Measuring the Resonance Structure of Accelerator Impedance with Single Bunches

We describe a new method used in the CERN SPS accelerator to measure the longitudinal impedance in the frequency range 100 MHz to 4 GHz. Single high intensity proton bunches were injected and their spectrum observed during slow debunching. The presence of different resonant impedances leads to line density modulation at the resonant frequencies. This instability reaches some maximum modulation amplitude which was recorded as a function of frequency for many bunches. Using sufficiently long bunches the SPS impedance structure was observed and previously unknown sources were identified.