Commissioning and operation of the LEP Pretzel scheme

From its start-up in 1989, LEP ran, as designed, with 4 bunches per beam and flat orbits. After 4 days of commissioning activity in October 1992, the regular mode of operation became 8 bunches per beam, thanks to a horizontal separation scheme of the “pretzel” type. Here we describe this transition, starting from the final stages of the machine studies which preceded it and concluding with the performance achieved by the end of 1992 and the factors limiting it. Some of the important steps were: the redistribution of betatron phase advance, minimisation of tune-splits between the beams, chromaticity changes, reduction of residual separations at the interaction points when running for physics at the Z⁰ energy, optimisation of pretzel separations at injection and in collision, and equalisation of the bunch currents. A number of changes to operational procedures and instrumentation were made. It was shown that, with suitable optics, the full beam polarization could be preserved on pretzel orbits. This allowed energy calibration by resonant depolarization and a measurement of the energy shift on pretzel orbits