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

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 Nb3Sn 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 dB/dt and the increment speed enhances at a certain value of dB/dt, where more strands are thought to participate in current induction. It is found that the emf voltage cancellation between double loops can occur and then the current induced by the remaining 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 no.1 scenario and the test no.2 scenario are additionally developed and assessed. Local quench is anticipated to occur during the test no.1 scenario operation. The test no.2 scenario as a realistic one to achieve an advanced tokamak mode operation is found to be acceptable in both current non-uniformity and AC loss, and recommendable instead. It is found that the relaxation length of the induced current does not depend on an external magnetic-field ramping slope but on the field ramping time. The relaxation length in the busline NbTi CICC is 10 ∼ 17 times longer than in the TF Nb3Sn CICC. The results of quench analysis using Gandalf code show that the TF conductor is recovered in every operation scenario. The effect of AC loss is found to be substantially low compared to that of current non-uniformity. Recommendations to ensure stable operation are also described in this dissertation. The analysis method developed throughout this study can be applied to similar systems of other superconducting tokamaks