New Methods of Improving the Orbit Determination and Stability at LEP

The orbit quality is of particular importance for a good performance of an accelerator. Beam parameters like the interaction rate of a collider or the level of spin polarization strongly depend on the orbit. Two new methods to improve the orbit measurement and stability are demonstrated. The beam position monitors in an accelerator are carefully aligned to the centres of the quadrupole magnets. A residual mechanical but also electronical offset can persist. This offset is measured by a beam based alignment method developed at LEP. The method and its installation are described. Measurement procedures as well as results for different magnet types and BPM electronics are shown. The impact of the BPM offsets on the achievable spin polarization level is outlined. The vertical orbit has shown large variations during LEP operation. Frequent orbit corrections were required to avoid a decrease of the interaction rate. Low-beta insertion quadrupoles were suspected to be the origin of the drifts. The positions of these magnets have been monitored by different systems. The movements are compared to the orbit variations and the correlation is shown. An orbit feedback based on the mechanical measurements has been put into operation to keep the LEP orbit stable.