Design of the PEP-II interaction region septum quadrupole

The PEP-II QF2 magnet is one of the final focus quadrupoles for the Low-Energy Ring (LER) and utilizes a septum aperture to accommodate the adjacent High-Energy Ring (HER) beamline. The LER lattice design specification calls for an extremely high field quality for this magnet. A conventional water-cooled copper coil and laminated steel core design was selected to allow adjustment in the excitation. The close proximity between the LER and HER beamlines and the required integrated quadrupole strength result in a moderately high current density septum design. The QF2 magnets are imbedded in a confined region at each end of the BaBar detector, thus requiring a small magnet core cross section. Pole face windings are included in the QF2 design to buck the skew octupole term induced by the solenoidal fringe field that leaks out of the detector. Back-leg windings are included to buck a small dipole component induced by the lack of perfect quadrupole symmetry in this septum design. 2D pole contour optimization and 3D end chamfers are used to minimize harmonic errors; a separate permanent-magnet Harmonic Corrector Ring compensates for remaining field errors. The design methods and approach, 2D and 3D analyses, and the resulting expected magnet performance are described in this paper