Optimization of poloidal field system and advanced divertor equilibrium configuration design of EHL-2 spherical torus

The EHL-2 (ENN He-Long 2) project aims to achieve and validate the p-11B thermal fusion reaction by establishing a high-ion-temperature scenario plasma regime and developing the necessary physics and engineering foundation for next-generation spherical torus (ST). This study presents an improved equilibrium configuration and poloidal field (PF) system design for EHL-2, focusing on achieving stable X-point target (XPT) operation under stringent thermal and structural constraints. A self-consistent iterative framework integrating equilibrium, transport, and waveforms design was developed to ensure physical feasibility and engineering robustness. To enhance XPT stability, the secondary X-point was repositioned out of the horizontal extent of central solenoid (CS) coil, and divertor coil cross-sectional areas were increased to mitigate excessive temperature rise. The resulting PF system, comprising a CS coil and twelve PF coils supports high-ion-temperature scenario. The equilibrium optimization, based on a high-ion-temperature scenario current profile, revealed strong bootstrap currents near the plasma edge that sustain high elongation and equilibrium stability. Validated waveforms were derived with Shape Editor (SE), ensuring compliance with voltage-second balance, vertical stability, and thermal limits across all waveform phases. Furthermore, the feasibility of the fishtail divertor concept was explored through periodic modulation of selected PF coils, demonstrating potential for dynamic strike-point sweeping and improved heat load management. These results collectively provide a comprehensive design framework for achieving stable, high-performance operation in the EHL-2 spherical torus, advancing the pathway toward compact, high-temperature fusion reactors.