Excitation properties and cryogenic stability of helical coils for the LHD

The helical coils for the Large Helical Device are the world's largest pool-cooled superconducting coils in operation. These were expected to be cryostable up to 13.0 kA at 4.4 K on a basis of the measured recovery currents in all the short samples. However, a normal-zone was induced at higher than 11 kA repeatedly. It propagated to the finite length and recovered within several seconds except at 11.45 kA. Because of slow current diffusion into a pure aluminum stabilizer, a normal zone can propagate dynamically below the cold-end recovery current. The excitation tests have been carried out, and average 11.65 kA has been achieved by grading the current in the three blocks of the helical coil. The disturbance during excitations and the cryogenic stability of the coil are presented.