Development of a 1/2-length prototype high field Nb3Sn magnet for the 4th generation ECR ion source第四代ECR(电子回旋共振)离子源高场Nb3Sn磁体1/2长度样机的研制

A 4th generation ECR (electron cyclotron resonance) ion source FECR (first 4th generation ECR ion source) is under construction at IMP (Institute of Modern Physics). Aiming to be operated with the microwave power of 20 kW at 45 GHz, a fully superconducting Nb3Sn magnet is to be developed to realize the optimum ECR plasma confinement. As the first superconducting ECR magnet that utilizes single-strand Nb3Sn (niobium three tin) wire for the winding of coils consisting of 4 sets of axial solenoids and one sextupole, it poses many challenges in the coil fabrication, cold mass assembly, magnet quench protection, and so on. With a shell-based structure and bladder-and-key technology, we have completed a 1/2-length FECR cold mass prototype. It has been fully tested when immersed in 4.2 K liquid helium. The critical issue in this work is to keep the brittle Nb3Sn wire safe during the complicated mechanical process and protect it against the high current high magnetic field excitation. In this paper, we will report on the details of how to design, fabricate, assemble, and test a high-performance Nb3Sn sextupole with the existence of sophisticated magnetic fields and stresses. The successful test of a high-current sextupole coil in a so-called mirror structure will be reported. The application of the bladder-and-key structure to the prototype and the performance will be presented. This research has also tackled the solutions to mitigate very intense flux jump that has caused severe challenges to quench detection and protection.