Eddy current control in the AGS Rapid Cycling Booster accelerator magnets

The Booster requires highly variable magnet cycles. When B˙ is large, eddy current induced sextupole, etc., in the dipole vacuum chamber (VC) is large, with a much smaller contribution from magnet ends. Simple passive coils excited automatically by transformer action cancel the B˙ induced sextupole. A self correction coil is not required for the quadrupoles, since B˙ induced aberrations are very small (<1.0×10^-4 at full aperture). Iron magnetization does not produce dipole or quadrupole magnet multipole aberrations, so these magnets have been effectively made independent of unwanted multipoles for all cycles. However, variations in the transfer functions and thus the Booster tune have not been automatically eliminated. Iron magnetization contributions are almost matched, but the B˙ induced field retardation in the dipoles VC is larger than in the quadrupoles. Results of measurements will be presented, plus a simple system to overcome the mismatch and make the tune independent of B˙. Properties of special lattice magnets and their corrections will also be described