Longitudinal impedance monitoring in the SPS with single short bunches at 26 GeV/c (RF on)

In the course of monitoring the low frequency impedance of the SPS over the years usually the quadrupole frequency shift as a function of intensity is measured with single bunches at 26 GeV/c with RF on using the peak detected signal. The additional acquisition of longitudinal bunch profiles allows the evaluation of various parameters of the injected beam, details of its quadrupole oscillation and the evolution of the bunch length as a function of time. Data acquired between 1999 and 2007 will be analysed in this respect. It will be shown that the bunch length data at 600 ms indicates clearly the effect of the SPS impedance reduction programme realised in 2000/2001 and that from then on the absolute value of the quadrupole frequency shift shows a tendency to increase over the years, indicating an impedance increase. However, it does not allow to monitor unambiguously the changes from one year to the next. The reason that the quadrupole frequency shift is not very well determined is attributed to the lack of reproducibility of bunch length and emittance at injection. In the future more attention will have to be paid to make these parameters reproducible.