Normal-zone propagation in helical coils of Large Helical Device

Large Helical Device (LHD) has been in operation since 1998. Propagation of a normal zone has been observed 26 times in a pair of helical coils, named H1 and H2, of the LHD during the 21 years of operation. The only fourth propagation resulted in quick discharge due to the imbalance voltage higher than the preset value of 0.2 V, whereas the propagation in the other cases stopped within a few seconds. Each the coil is divided into three blocks, named H-I, H-M, and H-O, from the inside. Since the conductor of the helical coils consists of NbTi/Cu strands, a pure aluminum stabilizer clad with a Cu-2%Ni layer, and a copper sheath, the current center shifts from the superconducting wires to the pure aluminum stabilizer at the normal zone. Therefore, imbalance voltages between H1 and H2 are induced in all the blocks during propagation of a normal zone. The cross-sectional position of the conductor in which the normal zone propagates can be estimated from the difference of the imbalance voltages among the blocks. In 2001, pickup coils were installed along the helical coils by the pitch of 30 degree of the poloidal angle in order to detect the position of a propagating normal zone. The pickup coils detect the change in magnetic field by a shift of current center at the normal zone. The position and velocity of propagating normal zones were detected successfully 15 times after the 10^th propagation. Most of the normal zones were induced at the bottom of the coils, and all of them propagated to one side, which is downstream of the transport current, with recovery on the opposite side. As the results of investigation of all the data, normal zones are considered to have been induced in the conductor in the first or last turn of the first or second layer of the H-I block. Therefore, normal zones should be induced at the position under the worst cooling condition with large disturbance due to slippage of the conductor against the helical coil case. (author)