Measurement of the Electric Properties of the KSTAR CS Model Coil

Abstract
The maximum current, the self inductance and the lap joint resistance of a split-pair superconducting pulse magnet were experimentally inspected. The pulse magnet mainly consists of two KSTAR central solenoid (CS) model coils, which are wound, continuous Nb$_3$Sn cable-in-conduit conductors (CICC) with Incoloy 908 jackets. The pulse magnet initially could not achieve its rated current over several days whereas the temperature of the magnet surfaces and coolant outlets stayed at 5 $sim$ 8 K. A training effect and a ramp rate limitation were suspected of being the cause at the beginning. However the magnet's supporting structure was found to have been exceedingly warm in that period, and the maximum current clearly corresponded to the temperature of the supporting structure. The self inductance of the pulse magnet was measured with an LCR meter at 0.1 $sim$ 1 kHz at room temperature, but the inductance was nearly double the design value. The self inductance was also measured by means of a decay time constant method at 5 K and, at values of current above 200 A, was found to be originally the same as the design value, however, it dynamically increased below that current. Supplemental calculation showed that the permeability of Incoloy 908 could have a detrimental impact upon the dynamic variation of the self inductance. The lap joint resistance was successfully obtained by measuring the voltage drops between two CICCs at 5 K. Another trial measuring the voltage drops between the two outer surfaces of the lap joint resulted in a nonphysical result that violated Ohm's law.
Keywords: KSTAR, CS model coil, Incoloy 908, Lap joint resistance, Self inductance, Maximum current

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