Design and optimization of a dual-ring adjustable permanent magnet strong focusing system based on Halbach array

Abstract Electron beam additive manufacturing (EBAM) requires a focusing system capable of both precise focusing and dynamic adjustability. This paper presents the design, optimization, and experimental validation of a dual-ring adjustable permanent magnet strong-focusing system based on a Halbach array, intended to replace conventional electromagnetic solenoids in EBAM. To accommodate the distinct requirements of high-energy-density melting and uniform preheating/annealing, a system comprising three permanent magnet quadrupole units was developed. Each unit features two concentric rings of octagonal magnets arranged in a Halbach configuration, enabling dynamic beam spot adjustment through the relative rotation of the outer ring. The mechanical structure, including rotatable yokes and shielding end plates, was optimized to ensure precise motion and suppress fringe field interference. Beam matching simulations determined the optimal magnetic field parameters, and a prototype was fabricated using Samarium–Cobalt magnets. Comprehensive magnetic field measurements and particle tracking simulations confirmed the system’s performance: achieving a fine focus spot with a radius of 0.0905 mm for melting and a larger, uniform spot of 0.4418 mm for preheating/annealing. The measured field distributions deviated from simulations by less than 1%, validating the efficacy of the proposed quadrupole field adjustment scheme. This work demonstrates an energy-efficient focusing solution for EBAM, eliminating the need for high-power, water-cooled solenoid systems.