Effects of Subcooling on Lengths of Propagating Normal Zones in the LHD Helical Coils

Propagation of a short normal zone was observed in a helical coil of the Large Helical Device, when the coil was cooled with subcooled helium, of which the inlet and outlet temperatures are 3.2 K and below 4.0 K, respectively. The normal zone was induced at the bottom position of the coil. It propagated to only the downstream side of the current with recovery from the opposite side, and stopped after passing the outer equator of the torus. The induced balance voltage is obviously lower and the propagating time is shorter than those of propagating normal zones observed in the helical coil cooled with saturated helium at 4.4 K. According to the simulation of the propagation of a normal zone, it is considered that such a short normal zone at the current close to the minimum propagating current propagates without full transition to film boiling.