Use of an elliptical aperture to control saturation in closely-coupled, cold iron, superconducting dipole magnets

The high fields permitted by superconducting windings result in saturation of closely-coupled iron in dipole and quadrupole beam transport magnets. Coupland suggested using a triangular cutout at the poles to reduce the change in the sextupole (b2) term due to saturation. The use of an elliptical aperture in a close-coupled dipole for the Relativistic Heavy Ion Collider (RHIC) has been studied using the BNL computer program MDP (a version of GFUN). The ellipse aspect ratio was varied while holding the horizontal (minor) radius constant. The proper aspect ratio gives no shift in b2 due to saturation, and a reduction in the b4 shift. A modification of the ellipse also reduces b4. The elliptical aperture introduces a large b2 term at low field which must be compensated for by the coil design. A practical coil design which does this for the RHIC magnet is presented