Development of 1.8 K HTS current feedthrough using large-sized YBCO bulk conductors

In Phase II experiment of a Large Helical Device (LHD) of the National Institute for Fusion Science (NIFS), the helical coils are planned to be operated at 1.8 K by employing pressurized superfluid cooling to raise the magnetic field up to 4 T using a current of 17.3 kA. It is important to develop a 20 kA-class current feedthrough into the 1.8 K region with high current capacity and low heat leakage in the maximum leakage magnetic field of 1 T. The YBCO bulk conductors of 20 mm in width, 140 mm in length and 10 mm in thickness were assembled into a prototype 1.8 K current feedthrough. The current transport tests of the prototype 1.8 K current feedthrough were carried out successfully for currents up to 20 kA with liquid helium bath cooling at 4.2 K. The transport current was held at 20 kA for longer than 300 seconds. During the 20 kA operation, the current transport section of the YBCO bulk conductor remained in the superconducting state and the voltage drop between the YBCO bulk conductors and the copper electrode was constant. The contact resistance and the Joule heat generation in the joint region between the YBCO bulk conductors and the copper electrode were 1.8 nΩ and 0.72 W, respectively when operated at 20 kA. We demonstrated the feasibility of a 1.8 K HTS current feedthrough using large-sized YBCO bulk conductors for the Phase II experiment of the LHD.