Fabrication of Three‐Dimensional HTS Coils for Accelerator Magnets

A project to develop fundamental technologies for accelerator magnets wound with high-T_c superconductors (HTS) is now in progress. The target applications of this project are fixed-field alternating-gradient (FFAG) accelerators for carbon cancer therapy systems and accelerator-driven subcritical reactors (ADSRs). A conceptual design study of superconducting coils for FFAG accelerators has been carried out. Minimizing the coil volume results in a complicated coil shape. The coils designed for FFAG accelerators consist of two characteristic sections: an aperture section and a side section. The aperture section has a negative-bend part and the side section has a saddle-like three-dimensional (3-D) bent part. When using tape-shaped HTS conductors, it is difficult to form complicated 3-D coil windings without adding strain to the conductors. A large strain results in deformation of the coil shape and a drastic decrease in superconducting properties, such as the critical current and then-index. To evaluate the superconducting properties of a 3-D coil, a small test coil having a negative-bend part and a 3-D bent part was designed and fabricated. The dimensions of the test coil were about 400 mm long and 440 mm wide. The total length of a 5 mm-wide YBCO-coated conductor used for the test coil was 14 m. The voltage-current (V-I) characteristics of the test coil were measured in liquid nitrogen. The n-index of the test coil was evaluated to be 37 in an electric field range of 10^-4-10^-7 V/m, indicating that the superconducting properties of the test coil did not decrease.