Beam dynamics effects in the CERN SPS used as a lepton injector

When used as LEP (Large Electron Positron collider) injector, the CERN SPS (Super Proton Synchrotron) accelerates dense bunches of electrons and positrons. Due to the low injection energy the beams are prone to collective phenomena. However, the large spacing between bunches ensures that single-bunch effects are the only limitation. Measurements of the evolution of the longitudinal and transverse emittances, as well as observations of the transverse mode coupling instability and of the longitudinal microwave instability, are reported and compared to calculations and computer simulations. It is concluded that, by observing the transverse mode coupling instability and the longitudinal microwave instability of lepton bunches in the SPS, it is possible to evaluate the machine coupling impedance in a way which complements the measurements done previously with protons. The transverse mode coupling instability limits the maximum intensity per bunch, which, however, can be increased to three times the nominal value by artificially diluting the bunch longitudinal emittance