Energy dependence of beam-beam interactions at LEP

LEP has been operated for several years at beam energies around 45 GeV and more recently at beam energies of 65 and 86 GeV, thus covering about a factor of two in beam energies. Vertical beam-beam tune shifts exceeding 0.04 were reached at all energies. At the lower energies, emittance increase using wigglers was used successfully to avoid excessive beam blow-up and flip-flop from beam-beam interactions and allowed to operate safely at the beam-beam limit throughout fills lasting typically 10 hours. Significant non-Gaussian tails were observed in the vertical plane and have limited the maximum current in collisions. At the higher energies, the best performance in terms of luminosity and beam-beam tune shift demands higher currents per bunch and minimization of emittances. Vertical to horizontal emittance ratios below 0.5% have been achieved.