An R&D approach to the development of long Nb3Sn accelerator magnets using the key and bladder technology

Building accelerator quality magnets using Nb₃Sn for next generation facilities is the challenge of the next decade. The Superconducting Magnet Group at LBNL has developed an innovative support structure for high field magnets. The structure is based on an aluminum shell over iron yokes using hydraulic bladders and locking keys for applying the pre-stress. At cool down the pre-stress is almost doubled due to the differences of thermal contraction. This new structure allows precise control of the pre-stress with minimal spring back and conductor over-stress. At present the support structure has been used with prototype magnets up to one meter in length. In this paper, the design of a 4-meter long, 11 Tesla, wind-and-react racetrack dipole will be presented as a possible step toward the fabrication of long Nb₃Sn accelerator magnets.