Design and fabrication of the R&D facility for Large Helical Device

Two different forced-cooled superconducting coils (TOKI-TF and TOKI-PF) were designed and fabricated to develop field coils for the Large Helical Device (LHD). The conductor used in the coils was the NbTi/Cu cable-in-conduit superconductor (CIC) encased in a SUS316L conduit and cooled with 4.5 K supercritical helium. The TOKI-TF is a helical coil with a field period m=4 wound in ten layers with the CIC conductor of 8 kA at 2.77 T. Its major and minor radii are 0.9 m and 0.25 m, respectively. One of the aims of the TOKI-TF was to demonstrate helical winding of the CIC conductor with good accuracy required for the LHD helical field coil. The TOKI-PF is a ring coil with a radius of 0.71 m, consisting of two double pancakes wound with the conductor of 25.6 kA at 2.76 T. The TOKI-PF was successfully energized at full current and provided useful data on the coil performance under a ramp rate of 2000 A/s and a long dump with a constant of 20 s