Numerical Evaluation on Current Behavior of No-Insulation Coils With Parallel HTS Tapes

No-insulation coil (NIC) has been proposed for improvement of thermal stability. NIC can reduce heat generation due to a current distribution that bypass the defect when a local hot-spot is formed. However, a charge delay because of no turn-to-turn insulation is one of the important characteristics of NICs. To decrease the charge delay of NIC, a winding technique with parallel HTS tapes without insulation between layers has been proposed. However, the increase of the number of parallel layers causes the significant current imbalance between parallel tapes. In this paper, the relationship between the number of parallel tapes and the effect on charge delay reduction is investigated. In addition, the effect on the thermal stability and magnitude of central magnetic field and thermal stability due to the current imbalance is also verified. Both analyses are carried out for both NIC with and without defects. The analysis results show that charge delay of NICs is reduced according to increase of number of parallel tapes. Current imbalance between layers increases in accordance with the increase of the number of parallel tapes. However, the analysis results also show that thermal stability and central magnetic field are not degraded by this imbalance current. In terms of NIC with the defect, thermal stability and fluctuation of central magnetic field are most improved by the effect of current sharing between parallel tapes when the number of parallel tapes is largest.