Results of Stability Test in Subcooled Helium for the R&D Coil of the LHD Helical Coil

Helical coils of the Large Helical Device are pool-cooled superconducting magnets. The operating current is restricted below about 90% of the design current because a normal-zone has propagated dynamically at several times at almost the same current. In order to estimate the effect of lowering temperatures on the cryogenic stability, an R&D coil was made of the same conductor. The cryogenic stability of the R&D coil was examined in saturated and subcooled helium. A normal-zone was initiated by a heater inserted between the conductor and the layer to layer spacer. The propagation was detected by voltage taps. In saturated helium of 4.4 K and 0.12 MPa, the minimum current to begin propagation is 10.7 to 10.8 kA. It becomes higher at the lower temperature, and it exceeds 11.7 kA in subcooled helium of 3.5 K as a temperature inside the R&D coil.