Stabilized operation of 30 kA NbTi Demo Poloidal Coil (DPC-U) with uniform current distribution in conductors

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Two 30 kA NbTi Demo Poloidal Coils (DPC-U1, -U2) were fabricated and tested in the Demo Poloidal Coil Project. DPC-U1 and -U2, referred to collectively as DPC-U, exhibited instability in pulsed and even in d.c. charge, such as coils quenching at much lower current than the design current which itself is still below the conductor critical current. It was found from the previous paper published by the authors that non-uniform current distribution is established in a large cable-in-conduit conductor, such as the DPC-U conductor, whose strands are electrically insulated from one another, due to an imbalance in the inductances of the strands. Also, it was clarified that the instability of DPC-U is caused by the current imbalance. However, it was also shown that the imbalanced distribution could be made uniform by creating a small resistive zone in the conductor by applying inductive heating pulses. In this paper, it is shown that the coil exhibiting instability due to imbalanced current distribution could be operated stably using this effect. In addition, it is shown that the DPC-U strands exhibited no deterioration in their designed critical current. This result indicates that the instability of DPC-U is entirely attributable to the current imbalance.