Effect of the Higher Field Multipoles in Superconducting Magnets on the Good Field Aperture

Abstract

The magnetic field in superconducting dipoles and quadrupoles have undesired higher multipoles present. These higher multipoles are caused by deviations from the ideal configuration of current and iron that produces a pure dipole or a pure quadrupole field. End effects are a source of higher multipoles. Correction coils may be provided which can be used to correct the effects due to some of the undesired multipoles, particularly the lower multipoles. The higher undesired multipoles, for which there are no effective correction coils, may limit the good field aperture of the magnet. The higher multipoles will curve the working line, the dependence of the ..nu..-value on the momentum. The effects, that may limit the good field aperture, will depend on how the accelerator is operated. One effect that may limit the good field aperture is that the curvature of the working line due to the higher multipoles may make it difficult to control the transverse collective instability. For the case of the ISABELLE accelerator, one preliminary magnet design would lead to higher multipoles being present that could limit the good field aperture to a momentum acceptance of ..delta..p/p = 2%. It is found that the 20-pole multipole in the quadrupole playsmore » a major role in limiting the aperture, and a modification of the quadrupole design could lead to a good field aperture of about ..delta..p/p = 3%, due to this higher multipole effect.« less