Mechanical Responses of a Combined Support Structure for a Nb3Sn Sextupole Magnet During Assembly and Thermal Cycle

The Institute of Modern Physics (IMP) in Lanzhou, China, is currently developing a complicated superconducting magnet system composed of four solenoids and six sextupole coils based on a sextupole-in-solenoid configuration. The magnet is made of a single Nb3Sn wire and designed for a fourth-generation ECR ion source operating at frequency of 45 GHz. To effectively prevent the wires motion from performance degradations and a quench, a pre-stress is essential for the superconducting magnet winding and strengthening. Focusing on the sextupole magnet's assembly and the pre-load procedure, a combined support structure with temporary bladders, load pads, and load keys, as well an aluminum shell was designed and fabricated in this work. A whole assembly process was fulfilled with a dummy structure at room temperature and a cooling-down and warming-up cycle to show its validity. Some low-temperature resistance strain gauges, together with a half-bridge compensation, were used to monitor strain of the aluminum shell during the pre-load control performance and thermal cycle process. A parametric study on the equivalent friction of the combined structure was carried out for the mechanical characteristics based on FEM simulation to show a reasonable agreement with the experiment measurements as a proper friction coefficient chosen. Finally, a real sextupole magnet was assembled with the combined support structure and tested for excitation. It was illustrated that a proper pre-load state realized by the support structure and a high excitation current about 800 A was reached so that the excited magnetic field gained 7 T as the designed value.