A 1.8 K current feedthrough using YBCO bulk conductor for supplying 20 kA

https://doi.org/10.1016/j.physc.2005.02.079Get rights and content

Abstract

Rectangle-shaped YBCO bulk conductors 20 mm wide, 140 mm long and 10 mm thick were manufactured from square-pillar-shaped YBCO bulk materials for supplying a 20 kA current into a pressurized superfluid cooling region at a temperature of 1.8 K. An assembled 1.8 K current feedthrough was mounted in the λ-plate of a pressurized superfluid cooling cryostat. Current-feed experiments 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 1200 s. Contact resistance and Joule heat generation in the joint region between the YBCO bulk conductors and the copper electrode were 1.45 nΩ and 0.72 W, respectively, in the 20 kA operation.

Introduction

The large helical device (LHD) is a fully superconducting heliotron type experimental fusion device constructed at the National Institute for Fusion Science (NIFS). In Phase I of the LHD project, superconducting helical coils in the LHD produce a magnetic field of 3 T when operated at 13 kA by cooling with conventional liquid helium at a temperature of 4.4 K. In Phase II, it is planned for the helical coils to be cooled with pressurized superfluid helium at a temperature of 1.8 K, and so raise the magnetic field to 4 T for an operating current of 17.3 kA. It is important to reduce heat leakage through the current bus line between the current leads in the conventional liquid helium bath and the helical coils operated at 1.8 K.

Current-feed devices are required to have high current transport densities and low thermal conductivities. In the design of the superfluid cooling system for the LHD, the maximum permissible heat load is specified as being 1 W for each current-feed device with an operating current of 18 kA. It is also desirable that the current-feed device be compact.

High temperature superconductors (HTSs) are promising materials for high current-feed devices in large scale superfluid cooling systems. Many types of HTS conductors have recently been developed [1]. A conductor based on the Bi2212 material was developed for a transport current of 10 kA in a magnetic field of 12 T [2]. Bi-based conductors for high current transport inevitably have large geometrical dimensions due to their relatively low current density. In contrast, because of the large pinning force, HTS materials based on YBCO have a critical current density that is about ten times higher than that of Bi-compounds. Devices based on a bulk YBCO conductor are expected to have a compact structure in their application to the transport of high currents above 10 kA. Porcar et al. demonstrated the feasibility of a high current transport device made of YBCO conductors by applying pulsed currents up to 6 kA [3]. Most YBCO bulk conductors have to date been developed for fault current limiters [4].

We have demonstrated high current transport of an “H”-shaped bulk conductor cut from a melt textured YBa2Cu3Ox disk of 65 mm diameter and 15 mm thickness for currents up to 25 kA [5]. However, the geometrical dimension of the “H”-shaped bulk conductor was insufficient for assembly into the 1.8 K current feedthrough. Mito et al. [6] reported test results of four kinds of prototypes of 1.8 K current feedthroughs, and set out guidelines for selecting large YBCO bulk conductors without internal defects or cracks.

In this work, YBCO bulk conductors 20 mm wide, 140 mm long and 10 mm thick were assembled into a 1.8 K current feedthrough mounted in the λ-plate of a pressurized superfluid cooling cryostat. Current-feed experiments into the 1.8 K region were carried out up to 20 kA by operating the 1.8 K current feedthrough.

Section snippets

Large-sized YBCO bulk conductors

After success in the 25 kA transport test of the “H”-shaped YBCO bulk conductor [5], YBCO bulk fabricated by the quench and melt growth (QMG) method is indicated as a promising material. The geometrical dimensions of the “H”-shaped conductor such as width, length and thickness were restricted to a 65 mm diameter and a 15 mm thickness by the disk-shaped YBCO bulk. A prototype of a 1.8 K current feedthrough was planned to be mounted in a λ-plate of 75 mm in thickness in a large pressurized superfluid

1.8 K current feedthrough

Fig. 4 shows a schematic drawing of the 1.8 K current feedthrough. Three bulk conductors were arranged side by side in a cylindrical GFRP plug of 75 mm in thickness made of 75% glass content GFRP laminated in parallel with the conductor. The gap between the conductors and the GFRP plug was sealed with an epoxy resin to stop super-leaks. Both ends of the YBCO bulk conductors were soldered to the upper and lower copper electrodes.

For mechanical support and the by-pass of electrical current in the

Current transport experiments

As shown in Fig. 5, the 1.8 K current feedthrough was mounted in a current transport test setup for a preliminary experiment with cooling using normal liquid helium at a temperature of 4.2 K. The cold ends of the gas-cooled current leads were connected to the feed and the return Nb/Ti stranded cables of the prototype 1.8 K current feedthrough. The test setup of the 1.8 K current feedthrough was inserted into the large cryostat and immersed in normal liquid helium at a temperature of 4.2 K. Current

Conclusion

Three YBCO bulk conductors 20 mm in width, 140 mm in length and 10 mm in thickness were assembled into a 1.8 K current feedthrough. Current-feed experiments into the 1.8 K region were carried out by operating the 20 kA current feedthrough. In the experiments, transport current was kept at 20 kA for longer than 1200 s. The 0.72 W experimental Joule heat generation in the joint region of the current feedthrough satisfies the criteria for a superfluid cooling cryogenic system of LHD coils.

Acknowledgements

The authors would like to thank Fuji Electric Co., Ltd for their collaboration in the development of the 1.8 K current feedthrough. We also wish to express our thanks to Nippon Steel Co. for their work in developing the large-sized YBCO bulk conductors. This work was performed with the support and under the auspices of the NIFS LHD Project Research Collaboration.

References (7)

  • A.P. Malozemoff et al.

    Physica C

    (2003)
  • L. Porcar et al.

    Physica C

    (1997)
  • L. Porcar et al.

    Physica C

    (2002)
There are more references available in the full text version of this article.

Cited by (0)

View full text