Numerical Investigation of Thermal-Fluid Characteristics and Helium Bubble Behavior in the 35.6 T All Superconducting User Magnet

In January 2026, a breakthrough 35.6 T user magnet was successfully developed by IEECAS. The thermal stability of such ultra-high field superconducting magnets is one of our critical concerns. When the magnets operate in liquid helium bath under high magnetic field, helium bubble may be trapped due to the diamagnetism of helium. This phenomenon degrades the heat transfer on the wall, leading to unknown temperature increases. In this study, we developed a coupled finite element method (FEM) of multi-physics model to evaluate the trapped region of helium bubble. For the electromagnetic part, we use the H-A formulation for no-insulation (NI) coil, incorporating the effects of both screening current and radial current on the thermal field. For the two-phase flow simulation, we combine the mixture model, the mass transfer model, and the RPI heat transfer model to conduct a detailed analysis of the helium boiling and bubble dynamics. This paper reports the computed thermal and flow field during the charging process of the 35.6 T ultra-high field metal-as-insulation (MI) magnet. It further evaluates the magnet loss and helium bubble evolution under different operating conditions, analyzing the influence of both loss and magnetic field on helium bubble behavior. The results indicate that screening current effect leads to a distinct mitigation on trapped helium bubble; magnet loss affects the trapped region size and gas-phase fraction of the bubble, while the magnetic field influences both the location and maximum extent. Through the simulation of helium bubble evolution, this study reveals the bubble-trapping mechanisms within high-field superconducting magnet system in liquid helium bath, thereby providing a theoretical foundation for developing the thermal management strategies of such magnet systems.