Development of cryogenic ferromagnetic iron for superconducting accelerator

The particle accelerator needs superconducting dipoles, which consist of beam tube, coils, yoke and helium flow channel in the container tube. The structural assembly is called cold mass and setup within a cryostat consisting of a cylindrical vacuum and thermal insulation. The dipole will be operated under the condition of 4K, a large magnetic field in space and a large current in the coil. The yoke material has a function of enhancing the central fields by returning magnetic path, magnetic-shielding to suppress disturbing field in the adjacent ring of magnets, and pre-stressing coils mechanically as collars. The yoke laminations were punched from 6.3mm in thickness from the newly developed ultra-low-carbon steel with less than 20ppm carbon concentration. The material performance is 220 MPa in 0.2% yielding stress, 300MPa in tensile strength, both at room temperature. To achieve quality control of the important high-field saturation magnetization, the chemical impurities were severely specified. As a result coercivity remained under control, ca. 0.6 at 4.2K. The manufacturing process was established by fine branking, carbon dioxide laser cutting method and magnetite-oxide formation. This material was adopted to the BNL-RHIC project and also the CERN-LHC project. (K. Kikuchi)