MEASUREMENT OF CHROMATIC EFFECTS IN LEP

The chromaticity produced by the insertions and the lattice of LEP is corrected in the FODO lattice of the arcs which has hor-izontal and vertical phase advances of 900 and 600 for the LEP optics presently used. This is done with two horizontal and three vertical sextupole families. To check this correction scheme the non-linear chromaticities have been measured by observing the betatron tunes as a function of RF-frequency, i.e. of momentum deviation. The differences between the results and the expected effects of the sextupoles are interpreted in terms of octupole and decapole components in the dipole magnets. To check the distri-bution around the ring of the chromaticities and their corrections the betatron phase advances have been measured as a function of momentum deviation. This was done by observing an excited betatron oscillation for 1024 turns in all beam position monitors for different RF-frequencies. The results clearly show the nega-tive chromaticity produced in the straight sections and its correc-tion in the arcs. The parasitic sextupole components are found to be spread uniformly in the arcs within the measurement errors. Under this condition one ®nds that they have a negligible effect on the dynamic aperture. I