Identification and reduction of the CERN SPS impedance

The first SPS impedance reduction programme has been completed in 2001, preparing the ring for its role as an injector of the LHC. This action has eliminated microwave instability on the SPS flat bottom and later nominal beam could be delivered to the LHC. The High Luminosity (HL-) LHC project is based on beam with twice higher intensity than the nominal one. One of the important SPS intensity limitations are longitudinal instabilities with minimum threshold reached on the 450 GeV flat top. In this paper the work which was carried on to identify the impedance sources driving these instabilities is described together with the next campaign of the SPS impedance reduction planned by the LHC Injector Upgrade (LIU) project.