-diameter room temperature bore is shown in Fig. 9. 4.2 Evaluation of the 5 T Y-based superconducting magnet Compact Disc Fig. 5. Photograph of an impregnated double pancake coil. Coil lC (A) Voltage (V/cm) 1¥10 -7 after impregnation : n=27 before impregnation : n=27 1¥10 -8 300 35 250 30 25 200 20 150 15 100 10 50 1¥10 -9 10 0 100 Current (A) 1000 Fig. 6. V -I characteristics of a pancake coil at 77 K. 28 5 1 2 3 4 5 6 7 9 11 13 15 17 19 21 23 8 10 12 14 16 18 20 22 24 0 Number of pacake coil Fig. 7. Measured coil I c and n-value of 24 pancake coils. n-value of pancake coils 260 mm Fig. 7 shows the time variation of the magnet temperature during initial cool-down. The magnet was cooled by a GM cryocooler. Four thermo sensors were attached on the 2nd stage of the GM cryocooler and on the cooling plates of the coil as shown in Fig. 8. The terminal voltage of the magnet was measured during cool-down. The superconducting transition was observed when the temperature of the cooling plates reached below 89 K. The coil temperature reached 23.7 K; it took around 260 h to reach this temperature. In 2012, we confirmed that the central magnetic field of the magnet was 5.0 T when the coil current was 333.4 A. It was demonstrated that the magnet was excited up to 5.0 T in 60 min. The increase of the coil temperature was measured approximately 1.3 K during the 60-min-operation. The load factor of the magnet was approximately 0.6 and the temperature margin of the magnet was approximately 25 K when the central magnetic field of the magnet was excited up to 5.0 T. We also confirmed that the axial central magnetic field drift of the magnet was stabilized by 1 % current reversal of the coil current. Moreover, we confirmed that the Y-based superconducting magnet could be excited up to 5 T in 720 min 9 months after the fabrication of the magnet, as shown Fig. 11. We have also confirmed that the 5 T Y-based superconducting magnet can be used as a background magnet for measuring Ic -B characteristics of the Ybased superconducting wires. 5. Conclusion We have succeeded in the development of the world’s largest 5 T Y-based superconducting magnet with a 20-cm-diameter room temperature bore, which stores energy of the magnet was 426 kJ. The magnet is composed of 24 pancake coils with an inner diameter of 260 mm. The total length of the Y-based superconducting wires is approximately 7.2 km. The superconductors were fabricated using ion-beam-assisted deposition (IBAD) and pulsed laser deposition (PLD) methods. The results indicate that Fujikura’s Y-based superconducting wires have reached to a practical level. We have also demonstrated that the 5 T Y-based superconducting magnet can be excited up to 5 T in 720 min 9 months after the fabrication of the magnet. The magnet can be used as a background magnet for measuring Ic -B characteristics of the Y-based superconducting magnet, too. GM cryocooler 300 6 5 250 Thermo sensors 56 cm Radiartion shield 110 cm 110 cm Fig. 9. Photograph of a 5 T Y-based superconducting magnet with a 20-cm-diameter room temperature bore. Fujikura Technical Review, 2014 3 2 100 89 K 23.7 K 0 260h 0 100 1 0 300 200 Time (h) Fig. 10. Time variations of the magnet temperature during initial cool down. Magnetic Field (T), Coil Voltage (V) 80 cm bottom cooling plate 150 Fig. 8. Schematic view of the 5 T Y-based superconducting magnet. 20 cm 4 central cooling plate 50 Cryostat GM cryocooler top cooling plate 200 6 500 Central Magnetic Field 5 400 720 min 4 333 A 300 Current 3 Temp. (top) Temp. (bottom) 2 200 100 1 0 Coil Voltage 0 150 300 450 600 750 900 0 Current (A), Temperature (¥10 -1 K) 2nd stage Temperature (K) 20 cm Terminal Voltage (V) 2nd stage Time (min) Fig. 11. Example excitation test results of the magnet at 25 K ; exciting up to 5.0 T in 720 min. 29 References 30 7) M. Daibo et al., : “Development of a 5 T 2G HTS magnet with 1) K . Kakimoto et al., : “Long RE123 coated conductors with a 20-cm-diameter bore,” IEEE Trans. Appl. Supercond., vol. high critical current over 500 A/cm by IBAD/PLD technique,” Physica C, vol. 471, pp.929-931, 2011. 2) M. 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