Control of field uniformity for a large superconducting storage ring magnet

A 1.45 Tesla, 14.2 meter diameter "superferric" magnet is in an advanced stage of construction at Brookhaven National Laboratory. This magnet will be used to store muons for a planned ultra-precise measurement of their anomalous magnetic moment g-2. This measurement requires a magnetic field uniformity of 1 PPM with a knowledge of the field over the muon orbits to 0.1 PPM. The methods built into the design to produce ultra-high field uniformity are described. Large deviations from the ideal circularly symmetric uniform shape of the iron flux path are required to accommodate transfer lines and superconducting current leads, as well as apparatus for beam injection. Shimming methods to correct for the perturbations due to these large holes are presented. The pole pieces consist of 36 closely fitting 10/spl deg/ arc sections butted together to produce a very good approximation to a continuous 360/spl deg/ ring magnet. However, in the case of a possible quench of the superconducting coils, significant eddy currents will be induced which will circulate within the confines of each 10/spl deg/ pole piece. At the great precision required, these eddy currents may leave very small but significant aberrations in the field even after they decay away, because of slight changes in the orientation of the magnetization. Surface coil possibilities to correct for this effect are described.<>