Analysis of the Mechanical Behavior of the KSTAR CS Magnet During Long-Pulse Plasma Discharges

The KSTAR central solenoid (CS) magnet consists of four pairs of coils stacked vertically and eight preloading structures that compress the coils in the axial direction. The strain and dis-placement of the CS magnet are monitored in real time, limiting the minimum compression as an important parameter of the supervisory interlock system for the safe operation of KSTAR. An equivalent vertical force method for a simplified multi-spring system was developed to analyze the preload and displacement variations of the CS magnet. The equivalent vertical force is based on the plasma and poloidal field (PF) coils currents during plasma discharge. The estimated displacements by the developed method agreed fairly well with the measured displacements of the long-pulse plasma discharges in 2017 (#18437) and 2018 (#21735) campaigns. The analyzed displacement can be used to simply estimate the remaining preload of the CS coils. This approach will contribute to the development of advanced operation scenarios for large plasma current and long-pulse discharges. By applying this algorithm to the plasma control system, the CS magnet can be controlled more safely during KSTAR operation.