Measurement of the LEP coupling source

Early during the LEP commissioning it was discovered that there was an abnormally large horizontal to vertical optical coupling in the vicinity of the resonance Qx-Qy=-8. Precision beam measurements of the closest tune approach, of the coupling of horizontal to vertical orbits and of the coupling of dispersion were performed. These measurements pointed to a weak source within the dipoles, which was independent of energy, not constant around the ring, and which showed the symmetries of skew quadrupole and skew sextupole. Comparison with magnetic measurements showed that the source could be identified as being a magnetized thin layer of nickel used to clad the lead shielding onto the aluminium chamber. The LEP optical model modified according to the measurements, could predict well the coupling resonance. Demagnetization of a short machine section gave some improvement. (author) 6 figs