The optics of the fast ejections of the CERN PS

The horizontal dispersion of the PS ring is squeezed in each of the proton and pbar ejection regions simultaneously by means of two pairs of quadrupoles in order to allow the entire beam to pass through the limited aperture of the ejection channels to the SPS (Super Proton Synchrotron) collider. The resulting nonlocal modifications of the beta values and phase advances make this configuration very sensitive to the closed-orbit distortion and the betatron tune. Nonzero chromaticities and orbit distortions introduce considerable dispersion variations across the large momentum spread of the short bunches required by the collider. This results in incoherent horizontal betatron oscillations upon their injection into the SPS ring. A recent upgrade of the pole face winding system and the introduction of two closed orbit correction dipoles allow the correction of both the closed-orbit and the chromaticity in the PS ring. These corrections increase the stability of the dispersion function, thus improving the ejection efficiency and suppressing the emittance blow-up