Multipole spilldown in the SPEAR 3 dipole magnets

The main SPEAR 3 dipole magnets are 1.45 m long with a pole contour designed to horizontally deflect and vertically focus the electron beam. At the nominal beam energy (3 GeV), the field and gradient along the magnet centerline are 1.3 T and 3.3 T/m (k=-0.33 m/sup -2/), respectively. Due to the straight core construction, the beam passes through each dipole with up to 16.6 mm trajectory offset relative to the centerline. This paper describes a method used to characterize the spilldown effect from magnetic multipole fields as observed by the beam traversing the dipole magnets. Results of tracking studies utilizing the longitudinal variation of multipole fields are discussed.