Accuracy of the LEP Spectrometer Beam Orbit Monitors

At the LEP e+/e- collider, a spectrometer is used to de-termine the beam energy with a target accuracy of 10−4. The spectrometer measures the lattice dipole bending an-gle of the beam using six beam position monitors (BPMs). The required calibration error imposes a BPM accuracy of 1 µm corresponding to a relative electrical signal variation of 2 · 10−5. The operating parameters have been compared with beam simulator results and non-linear BPM response simulations. The relative beam current variations between 0.02 and 0.03 and position changes of 0.1 mm during the fills of last year lead to uncertainties in the orbit measure-ments of well below 1 µm. For accuracy tests absolute beam currents were varied by a factor of three. The envi-ronment magnetical field is introduced to correct orbit read-ings. The BPM linearity and calibration was checked using moveable supports and wire position sensors. The BPM triplet quantity is used to determine the orbit position mon-itors accuracy. The BPM triplet changed during the fills between 1 and 2 µm RMS, which indicates a single BPM orbit determination accuracy between 1 and 1.5 µm.