Stability Analysis of the KSTAR PF Busline

The Cable-In-Conduit Conductor (CICC) for the KSTAR buslines is made of NbTi superconducting (SC) strands. A busline consists of several electrical joints, which are the major heat load contributors to the busline cryo-system. In the poloidal field (PF) busline helium circuit, the supercritical helium is fed to the electrical joint of current lead end and comes out to the magnet terminal joint. This helium flow configuration has been verified to maintain the cryogenic stability of the buslines through the KSTAR operation. During the normal operation of the KSTAR PF coil, the heated helium coming out to both the coil and the busline meets at the magnet terminal and exchange heat, but the busline outlet temperature still remained less than magnet outlet temperature. As the buslines for the electrical connection in series of the upper and lower coils for PF1 and PF2 have the helium path through the two terminal joints of magnet, they experience higher temperature than the other buslines mainly due to the larger heat exchange. In this case, the connection buslines are considered to have very low safety margin and have the strong possibility of quenches.