Saturation Effects in High-Field Superconducting Magnets

It appears desirable to surround a high-field superconducting dipole magnet with an iron shield. This gives about 45% more magnetic field for the same total current, and reduces the stored energy considerably. The leakage magnetic field is also reduced. Saturation of the iron shield at high fields introduces sextupole fields in the median plane which, although small, can introduce serious nonlinear effects in the particle orbits. The size of the sextupole fields can be reduced by proper choice of the thickness of the iron shield. Computer studies have shown that there exists an iron shield thickness which will make the sextupole field vanish at a particular field. This critical thickness of the iron shield is not too far from presently considered thicknesses which are based on other considerations.