Longitudinal Impedance Characterization of the CERN SPS Vacuum Flanges

This contribution describes the thorough studies carried out to characterize the longitudinal impedance of the CERN SPS vacuum flanges, which are believed to be the main source of LHC beam instability. Around 500 high-impedance flanges of 8 different types have been identified. Three factors play an important role in the characterization of these flanges: the type of vacuum chambers that the flange interconnects, whether or not both sides are electrically isolated (by means of an enamel coating) and, finally, the presence of damping resistors which damp high-Q resonances. Not only, full-wave electromagnetic field simulations, but also RF measurements have been used to evaluate the impedance of these elements. The R/Q of the relevant resonances was measured using the well-known bead-pull technique. In particular, a subset of around 150 flanges has been found to be the source of a high-impedance resonance at 1.4 GHz, also observed in beam measurements. Guidelines on how to reduce the impedance of these elements are also presented.