Experimental Evaluation of Inductance and Its Impact on Quench Detection of the KSTAR Coils

Windings of Nb₃Sn cable-in-conduit conductors of the toroidal field (TF) and the 1st-5th poloidal field (PF) coils of the Korea Superconducting Tokamak Advanced Research (KSTAR) have Incoloy-908 jackets, which exhibit weak ferromagnetism. The inductance of the PF coils was measured by monitoring the induced voltages on the coils while TF coils were steadily energized at 20 kA, and one of each pair of PF coils was charged up to 4 kA at 2 kA/s. At a large PF current, the measured self inductance was very close to the design value. In contrast, the measured self inductances of the PF1-PF5 coils increased 90-135% by decreasing their current below 2 kA; whereas those of the PF6-PF7 coils almost stayed at the same levels. On the other hand, the measured self inductances were much smaller than the other measurements obtained at no TF current. The self induced voltage was almost cancelled out by using a single Wheatstone bridge of the quench detectors; whereas, the mutually induced voltage was cancelled out by numerically subtracting outputs of two Wheatstone bridges for coils, which were symmetrically located in the assembled coils.