Disruption simulations for 3D electro-magnetic load assessment of EXL-50U spherical torus towards full-performance operation

This study presents disruption simulations for the EXL-50U spherical tokamak in support of the 1-MA experimental campaign conducted in 2025. A two-stage approach, available through the PECAN framework, was employed to capture plasma dynamics and the resulting 3D electromagnetic (EM) loads. The assumed current quench (CQ) durations, bounded by limits derived from the ITPA Disruption Database, were later validated against the newly established EXL-50U disruption database, showing good agreement with experimental observations. The analysis revealed a distinct nonlinear behavior in the vertical displacement event (VDE) evolution, attributed to the combined effects of plasma shape variation and the stabilizing effect of the passive stabilizing plates (PSP). The simulations predict plasma–PSP contact consistent with observed PSP damage, emphasizing the need for enhanced vertical control. The computed EM loads highlight significant contributions from poloidal eddy currents interacting with the toroidal field, producing peak forces in the inboard vessel leg up to twice those from toroidal eddy currents—an interaction often overlooked in similar analyses in the field, yet one that should be carefully assessed in spherical tokamaks due to their inherently compact design. Overall, these results provide a detailed 3D EM load dataset for structural analysis and offer key feedback for disruption control and mechanical design for EXL-50U, while informing the development of ENN’s next-generation EHL-2 spherical tokamak.