Development of high field Nb-Ti accelerator dipoles

A four layer, 5 cm beam tube aperture, 1-m, long model accelerator dipole has been built and recently tested at the Lawrence Berkeley Laboratory. The conductor for this dipole is graded. The cable used for the inner two layers has about 30 percent more superconductor than that in the outer two layers, so the conductors reach the short sample limit at nearly the same current. This magnet is the third of a series of high field dipoles under development at LBL and has been tested at 1.8 and 4.2 K in liquid helium at one atmosphere pressure. Because of the large forces exerted at high field the magnitude and distribution of prestress in the assembled coil is quite important. The stress in each layer was measured and adjusted quite closely during the assembly process. The magnet achieved 9.08 T at 1.8 K and 7.15 T at 4.4 K. These fields appear to correspond to the critical current limits of the conductors in the region of the splice between layers 3 and 4. Training behavior, ramp rate sensitivity and magnetic field measurements are described.