The High Luminosity LHC project requires 2.3e11 protons per bunch (ppb) at injection. The goal of the LHC Injectors Upgrade (LIU) program is to upgrade the whole LHC injector chain to deliver this high intensity beam. For the SPS, the target after the ongoing Long Shutdown 2 (LS2) is to capture beam with an intensity of 2.6e11 ppb with a loss budget of no more than 10%. This is a challenging goal for the SPS, since it means a doubling of the nominal LHC intensity. Particle losses during injection and on the flat bottom of the SPS are a strong limitation for reaching these high intensities. Furthermore, beam instabilities at flat bottom and during ramp may deteriorate the beam quality.
Beam measurements were conducted to study the main sources of these limitations. These measurements were also used to benchmark macroparticle simulations performed with the CERN longitudinal tracking code BLonD. Reliable simulations are required to predict beam performance after the ongoing SPS upgrades. In this seminar, we discuss various effects that need to be taken into account to correctly reproduce beam measurements in simulations. They include the realistic injected bunch distribution, the effect of the SPS low-level RF system (feedback and phase loop), and the full SPS impedance model. Finally, we present simulations of particle losses and instability thresholds for the high-intensity beam in the post-LS2 SPS.