Comparison Between Active and Passive Shielding Designs for a Large Acceptance Superconducting Dipole Magnet

Comparison between active and passive shielding magnets for a large acceptance superconducting dipole magnet has been carried out. The two magnet designs have been studied to answer to the new requirements of Nuclear Physics experiments in order to get a momentum resolution of up to 10^-3 with 1 GeV/nucleon heavy-ion beams: A field integral of about 5Tmiddotm]], a very low fringe field, especially in the target region about 1 meter upstream from the magnet entrance, and a large acceptance defined by a maximum bending angle of 40deg together with an opening horizontal and vertical angle of +/-80 mrad for the neutrons and the charged fragments. To fulfil these requirements, we find that the iron shielding magnet needs a heavier mass, about 300 tons, compared to the active one, even though the total device volume is not so different (4 meter diameter). With iron the coil volume is divided by two (compared with the same current density value) but the stored energy decreases only of less than 20%. The trajectory reconstruction algorithms give results within the specifications for the kinematical reconstruction at the target in both cases, either with iron pole and yoke or with innovative shielding coils, as shown by the given figures of merit. The conclusion of this study is that we find here the limits of a traditional iron design and that the physics requirements open the way to the choice of an active shielding design