Studies for the SPS Travelling Wave Cavities Upgrade

The Super Proton Synchrotron\,(SPS) 200\,MHz accelerating system poses an intensity limitation for the planned High Luminosity\,(HL)-LHC upgrade due to excessive beam loading as well as higher-order modes\,(HOMs) contributing to a longitudinal multi-bunch instability. The mitigation of these HOMs together with a shortening of the cavities is therefore an essential part of the LHC Injectors Upgrade\,(LIU) project. A brief introduction to the accelerating structure as well as its present and future cavity configurations is given. First conclusions are drawn from lab measurements performed on three spare cavity sections regarding tuning and beam impedances. The following studies on longitudinal impedance are targeted towards the shorter cavity configuration used in the future. The particular difficulties inherent to the HOM-damping of this configuration are identified and illustrated. Taking these findings into account, a first improved HOM-damping scheme with regard to the scheme in use today is developed.