Mechanical Design And Experiment On The Nb₃Sn High-field Magnet Of A Superconducting Ion Source

The HIAF?High Intensity heavy ion Accelerator Facility?as one of the national projects for science and technology for the 12th 5-Year-Plan and the National Science Foundation's“Low Energy heavy ion Accelerator Facility?LEAF?”are undertaken by the Institute of Modern Physics of Chinese Academy of Sciences?IMP?.And the FECR?a Fourth-generation ECR ion source?,whose goal is to achieve the magnetic field required for operating at the microwave frequency of 45 GHz,is currently in development at IMP to meet the requirement of highly charged heavy ion beam's generation for HIAF and LEAF.The device will comprehensively challenge the existing bottleneck of the Nb₃Sn superconducting magnet technology.This paper presents the R&D work related to the superconducting FECR magnet,which focuses on some mechanical design and experiment on the Nb₃Sn high-field magnet.The main research contents and results are as follows.Because the FECR magnet is made from Nb₃Sn which is sensitive to the strain and there are strong Lorentz forces between the solenoids and sextupole coils,the mechanical structure design is a big challenge.In order to validity the reasonability and feasibility of the structure design and its simulation results,a half-length prototype of the FECR magnet was developed.Mechanical analysis of the half-length prototype for a superconducting FECR ion source with the ANSYS Parameter Design Language?APDL?and the magnetic field calculation software OPERA 3D was carried out.Its stress distribution and variation of the prototype during assembly,cooling down and charging were obtained.To test the performance of the individual sextupole coil after the process of winding,heat treatment and vacuum-impregnation,a mirror structure based on a Bladder&Key technology and aluminum shell-based structure which proved can be used to apply uniform pre-stress to the sextupole coil was devised.The contact analysis,multi-step load and the technology of“birth and death of element”in ANSYS were used to simulate the optimization process of the mirror structure during room temperature assembly,cool-down and magnet excitation with the 2-D electromagnetic-structural coupling finite element model.Furthermore,the 3-D analysis and optimization of the magnet mirror structure for testing Nb₃Sn sextupole coil were carried out.Based on these results,the engineering design of optimized mirror structure was drawn according to producing experience.The pre-loads system of the mirror structure is based on the use of bladder and key.In order to simulate the assembly procedures from bladder and key pre-stress at room temperature through cool-down and verify the proposed method with ANSYS,a dummy coil according to the size of the 1/2-length sextupole Nb₃Sn coil was got.The3-D finite-element mechanical model of the magnet Mirror structure for dummy coil was established and then FEA analysis was provided.Furthermore,based on the appropriate preload target and the low temperature strain gauge,the magnet mirror structure assembly and preload process were carried out,and compared with the simulation results.The results of the strain in the middle of shell and dummy coil exhibit a very good agreement with expected results calculated with the structural analysis program ANSYS,and the maximum error is less than 10%.The results verify the rationality of the Mirror structure design and the credibility of simulation results.The process of winding,reacting and impregnating for the half-length Nb₃Sn sextupole coil was described.Furthermore,based on the geometry measurements of the sextupole coil,influence of manufacturing tolerance on the pre-stress in the sextupole coil of the mirror structure was analyzed and assessed using the 2D finite-element mode.The radial pre-load system of the mirror structure is based on the use of the bladder and key,and the axial pre-load system is based on the use of the piston.Guided by the full range simulation of the 3D analysis results and the 1/2bridge strain measurement system,the mirror structure was assembled.Then the sextupole magnetic mirror structure were cooled and energized.The results showed that the magnet was energized to 800 A at one time ad sustained for more than one hour.And the prediction strain results on the outer aluminum shell as well as the axial rods are in better agreement with experimental ones.The successful test verify that this support structure is capable of applying the required preloads to the Nb₃Sn sextupole coil and the simulation results is reasonable.Meanwhile this will provide referece information for the new mirror structure design of the full-length Nb3Sn sextupole coil which is composed of the Nb3Sn–based superconducting magnet system for a 4th generation ECR ion source.