New tune measurement system for the ESRF booster

The injection of electrons in the ESRF storage ring is performed at full energy, i.e. 6 GeV. A linear accelerator provides the booster with a beam at an energy of 200 MeV. During the accelerating cycle of the booster, from 200 MeV to 6 GeV, the tune of the electron beam varies according to the non-proportionality of the magnetic field in the quadrupole magnets as compared to the dipole field. This is mainly due to the harmonic content of the current in the magnets which differs with the load of these systems resonating at 10 Hz and their saturation level. In order to measure the fractional part of the tunes all along the accelerating cycle, (50 ms) it is necessary to acquire the beam position at a rate of at least one sample per turn, each μs. A set of 48 tune values can be extracted from this record with an accuracy of better than 10-3. (authors)