Characterization of a permanent magnetic dipolar system for the FOOT experiment

The FOOT (FragmentatiOn Of Target) experiment aims to measure double differential fragmentation cross-sections for applications in Particle Therapy and space radiation protection. A critical component of the apparatus is its magnetic spectrometer, composed of two Halbach-configured dipole magnets (M1 and M2) using NdFeB permanent magnets, designed for high field uniformity and stability. A full 3D magnetic model was developed using the OPERA solver, incorporating detailed BH curves and a refined meshing strategy to ensure precision along the beam axis. The resulting field map, essential for Monte Carlo simulations in FLUKA, was validated through high-resolution magnetic measurements along the longitudinal and radial axes using a Hall probe. The comparison revealed agreement within 1.4% over most of the field region. Radial scans confirmed uniformity within 1% up to 10 mm from the axis, with minor misalignments attributed to mechanical tolerances. These results confirm the accuracy of the magnetic model and its suitability for precise momentum reconstruction in the FOOT spectrometer.