Experimental analysis on initial current decay characteristics of persistent-mode HTS coil by external alternating magnetic field

This paper deals with the current decay characteristics of a high-T_c superconducting (HTS) coil operated in persistent current mode (PCM) under alternating magnetic field. In superconducting synchronous machine applications, such as linear synchronous motor (LSM), the superconducting coils are designed to operate in PCM to obtain steady-magnetic field with DC transport current. This superconducting coil operates on a direct current, but it can be exposed to alternating magnetic field due to the armature winding. When the field coils are subjected to an external time-varying magnetic field, it is possible to result in a current decay for those coils in PCM operation due to electrical AC loss. In this research, a current decay measurement system, which consists of field coil and armature coil, was fabricated to verify current decay characteristics of field coil in PCM operation against external alternating magnetic field. The current decay rate for the field coil was measured by using a hall sensor as a function of amplitude and frequency of armature coil.