Improvements of Current Decay Behavior of HTS Coils in Persistent Current Operations

In order to improve the current decay behavior of high-temperature superconducting (HTS) coils using silver-sheathed Bi-2223 tapes in persistent current operations, the effect of the shielding current on the current decay behavior has been studied for magnetically levitated superconducting coils in internal ring devices. Generally, the current decay behavior of HTS coils is evaluated by the inductance of the HTS coils, the joint resistance and electric field versus current density relationship of the HTS tapes. In this paper, the effect of shielding currents in HTS tapes is discussed. It is shown that improvements of their properties of HTS tapes, which are the n-value and the critical current, are not so effective to decrease the shielding current in HTS tapes. Numerical electromagnetic analysis using finite element method and experiments have been carried out. Furthermore, improvements of the current decay behavior using excitation techniques are discussed.