Modifications to the excitation characteristics of Fermilab Main Injector dipoles by machining

Twelve dipole magnets were built under the R&D phase of the Fermilab Main Injector project using steel from a different vendor than the production vendor. These dipoles exhibited an excitation-dependent difference in strength relative to the production magnets, with a maximum difference of about 1% observed at high field. This difference was too large to allow them to be used in the project. From calculations based on the differences in the B(H) curves of the two types of steel, the high-field strength of the twelve R&D dipoles was reduced by machining the back-legs of the cores. The machining technique will be described, and excitation curves before and after machining will be compared.