Consideration of conductor motions in the helical coils of the Large Helical Device

A pair of helical coils of the Large Helical Device is a large pool-cooled superconducting magnet. Conductor motions are induced repeatedly at higher than about 60% of the design current of 13 kA in charging, and these disappear while holding the current or discharging halfway from the high current. This hysteresis could be caused by the friction. A normal zone can propagate at higher than 87% of the design current because of the excess heat generation by the slow current diffusion into a pure aluminum stabilizer. This propagation has been observed nine times in four years' operation. From the comparison of balance voltages of the three pairs of coil blocks, the positions of the conductor motions and the induced normal zones can be evaluated. The behavior of the conductor motions is discussed.