MHD equilibrium reconstruction and a novel method to estimate plasma pressure by using sawtooth oscillation in EXL-50U spherical torus

In support of the strategic development of the EHL-2 (ENN He-Long 2) spherical torus, an MHD equilibrium reconstruction system based on EFIT (Equilibrium Fitting) code has been developed and validated for the EXL-50U device. By integrating calibrated external magnetic measurements with diagnostic benchmarking using distortion-corrected CCD (charge-coupled device) imaging and Langmuir probe signals, together with numerical sensitivity analysis, the system achieves a Last Closed Flux Surface (LCFS) reconstruction accuracy within ±2 cm relative to CCD and within 1 cm relative to Langmuir probe data. Analysis of divertor discharges exhibiting sawtooth oscillations indicates that while magnetic-only reconstructions are consistent with internal kink-mode instability criteria (q_0< 1), they tend to systematically overestimate the central plasma pressure relative to kinetic data. By introducing the sawtooth instability threshold as a supplementary physical constraint, this study resolves the inherent discrepancies between magnetic and kinetic datasets, enabling a higher-fidelity determination of core pressure profiles and establishing a robust foundation for advanced physics analysis in EXL-50U.