The Large Helical Device (LHD), the largest stellarator, has been operating for the research of fusion plasma since 1998. A toroidal field of almost 3 T is produced by a pair of pool-cooled superconducting helical coils. A normal-zone had been induced several times at the bottom of the innermost layers of the coil at currents higher than 11.0 kA. Upgrading the cooling system was investigated to improve the cryogenic stability. The effect of subcooling was examined using a model coil. The minimum current to begin propagation of a normal zone was increased from 10.8 kA in saturated helium to 11.7 kA in subcooled helium of 3.5 K. On the basis of these results, an additional cooler with two-stage cold compressors was installed at the inlet of the LHD helical coil in 2006. The inlet and outlet temperatures of the coil were successfully lowered to 3.2 K and 3.8 K, respectively, with a mass flow of 50 g/s. However, a normal-zone was induced near the top of the coil at 11.45 kA during continuous excitation from the zero current. In order to remove the additional temperature rise caused by AC losses during charging, the excitation method was revised to waiting cool-down at 11.0 kA, and excitations equivalent to 11.5 kA were attained. In addition, an average of 11.83 kA was attained by the current grading method among three blocks of the helical coil.
