Aiming to develop a combined superconducting magnet for a fourth-generation ECR source operating at 45 GHz at the Institute of Modern Physics (IMP) in Lanzhou of China, a significant gain in performance can be achieved by using Nb3Sn to allow solenoids and sextupole coils to reach a high field of 12 T. In consideration of special design of the sextupole-in-solenoid shape, the supporting structures with pre-tensioned aluminum cylinders named Bladder are used to bear the large loading and maintain the configuration during the magnet assembly and operation in cryogenic and electromagnetic environment. A dipolar structure based on an aluminum shell and home-made bladders were designed in this work. The mechanical characteristics of the dipolar support structure were explored numerically and experimentally. A 3D finite element modeling was developed to analyze the deformation and stresses in the structures during its assembly, cool-down and warm-up for simulating the practical operation conditions. The measurements on the strains profiles in the aluminum shell in the dipolar support structure also has been conducted using low-temperature resistance strain gauges combined with a half-bridge compensation method for temperature. Our results show that the pre-stresses induced in the support structures during the two stages of gas loading in the bladder and shrinkage during cool-down process reach the level for efficiently hustling the 12T superconducting ECR magnet. The numerical analysis is in good agreement with the measurements. Additionally, the mechanical behavior of the bladders during gas loading and cool-down is obviously dependent with the friction property between contact surfaces, which gives the main concerns in the design and optimization such special support structures under cryogenic environment.
Key words: Superconducting magnet, dipolar support structure, bladder, cryogenic environment, mechanical analysis