Reconsideration of Evaluation of Balance Voltages During a Normal Zone Propagation in the LHD Helical Coils

One-side propagation and recovery of normal zones were observed more than a few times in a pair of helical coils (H1 and H2) of the large helical device. Each coil is divided into three blocks ( H-I, H-M, and H-O) from the inside. When a normal zone propagates, balance voltages between H1 and H2 are induced in all the blocks because of a shift of current center from the superconducting wires to a pure aluminum stabilizer at the normal zone. The cross-sectional position of the conductor in which the normal zone propagates can be estimated from the difference among the balance voltages of the blocks. In the previous study, the conductor positions were estimated at the first or last turn in the third and fourth layers of the H -I under the assumption that the resistive voltage is in proportion to the normal zone length. This assumption is not precise because the transient high resistance is induced by slow current diffusion into the stabilizer. Considering the transient resistance of the normal zone, the position of the normal zone propagation is reconsidered. The estimated positions are in the first layer of the H-I for most of the cases.