Measurement of the Effect on Single Bunch Stability of Changing Transition Energy in the CERN SPS

The SPS is the final accelerator in the chain which will be used to fill the Large Hadron Collider (LHC), at present under construction at CERN. The most critical limitation for single bunch intensity in the SPS comes from the longitudinal microwave instability. One of the ways to raise the instability threshold is to decrease the transition energy which can be obtained by perturbing the dispersion function. In order to test expectations an experiment was performed in the SPS using the existing lattice. By adjusting the betatron tune at 26 GeV to be close to a multiple of the machine super perio d, a situation not acceptable for regular operation, the transition energy was reduced from 22 to 19 GeV. The results of this change on the beam were measured directly from the bunch spectrum.