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.

I. Introduction

The superconducting coils and bus lines of the Korea Superconducting Tokamak Advanced Research (KSTAR) are interconnected with lap or strand-to-strand (STS) joints made of normal conductors; accordingly, an electric resistance of the joints still remains at cryogenic temperature. The arrangement of the coils and bus lines is illustrated in Fig. 1. The resistance of the lap joint is designed to be on the order of in order to reduce Joule heating at a large current, and actually the Joule heating of the joints seems too small to measure the joint resistance by calorimetric methods due to much larger AC loss according to measurements in the past campaigns. On the other hand, an inductance of the superconducting coils is significantly large in contrast to the resistance. Additionally, the inductance of the KSTAR coils may non-linearly increase at small electric current because the cable-in-conduit conductors (CICCs) of the KSTAR Nb3Sn coils are jacketed with Incoloy 908 exhibiting weak ferromagnetism [1], [2]. Fig. 1.

Arrangement of superconducting PF coils and bus lines. Large coils are installed in a cryostat, and connected to current leads cooled by liquid He with long superconducting bus lines. The superconducting coils and bus lines are interconnected with lap joints made of normal conductors.