Dynamic beam based calibration of orbit monitors at LEP

The properties of an electron positron staorage ring are mainly determined by the quadrupole magnets. Highest luminosity and best performances are obtained when the beam is well centered in the quadrupole magnets. The quadrupole magnets are accurately aligned to form a reference system. Beam position monitors (BPM) are installed with tight mechanical tolerances with respect to the magnets to measure the closed orbit relative to this reference. Despite careful alignment and electrical calibration of the monitors, there can still be a residual offset between the monitor and the magnetic centre of the quadrupole. The system described measures this offset by a beam-based method (1,2,3).