Fabrication and Excitation of a Model Magnet Using Coated Conductors for Spiral Sector FFAG Accelerators

With the aim of verifying the winding technology required for fixed-field alternating gradient (FFAG) accelerator magnets, and also to verify the magnetic field accuracy as compared with the designed field distribution, a reduced-size test magnet, referred to as a “model magnet,” has been developed. The model magnet was designed to incorporate multiple REBa2Cu3Ox (REBCO) coils with characteristic shapes, namely negative bend parts and 3-D winding shapes, derived from a full-scale spiral-sector FFAG accelerator design which was accomplished in this study. The coils were wound using 4-mm-wide REBCO-coated conductors made by the ion-beam-assisted deposition metal organic chemical-vapor deposition method, and the total length of the conductor for the model magnet was about 2000 m. After winding, the coils were installed in heat-conduction shells made of an aluminum alloy and were directly cooled by a Gifford-McMahon (GM) cryocooler in a vacuum vessel. It was essential to wind the REBCO coils without degradations, such as local delamination of the superconducting layer, because the voltage generated at such degraded parts frequently increases during operation, and eventually results in thermal runaway. The superconducting properties of all coils were verified before and after installing them in the aluminum shells. This paper describes the assembly of the REBCO coils, as well as testing procedures and test results of the model magnet.