Generation of magnetic fields over 21 T in a 61 mm clear bore using low copper ratio (Nb,Ti)3Sn conductors

A 21 T superconducting magnet system has been developed and improved at the National Research Institute for Metals in Japan. As the steady progress of high-T_c superconducting coils required a larger bore for testing their performance, a new innermost coil has been developed. We increased the available bore from 50 mm to 61 mm. The new coil employed three kinds of rectangular (Nb,Ti)₃Sn conductors. External copper stabilized these conductors, with a Cu:SC ratio of only 0.25. The new coil operated in a 160 mm diameter clear bore with a background field of 18 T in saturated superfluid helium, generating a central field of 21.5 T without any training behavior. A double walled dewar, the outer diameter of which was 60 mm, was also developed. With this dewar, the new system provides a temperature-variable bore of 50 mm