A permanent magnet dipole correction element for the Tevatron

We have constructed four Halbach-type permanent magnets for use as correction dipoles in the Tevatron. Each magnet consists of 16 trapezoidal prisms of Ferrite, arranged in a cylindrical shell. The magnets may be rotated freely about the beamline by a stepper motor, and are used in pairs to provide any field integral between 0 and 0.6 T-m, in any plane. The correctors buck the dipole component of the static magnetic field caused by the various toroids of the D0 detector, and thus prevent a closed orbit distortion in the Tevatron. The tight space constraints (6" square by 12" long for each pair) and difficulty of getting utilities beyond a dc control cable to the location led us to choose a permanent magnet rather than a Copper-Iron one. We describe briefly the principle of their design and operation. Next the mechanical design, motion control system and field quality are discussed. We conclude with a summary of our operating experience.< >