Synchronous phase shift measurements for evaluation of the longitudinal impedance model at the CERN SPS

The High Luminosity LHC (HL-LHC) requires 2.3 × 1011 protons per bunch (ppb) at LHC injection. For the SPS, the injector to the LHC, this goal requires a doubling of the injected intensity to 2.6 × 1011 ppb. Longitudinal instabilities were observed in the SPS for intensities below the required 2.6 × 1011 ppb. Identifying, and ultimately mitigating, the impedance sources driving the instabilities requires an accurate impedance model. Here, we report on measurements of the synchronous phase shift with intensity and corresponding energy loss at the SPS injection. Using the loss factor to compute the energy loss from the measured bunch spectrum and the SPS impedance model leads to significant disagreements with measurements. This issue is investigated for the simplified case of a single resonator. However, simulating matched bunches using the SPS impedance model yields better agreement with measurements.