Measurements of impedance distributions and instability thresholds in LEP

The distribution of reactive and resistive components of the transverse impedance around the circumference of LEP has been obtained by exciting the beam and measuring the variations of phase advance and of closed orbit displacements with beam current. Tune shifts with current and instability thresholds were determined for two machine lattices with different beta functions and momentum compaction for various values of bunch length and synchrotron frequency. These measurements are particularly interesting for LEP2, which will require larger bunch currents to achieve the design luminosity.