Convergence studies of D-shaped coil/bobbin interactions in a sweeper magnet system

For high field solenoids and beamline dipole magnets, structural reinforcement, while complicated, is relatively straightforward due the high degree of symmetry. In high field, large gap dipoles there frequently is a lower order symmetry. Consequently, the stress state is likely to be more complicated and to require higher precision analysis than is frequently available. Results of a systematic parameter study of a large gap superconducting dipole being constructed by the NHMFL are presented. The magnet consists of two D-shaped NbTi coils and will provide a peak mid-plane field of 3.95 T in a 140 mm gap resulting in a 6.25 T peak field in the conductor and net forces on the coils legs of up to 1.45 MN each. Extensive model-based computer analyses have been applied for optimizing the shape of the coils and the stainless steel bobbin to reduce the strain to an acceptable level while maintaining overall field quality and a reasonable fraction of critical current. Results obtained by applying common engineering practices are compared with those obtained by introducing additional modeling details. Thus, the data presented provide a justified basis for evaluating the adequacy of the alternative modeling approaches for the sweeper magnet and similar magnet systems.