Design of special deflection magnet for 120 keV neutral beam injection system

The magnetic deflection system is a crucial component of the Neutral Beam Injection (NBI) system in magnetic confinement fusion, directly determining the safety and reliability of the entire NBI system. With the increasing requirements for neutral beam injection heating in magnetic confinement fusion, the development of deflection magnet with higher ion deflection efficiency and longer pulse adaptation capabilities has become a key focus of current research. The 80 keV injection beam energy of EAST-NBI faces certain challenges in addressing future high-parameter injection requirements. Under the demands of high energy, long pulse, and multiple beam currents, this paper proposes a novel deflection magnet design at 120 keV parameters. A detailed three-dimensional model of a specially-shaped deflection magnet based on six D⁺ beam source channels is developed. Additionally, the deflection performance of stray ions and the thermal-mechanical behavior of high-heat-flux components are analyzed. Finite element analysis has been employed to systematically investigate the trajectories of 120 keV mixed-energy particle beams through the deflection magnet and the corresponding three-dimensional spatial magnetic field distribution. Simulation results confirm that high-energy stray ions undergo effective magnetic deflection and are fully intercepted by the optimized ion dump structure. Concurrent thermo-mechanical analysis of high-heat-flux components reveals peak operational temperatures of 214 °C and maximum equivalent thermal stresses of 113 MPa, both well below the material safety limits. The integrated analysis of magnetic field distribution, beam trajectory, and thermo-mechanical results demonstrate that the proposed deflection magnet design successfully fulfills multi-objective requirements for residual ion management in high-power neutral beam injection systems. Moreover, the system exhibits robust operational stability under extended pulse durations, confirming its readiness for experimental deployment following manufacturing and commissioning phases.