RE-Ba-Cu-O bulk superconductors fabricated by melt-process have the distinctive feature of a high critical current density of 77K in strong magnetic fields, and consequently are expected to be used for next-generation current leads. We have made proto-type of RE-Ba-Cu-O bulk current leads and investigated the transport properties, the heat leak, the thermal cycle toughness and the mechanical properties. This paper presents the structure of the bulk current leads and the test results obtained so far.
High temperature superconductors (HTS) have been successfully applied to current leads. Low thermal conductivity and high critical current density (Jc) are the characteristics desired for such applications. We have developed Y-Ba-Cu-O current leads for MAGLEV. The mechanical properties of Y-Ba-Cu-O bulk superconductors are improved by epoxy resin impregnation. In order to reduce the stress caused by the relative displacement between the inner vessel and the radiation shielding plate of superconducting magnet, flexible copper cables were connected in series with a Y-Ba-Cu-O current lead. We were able to feed a current up to 1000 A at 77 K, and then we measured V-I characteristics. The present results show that the bulk Y-Ba-Cu-O can be applied to the current leads for MAGLEV.
This paper introduces the development progress regarding the current lead for SMES. Large-current carrier capacity has been attained by connecting two or more 1kA-class YBCO superconductivity current lead elements in parallel. Miniaturization of the SMES current lead for bridging instantaneous voltage dips has been achieved by cooling the YBCO bulk elements with a small direct cryocooler, and a 10kA-class current lead has been produced by connecting 1kA-class current leads in parallel. This current lead, which supplies electric power to the SMES model coil for load fluctuation compensation and frequency control, was further developed as a national project of the Agency of Natural Resources and Energy.
The development of a current leads using YBCO superconducting rods was reviewed. The textured YBCO rods were fabricated by a zone-melt process. A high Ic value of 2,000 A was obtained by the structural control of texturing and pinning centers by Y211 particles. The highly textured structure was realized by the growth conditions of high G/R (G: Temperature Gradient and R: Growth Rate). The fine Y211 particles were obtained by the addition of the Pt powder to reduce a coarsening rate of Y211 in the liquid. The current leads were formed by the development of packaging technique for high mechanical strength and connection with the electrodes. Concerning the mechanical strength, no degradation of Ic performance due to a vibration of 15 G was achieved. Consequently, the current leads with not only high superconducting current but low thermal invasion and Joule heating was realized. It was confirmed that the total heat load could be suppressed to be 0.21 W/pair for 500 A where the high and low temperatures were controlled to be 80 and 20 K, respectively.
The use of HTS for current lead is one of the most important issues to improve the thermal performance such as minimizing the heat load and electrical power for cooling. Large currents of more than 10kA for capacity current leads are necessary for recent large-scale superconducting facilities like fusion application. We introduce several key technologies to realize such a large current capacity and high thermal performance. Theoretical and experimental results of HTS current leads show that the use of HTS introduces significant improvement on their thermal performance.
Bi-2223 bulk current lead has been developed by the sintering technique. The critical current (Ic) and the critical current density (Jc) of the Bi-2223 current lead indicate 1000 A and 1000 A/cm2 at 77 K in a self-magnetic field, respectively. The magnetic field dependence of the Ic values for the developed current lead was measured at several temperatures. The Jc value of more than 600 A/cm2, which corresponds to the Ic value of 600 A, was shown in 5 K, 1.2 T and 50K, 0.6 T. The heat leakage of 183 mW was calculated for a pair of the current leads at temperatures from 50 K to 5 K. The cantilever bending strength of 29-47 MPa was obtained for the Bi-2223 bulk leads. The developed Bi-2223 lead was applied to the cryocooler-cooled superconducting magnet. The Bi-2223 current lead is a key component for the magnet system. About 160 of the current leads have been already applied to the magnets, indicating high reliability.
In the phase II experiment of the Large Helical Device (LHD) of the National Institute for Fusion Science (NIFS), it is planned to operate the helical coils at 1.8 K by employing pressurized superfluid cooling to raise the magnetic field to 4 T with 17.3 kA. It is important to develop a 20kA-class current feedthrough into the 1.8 K region, but it must have a high current capacity and low heat leakage in the maximum magnetic leakage field of 1 T. Rectangle-shaped YBCO bulk conductors measuring 20 mm wide, 140 mm long and 10 mm thick were manufactured from square-pillar-shaped YBCO bulk materials for a 20 kA current. To check the quality of the bulk conductors, internal defects or cracks were detected by carrying out a precise survey of trapped magnetic flux. An assembled 20 kA current feedthrough was mounted in the λ-plate of a pressurized superfluid cooling cryostat. Experiments of current feeding into the 1.8 K region were carried out by operating the 20 kA current feedthrough. In the experiments, the transport current was kept at 20 kA for longer than 1,200 s. During the 20 kA operation, the current transport section of the YBCO bulk conductors remained in the superconducting state and the voltage drop between the YBCO bulk conductors and the copper electrode was observed to be constant. A contact resistance and the Joule heat generation in the joint region between the YBCO bulk conductors and the copper electrode were obtained as 1.45 nΩ and 0.72 W, respectively in the 20 kA operation. We have demonstrated the feasibility of using a 20 kA current feedthrough for the phase II experiment of the LHD.
Bi2212 HTS bulk cylinders have been prepared by the diffusion process for current lead application. The Bi2212 HTS layer is synthesized through a diffusion reaction between a Sr-Ca-Cu oxide substrate and a Bi-Cu oxide coating with Ag addition. The HTS diffusion layer is about 150 μm in thickness formed around the cylinder, 37/29 mm in the outside/inside diameter and 200 mm in length. The transport current of the cylindrical specimen exceeds 7,500 A at 4.2 K and self-field, which corresponds to a transport current density of 24,000 A/cm2. The total heat loads, including heat leakage conducted through the cylindrical specimen between 20 K and 4.2 K and between 40 K and 8 K, are estimated to be 90 mW at 5,000 A and 300 mW at 3,000 A, respectively. Present Bi2212 HTS cylinders with large transport currents as well as low joint resistance and thermal conductivity, show promise as current leads for superconducting magnets.
JAERI has been developing a large-capacity high-temperature superconductor (HTS) current lead for fusion application, and succeeded in fabricating and testing a 60 kA HTS current lead satisfying ITER requirements. Targets of performance are 1/10 heat leak and 1/3 electric power consumption for the cryogenic system as compared use with a conventional lead. To achieve these targets, selection of HTS sheath material, optimization of the Cu part, reduction of joule heat at the joint between HTS and Cu parts and improvement of heat transfer between the HTS and stainless-steel tube were investigated. The 60 kA HTS current lead developed satisfied the design conditions and nearly achieved the targets. Adoption of the HTS current lead reduces the total electric power consumption of the ITER's cryogenic system by 13%.