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.
Flux pinning properties in high-temperature superconductors are significantly in fluenced by flux creep, especially at high temperatures and in high magnetic fields. One of the results of flux creep is an appreciable degradation of the maximum magnetic field for the non-resistive transport current from the upper critical field. This characteristic field, called the "irreversibility field", can be theoretically analyzed using the flux creep model. The important parameter that determines the creep phenomena is the pinning potential given by the product of the pinning energy density and the volume of the flux bundle. In this article, various dependencies of the irreversibility field on temperature, flux pinning strength anisotropy and size of the superconductor, and electric field at the time of measurement are reviewed according to the prediction of the flux creep model. These aspects are compared experimentally. An example of theoretical estimation of the irreversibility field is shown.