Design and fabrication of pool cooled helical coil as an R&D program for Large Helical Device

A pool-cooled NbTi superconducting helical coil system (named TOKI-HB) has been fabricated as part of one of the research and development programs for the Large Helical Device (LHD). The main purpose of the construction of this device is to build an entire pool-cooled superconducting helical coil system which is large enough to be extrapolable to LHD. The helical coil has a major radius of 0.8 m and a minor radius of 0.2 m with a helical pitch number of 3. An operating current of 8.9 kA produces a maximum magnetic field of 0.75 T at the geometrical center of the poloidal cross section and 3 T at the coil surface. Basic design concepts of the TOKI-HB device are described together with some issues of the fabrication.