Measurement of chromatic effects in LEP

Abstract

The chromaticity produced by the insertions and the lattice of LEP is corrected in the FODO lattice of the arcs which has horizontal and vertical phase advances of 90 0 and 60 0 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 distribution 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 negative chromaticity produced in the straight sections and its correction in the arcs. The parasitic sextupole components are found to be spread uniformly in the arcs within the measurement errors. Under this condition one finds that they have a negligible effect on the dynamic aperture.

Applications of the 1000-turns orbit measurement system at LEP

“…Betatron phase measurements have been applied to check the chromaticity correction of the magnetic lattice of LEP [7] and the distribution of impedance of the vacuum chamber around the circumference of LEP [8]. The first was carried out by shifting the RF-frequency (which changes the length of the closed orbit and consequently the beam energy) and taking phase measurements at various beam energies.…”

“…Thus, precise measurements of the phase advance are obtained by harmonic analysis of the signal at each pick-up. This technique has been used to measure the beam energy dependence of the phase advance due to chromaticity [7] and the bunch current dependence of the phase advance due to impedance [8]. Finally, we describe a method to obtain the betatron function from the measured phase advance and report about its accuracy and applications at LEP [9,10].…”