Low-sextupole steering dipoles for the MIT-Bates SHR

The South Hall Ring (SHR) requires use of iron-core steering dipoles in the ring lattice, each capable of bending 1-GeV electrons by 1 mrad. Acceptable second-order aberration effects require a very low integrated sextupole strength. The authors have studied double-axis cos theta coils, single-axis current sheets with rectangular box frames, and conventionally wound box frames. The multipole content of each unit was determined by a fit to the measured two-dimensional field map. In conventional box frames, lower sextupoles were achieved by increasing the ratio of width/gap and by optimizing the shape of the pole plates of the frame box. The latest design of a box frame steerer meets the sextupole requirements for the SHR and has removable pole plates with convex edges with R=200 cm, where R is the radius of curvature of the pole plate ends. However, in order to minimize sensitivity of the coils to the hysteresis effects, all plates need to be annealed or steel of higher quality should be used.<>