Electric Resistance Measurement of the KSTAR Superconducting Coils With Small Resistive Joints

An electric resistance of lap joints assembled with Korea Superconducting Tokamak Advanced Research (KSTAR) poloidal field 1 (PF1) superconducting coils and bus lines was measured by 4-wire method with some improvements to reduce induced noise in measurements at cryogenic temperature. Generally, a large current has to flow through the joints in order to measure its resistance by electrical methods; however, an induced voltage generated by a DC power supply is much higher than a voltage drop on the joints. The improved method used a bypass resistor installed in parallel to the DC power supply and the coils in order to naturally reduce ripples of the coil current. Transitional waveforms of the coil current and tap voltages were approximated by non-linear curve fitting with constraints based on an electric circuit equation. The resistance and inductance were evaluated by solving simultaneous equations of an applied voltage on the conductors. The induced noise reduction by this method was helpful to evaluate the joint resistance.