Masanori Daibo

New 25 T Cryogen-Free Superconducting Magnet Project at Tohoku University

The new high magnetic field research laboratory network is recognized as one of the Japanese Master Plans of Large Research Project by the Science Council of Japan. Recently, the project of the 25 T cryogen-free superconducting magnet (25 T-CSM), which is operated under a conductive cooling condition by cryocooler, was approved under the high magnetic field research laboratory network. We adopted a high strength CuNb/Nb3Sn Rutherford cable with a prebending treatment for the middle section coils of the 25 T-CSM. The central magnetic field of 14 T is generated by the operational current of 851 A by the Nb3Sn middle section and NbTi outer section coils in a 300 mm bore. The induced maximum hoop stress in the CuNb/Nb3Sn section is about 250 MPa. In addition, the 11.5 T high temperature superconducting insert coil is also designed using Gd123 tapes. Therefore, a total central magnetic field of 25.5 T can be achieved.

Characteristics of REBCO Coated Conductors for 25 T Cryogen-Free Superconducting Magnet

A 25 T cryogen-free superconducting magnet is under development at the High Field Laboratory for Superconducting Materials (HFLSM), Institute for Materials Research (IMR), Tohoku University. Fujikura has manufactured and supplied REBCO coated conductors for the 25 T cryogen-free superconducting magnet. In-field critical current (Ic) properties in high magnetic fields at lower temperatures below 20 K and mechanical properties are important for design of these superconducting magnets. However, there is insufficient data of Ic properties in high magnetic fields above 15 T. In this work, in-field Ic properties at 20 K, 15 T of several samples cut from commercial coated conductors are evaluated at HFLSM, IMR, Tohoku University. The correlations between Ic at 20 K, 15 T and Ic at 77.3 K are compared. As a result, it is confirmed that Ic at 20 K, 15 T have a stronger correlation with in-field Ic at 77.3 K rather than Ic at 77.3 K, self-field. In addition, tensile strengths at 77.3 K of more than 30 samples of coated conductors are also evaluated.

Characteristics of Impregnated Pancake Coils Fabricated Using REBCO Coated Conductors

REBCO coated conductors are expected high performance in superconducting applications because of their high current density and high mechanical strength. However, in order to use these conductors in superconducting applications, a technique is required for fabricating impregnated REBCO coils without degradation.In this work, we fabricated six pancake coils without degradation using wounded REBCO coated conductors. These conductors were laminated with a copper tape as a stabilizer, and were fabricated by the IBAD/PLD method. In addition, we evaluated the V-I characteristics and n-values of the coils in liquid nitrogen and compared the calculated and measured values for the critical current (Tc) of the REBCO coated conductors. The measured Tc of the coils were in good agreement with the calculated ones. Furthermore, we fabricated a stacked coil using an approximately 190 m long REBCO coated conductor and evaluated the stability of the stacked coil at 77, 60 and 50 K under conduction-cooled condition. The thermal runaway current of the stacked coil was 166.4 A, and the central magnetic field was 1.27 T at 50 K.

Development for Mass Production of Homogeneous RE123 Coated Conductors by Hot-Wall PLD Process on IBAD Template Technique

Recent development of production technique for RE123 coated conductors was reviewed, using Hot-wall PLD/ IBAD process. Lot-to-lot distributions for both average value and longitudinal uniformity of Ic at 77 K, 0 T were summarized for 127 run of 300-500-m-long 10-mm-wide CC. By optimizing hotwall PLD process parameters we found a reproducible high Jc production condition (20% enhancement in field at 20 K), less angular dependent to applied magnetic field. Furthermore, new hermetic type Cu stabilization foil configuration was developed and general mechanical properties against tensile strain, etc. were surveyed.

AC Losses of an HTS Insert in a 25-T Cryogen-Free Superconducting Magnet

A 25-T cryogen-free superconducting magnet (25T-CSM) is being developed at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. In the case of a cryogen-free superconducting magnet, the coil temperature rises during a sweep of an operating current due to ac losses. Hence, the ac loss estimation is very important for the cooling design. The critical current density and the magnetization of Gd123 tapes at 4.2 K were measured in order to estimate the ac loss of a Gd123 insert of the 25T-CSM. The ac loss is actually the hysteresis loss, which are calculated from the Jc properties taking the magnetic field distribution in the Gd123 coil into account. Regarding the effect of tape stacking (or winding) in a pancake coil, the slab approximation can be used for hysteresis loss calculation. In the case of slab model, the full penetration field at the center of the tape becomes higher than the maximum applied magnetic field in the most part of the coil. As a result, the hysteresis loss increases with increasing a magnetic field when the magnet energizes. The hysteresis losses assuming the slab model, however, show an opposite field dependence to those calculated from the strip model without the stacking effect. Hence, the ac loss of 5 W is estimated when the magnet energizes to 25.5 T within 60 min.

Development of a 5T 2G HTS Magnet With a 20-cm-Diameter Bore

REBCO coated conductors known as second-generation (2G) high-temperature superconductor are expected to show high performance in superconducting applications because of their high mechanical strength and high current density in a magnetic field. Recently, there have been reports about fabricating impregnated REBCO coils. However, more characteristic data of REBCO coils with larger diameters are required in order to apply these conductors to practical superconducting applications. In this work, we fabricated a cryo-cooled high-temperature superconducting magnet, which was composed of 24 pancake coils with an inner diameter of 260 mm, using REBCO coated conductors. The total length of REBCO coated conductors was approximately 7.2 km. These conductors were laminated with a copper stabilizer and were fabricated by Fujikura Ltd. using ion-beam-assisted deposition and pulsed laser deposition methods. Before fabricating the magnet, all pancake coils were evaluated their V -I characteristics in liquid nitrogen, in order to confirm the characteristics without degradation. After the fabrication, the magnet was cooled down to 24 K by a Gifford-McMahon cryocooler. We excited the magnet up to 5 T, successfully.

Evaluation of the Normal-Zone Propagation Characteristics of REBCO Coated Conductors With Laminated Cu Tape

The quench characteristics, particularly the normal-zone propagation and increase in hot-spot temperature, of REBa2Cu3Ox (REBCO) coated conductors are important issues for safe operation in the superconducting applications. Although experimental data on the quench characteristics of REBCO coated conductors have been reported only limited data is available specifically on REBCO coated conductors laminated with Cu tape of thickness no less than 0.1 mm, which are expected to be used for high-current operation in future superconducting applications. Here we reported experimental measurements of the normal-zone propagation velocity (NZPV) in REBCO coated conductors laminated with Cu tape of 0.1 mm thickness at 77, 65 and 50 K in a background magnetic field. We also compared these results with the NZPV at 77 K of REBCO coated conductors laminated with Cu stabilizer tape of thickness 0.05 and 0.3 mm.

Design of YBCO Insert Coil for a Cryogen-Free 22 T Superconducting Magnet

A YBCO insert coil has been developed for upgrading a cryogen-free 18 T superconducting magnet installed in the High Field Laboratory for Superconducting Materials (HFLSM) at Tohoku University to a 22 T superconducting magnet. The YBCO insert coil is designed to generate 6.5 T at 200 A in 15.5 T outer LTS coils. The YBCO insert coil is composed of a stack of 50 single pancake coils wound with YBCO-coated conductors (0.23 mm × 4 mm). The inner and outer diameters of the YBCO insert coil are 96 mm and 178 mm, respectively, and the total conductor length is about 3 km. The maximum hoop stress of the YBCO insert coil was estimated to be 310 MPa when the central magnetic field was 22 T. The magnet system is cooled by a GM/JT cryocooler and two single-stage GM cryocoolers. Thermal runaway may cause burnout of the YBCO insert coil, and therefore, it is important to calculate the voltage-current characteristics of the coil from the superconducting properties of the YBCO-coated conductors. The coil should have no damaged area in the windings because a damaged area would generate heat locally, eventually resulting in thermal runaway. Therefore, a demonstration coil with almost the same size as the pancake coils used for the YBCO insert coil was fabricated and tested in conduction cooling conditions in order to evaluate whether the coil could withstand the thermal stresses and electromagnetic force.

10 T generation by an epoxy impregnated GdBCO insert coil for the 25 T-cryogen-free superconducting magnet

A GdBa2Cu3O y (Gd123) insert coil for the 25 T cryogen-free superconducting magnet was constructed, installed and tested. We succeeded in the generation of 10 T using a Gd123 insert coil without a background field. The temperature of the Gd123 coil increased from 4.5 K gradually and reached about 5.5 K, when the magnet was energized with 0.036 A/s, which corresponds to a 1 hour energizing mode. The calculated and measured central magnetic fields are 10.61 T and 10.15 T, respectively, because of the magnetization current effect in RE123 tape. The maximum heat load by the AC-losses estimated from the temperature rise is about 3 W, which is consistent with the slab model combined with tape stacking effect.

Evaluation of a 426 kJ Cryocooled Magnet and a Model Magnet With REBCO Coated Conductors

REBa2Cu3Ox (REBCO, RE = rare earth) coated conductors are expected to show high performance for superconducting applications because of their high current density and high mechanical strength. Fujikura has developed REBCO coated conductor over 570 A/cm width at 77 K, self field with over 800 m length. In addition, we have succeeded in developing a 5 T cryocooled REBCO magnet with a stored energy of 426 kJ. The magnet is composed of 24 pancake coils with an inner diameter of 260 mm, and the total length of the REBCO coated conductors is approximately 7.2 km. We confirmed that the REBCO magnet could be excited up to 5 T for 720 min 9 months after the fabrication of the magnet. It is important to validate the long-term operation test results of the REBCO magnets with larger diameters in order to use these conductors in practical superconducting applications. On the other hand, we have fabricated and evaluated a model magnet before we fabricated a 426 kJ cryocooled magnet. The model magnet is composed of 6 REBCO pancake coils and 18 “dummy coils” with an inner diameter of 260 mm and an outer diameter of 515 mm. The model magnet is about the same size and shape as the 426 kJ cryocooled magnet. We have confirmed that the calculated critical current (Ic) of the top pancake coil of the model magnet was in good agreement with the measured coil Ic from 30 to 77 K under the conduction-cooled conditions. In addition, we have evaluated the thermal runaway behavior of the model magnet under the conduction-cooled conditions. As a result, we have confirmed that the model magnet with a 0.3-mm-thick copper stabilizer had sufficient time to ramp down without degradation at 361 A transport current after the detected normal transition.