FEM-based plasma equilibrium reconstruction for transient and off-normal phases in KSTAR

In this study, we developed a finite element method (FEM)-based equilibrium reconstruction code for the KSTAR tokamak to analyze plasma behavior during highly transient phases, including early startup, pre-disruption, and vertical displacement events (VDEs), where conventional equilibrium fitting (EFIT) reconstructions often fail to converge. The code minimizes a weighted least-squares cost function fitting magnetic diagnostic signals, with the currents induced in the vacuum vessel and passive structures fitted simultaneously with the plasma current rather than prescribed through a separate eddy current circuit model. Validation against EFIT for stable limited and diverted discharges yielded plasma position agreement to within 20–40 mm, a level consistent with the intrinsic uncertainty of EFIT-based reconstruction during the current ramp-up and plasma shaping phase. Applied to the early startup phase, cross-validation with fast-frame camera imaging confirmed the code's ability to capture initial plasma breakdown well before EFIT becomes available. The reconstruction was further extended through the VDE and current quench phases, with the results cross-validated against halo current monitors, deuterium-alpha emission, and diamagnetic flux measurements, confirming the physical consistency of the reconstructed plasma evolution throughout the event. These results demonstrate the capability of the FEM-based reconstruction code to reliably track plasma behavior across a wide range of transient operational phases, and the framework can serve as a valuable tool for analyzing transient plasma phases in future tokamak devices.