Experimental results of the R&D forced-flow poloidal coil (TOKI-PF)

As research and development (R&D) of poloidal-field coils for the Large Helical Device (LHD), a forced-flow cooled cable-in-conduit-type NbTi superconducting coil (TOKI-PF) has been tested. The success of excitations assured us that the NbTi cable-in-conduit conductor can be adopted for the LHD poloidal coils. During cool-down, mainly the hydraulic characteristics were measured. The friction factor could be expressed by using an empirical formula. In the DC operations, training behavior was observed, like in a pool-cooled coil. The friction factor was also affected by the number of excitations, which may be related to strand movement. The stability margin and propagation velocity were also measured using an inductive heater. It became evident that the stability margin had a lower value when the operation current was higher than 15 kA. This current seems to correspond to the limiting current.