Lawrence Berkeley National Laboratory (LBNL) in collaboration with the Institute of Modern Physics (IMP) is developing a Nb3Sn superconducting magnet system for a fourth-generation ECR source operating at the microwave frequency of 45 GHz. This paper presents a mechanical design capable of supporting the magnet up to the required operational level, resulting in peak coil fields of the order of 12 T, without exceeding the stress limits of each component, in particular the brittle Nb3Sn conductor. The coil system, based on the sextupole-in-solenoid configuration, is supported by a shell-based structure that uses an aluminum cylinder pre-tensioned with water-pressurized bladders during the magnet assembly. High thermal contraction of the shell allows reaching the target preload level at a cryogenic temperature without over-stressing the coils during assembly. We present the optimization steps of the support structure, describe its main components and assembly procedure, and we analyze expected coil stress at each step of the magnet assembly and operation using three-dimensional finite-element mechanical model.
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