LEP1 operation, 1989-1995

In October 1995, the last run foreseen for dedicated Z production at CERN was performed in LEP, thereby bringing to a close the first phase of operation of the machine. A total luminosity of 200 pb-1 has been delivered to each of the four experiments, which together have recorded the decays of over 20 millions Zs. Machine performance has increased to the extent that a good weekend in 1995 saw as much luminosity delivered as in the whole of 1989. This improvement has been made possible by a combination of several things. Over and above general operational expertise, special care went into the treatment and stabilisation of the closed orbit in order to obtain reproducible high performances with vertical beam-beam tune shifts exceeding values of xy = 0.04. Both Pretzel and Bunch Train schemes have been introduced to double the number of bunches, and high-tune optics have been developed to produce low transverse emittances which allow operation at the beam-beam limit throughout physics runs. Included in the integrated luminosity are data taken off the peak of the Z resonance, to allow precise determination of the mass and width of this particle. Accurate measurements of the beam energy during these runs have brought to the fore some unusual effects.