Mechanical Analysis and Validation of the Prototype of Nb3Sn Superconducting Magnet Structure With Dummy Coils for FECR Ion Source

The Fourth-generation Electron Cyclotron Resonance (ECR) ion source (FECR) Nb
₃
Sn superconducting magnet is being developed by Institute of Modern Physics (IMP), Chinese Academy of Sciences. In order to reduce the risk of development and address key technical challenges, a half-length prototype is designed. The prototype consists of six sextupole coils and two solenoid coils. To validate the assembly and loading procedures, the structure was assembled with dummy sextupole coils (Al 6061 cylinder) and dummy solenoid coils (extend solenoid stainless-steel formers) that replaced the brittle Nb
₃
Sn coils, then cooled-down to 77 K with liquid nitrogen and warmed up. The dummy coils are different from Nb
₃
Sn coils, the mechanical behavior of the structure in each step is analyzed with finite element software ANSYS. The numerical analysis model must be cross-checked with the tests and validate it. The evolution of the mechanical behavior was monitored via strain gauges located on different components of the structure (Al shell, dummy sextupole coils, and axial rods). We focus on the expected stresses within the structure after assembly and pre-loading, cool down, warm up and thermal cycle. The expected stresses were determined from the 3D finite element model (FEM) of the structure. Some low-temperature strain gauges, were used to monitor strain of the structure during the pre-load control performance and thermal cycle process. A comparison of the 3D model strain/stress predictions with the strain gauge data measurements is made. The coherence between the predicted contact pressure/strain/stresses with Fuji paper and the experimental gauge measurements will validate the FEM model of the structure. Once robust finite element analysis (FEA) numerical results are available, we can estimate and improve the final results of the real assembly with Nb
₃
Sn coils. So as to realize the stress management of the magnet.