Impedance Reduction in the CERN SPS as seen from Longitudinal Beam Measurements

The longitudinal single bunch (microwave) instability of the proton beam in the CERN SPS, observed for the first time in 1977, became an important issue with the high intensity and high quality beams required for transfer to LHC. The main sources of impedance causing this instability were found in 1996 from measurements of the spectrum of unstable single bunches. During the 2000/2001 shutdown the major part of an impedance reduction programme was completed. Comparative beam measurements from before and after this programme are presented. While results obtained with short captured bunches allow us to see the overall bunch stability improvement, measurements with long bunches and RF off allow this impedance change to be distinguished at particular frequencies.