RF scheme to mitigate longitudinal instabilities in the SPPC

A large collider project CEPC-SPPC is under study with a global effort and a leading role from IHEP, with CEPC (Circular Electron-Positron Collider, Phase I) to probe Higgs physics and SPPC (Super Proton-Proton Collider, Phase II) to explore new physics beyond the Standard Model. The key design goal for SPPC is to reach 75 TeV in center-of-mass energy with a circumference of 100 km. As an important part of the SPPC conceptual study, the study on the longitudinal dynamics at SPPC has been conducted, with a focus on the RF scheme to meet the requirements for luminosity and mitigate the relevant instabilities. The longitudinal instability bottleneck associated with the longitudinal dynamics is the loss of Landau damping. To mitigate the beam instabilities, a higher harmonic RF system (800 MHz) together with the basic RF system (400 MHz) to form a dual-harmonic RF system is found helpful, which also increases the luminosity by producing shorter bunches. The preliminary results indicate that longitudinal impedance threshold can be increased.