Current non-uniformity and AC loss in superconducting tokamak coil joints, and their impacts on tokamak operation

Abstract

This thesis is about the study of non-uniform transport current by induced current in joints for superconducting tokamak coils made of multi-strand and multi-stage superconducting cable-in-conduit conductor (CICC). The induced current propagation into the CICC, AC loss generated during the propagation, and their effects on the stability of tokamak operation constitute the other main contents of the study. A strand-to-strand (STS) joint, which is a soldered joint of cylindrical shape for joining the KSTAR toroidal field (TF) coils, is a good material for this study. A system composed of the $Nb_3Sn$ CICC of a TF coil, the STS joint, and the NbTi CICC of an inter-coil busline is mainly investigated in this thesis. A numerical method adopting an infinite two-wire transmission line model is approached. The investigation has been carried out through two distinctive experiments using sample joint loop(s) connected to full-size NbTi CICC samples. Being different from the earlier expectation, the induced current does not distribute widely in the joint but exists locally. The flattop values of induced currents increase linearly with an increase of $\sl{dB/dt}$ and the increment speed enhances at a certain value of $\sl{dB/dt}$, where more strands are thought to participate in current induction. It is found that the $\it{emf}$ voltage cancellation between double loops can occur and then the current induced by the remaining $\it{emf}$ voltage in one loop propagates to the other. The stability around the joint is assessed with respect to current non-uniformity and AC loss due to the induced current under various KSTAR operation scenarios. Besides the existing reference scenario and the reference scenario accompanied by a plasma disruption, two other scenarios named as the test~#1 scenario and the test~#2 scenario are additionally developed and assessed. Local quench is anticipated to occur during the test~#1 scenario operation. The test~#2 scenario as a realistic one to ac...