Detection System for Propagating Normal-Zones With Pick-Up Coils in the LHD Helical Coils

Propagation and recovery of normal zones were observed several times in a pair of helical coils, which are H1 and H2 coils, of the Large Helical Device. In order to detect the position of a propagating normal zone, pick-up coils were installed along the helical coils by the pitch of 30 degree of the poloidal angle. The diameter and turn number of the pick-up coil are 99 mm and 10,000, respectively. They detect the change of magnetic field by current transfer between the superconducting strands and an aluminum stabilizer. The position and the velocity of propagating normal-zones were detected successfully 15 times since the pickup coils had been installed. They were induced 12 times in H1 coil and three times in H2 coil. Most of the normal zones were induced at the bottom of the coils, and all of them propagated to one side, which is the downstream of the transport current, with recovery from the opposite side. The propagation velocity can be estimated from the delay time of the peak voltage of the pickup coils. These obtained data are very useful to investigate the cause of the normal-zone propagation and cryogenic stability of the helical coils. Furthermore, this method is applicable for other superconducting magnets made of asymmetrical conductors.