Mechanical Design of a Nb3Sn Superconducting Magnet System for a 45 GHz ECR Ion Source
Lawrence Berkeley National Laboratory in collaboration with the Institute of Modern Physics, Lanzhou, China, has developed a Nb3Sn superconducting magnet system for a fourth-generation electron cyclotron resonance (ECR) ion 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 in the order of 12 T, without exceeding the stress limits of any of the components—particularly, 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 pretensioned using Bladder & Key Technology (with water-pressurized bladders) during the magnet assembly. High thermal contraction of the shell attains the target preload level at cryogenic temperatures during operation without overstressing the coils during roomtemperature assembly. We present the optimization progression of the support structure, describe its main components and assembly procedure, and examine the predicted coil stress at each step of the magnet assembly and operation using the three-dimensional finite-element mechanical model.