Performance Limitations of the CERN SPS Collider

The SPS has now accumulated more than 10 months of operation as a proton-antiproton collider spread over 3 long physics runs. During this time the peak luminosity has been pushed up to 3.5×1029 cm-2S-1 and the luminosity lifetime to almost 30 hours. Different physical phenomena limit the machine performance at various times during injection and storage. The peak luminosity is governed by the number of bunches per beam and the emittance and intensity per bunch as well as by the horizontal and vertical beta values at the experimental insertions. Although the transverse emittances and antiproton intensity are defined by the injector chain, the proton bunch intensity is mainly limited by the microwave instability in the SPS itself. The low-beta insertions have been pushed to the limit of available quadrupole strength and chromaticity correction capability at the maximum storage momentum of 315 GeV/c (ßH = 1m, ßV = 0,5m). The luminosity lifetime in the first few hours of storage is limited by the transverse emittance growth of the dense proton bunches due to intrabeam scattering1,2. As the store progresses, the proton bunch decay rate increases due to the growth of the longitudinal emittance (also through intrabeam scattering), finally becoming the dominant contributing factor in governing the luminosity lifetime. The beam-beam interaction limits the usable area in tune space to a very small region free of 10th order resonances3. Consequently, the total tune spread must be kept below 0.025, allowing only 3 bunches per beam without separation at the unwanted crossings.