Current imbalance due to induced circulation currents in a large cable-inconduit superconductor
Abstract It has been previously reported by the authors that 30 kA NbTi pulsed coils (Demo Poloidal Coils; DPC-U1 and -U2) exhibit instability such as quenching at much lower currents than their critical level as a result of current imbalance in the conductor. In this paper, a theoretical study for such an imbalance in a large cable-in-conduit (CIC) conductor consisting of insulated strands is presented. This study indicates that significant circulation currents are induced in large CIC conductors, such as the conductor of DPC-U1 and U2, and remain for a long time because of the superconductivity of the strands. A large current imbalance is produced by superimposing the induced circulation current onto the transport current. It is also shown that the existence of an external field induces larger circulation currents, resulting in the larger current imbalance. For justification of these indications, characteristics of current imbalance are investigated from the experimental results. The magnitude of the current imbalance is evaluated as the ratio of the maximum strand current to the average strand current. This ratio was estimated to be 7.1 when DPC-U1 was charged singly, and reached about 15 when DPC-U1 was subjected to an external field from DPC-U2 and a test coil was installed between DPC-U1 and U2. Also, the time decay constant of the induced circulation currents was estimated to be around 2 h. These figures are interpreted by calculating the current distribution in the DPC-U1 conductor based on the assumption of asymmetric strand transposition of about 0.1% deviation in self inductances. It seems impossible to control such small asymmetry of the strand transposition in a commercial manufacturing procedure. Therefore, such instability as a result of current imbalance is inevitable in large CIC superconductors consisting of insulated stands. A similar instability may be caused in a large CIC superconductor when strands are coated with highly resistive material.
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