Numerical Simulation of One-Sided Normal Zone Propagation in Large Helical Device Conductors

Abstract

The large helical device (LHD) at the National Institute for Fusion Science (NIFS) is the world's largest heliotron-type superconducting magnetic device for nuclear fusion. At the start of the LHD experiment, a phenomenon in which a finite length of the normal zone propagates only to one side, is observed in the region below the rated operating point. The LHD conductor comprises a NbTi/Cu Rutherford conductor, pure aluminum stabilizer, and copper sheath. The aluminum-stabilized conductor is known to exhibit anomalous magnetoresistance due to Hall currents flowing in its cross-section. To simulate the one-sided propagation phenomenon, a quench simulation is performed, considering the Hall effect. A difference is observed in the current-transfer length from the Rutherford conductor to the aluminum stabilizer and copper sheath between the left front and the right front of the normal zone, and the Joule heat in the Rutherford conductor immediately after the normal transition is asymmetric. Under He II cooling (2 K) and a magnetic flux density of 6 T, the one-sided propagation phenomenon is numerically simulated in the transport current range of 15–17 kA. These results are in good agreement with the experimental results.