The CERN SPS Low Level RF: Lead Ions Acceleration

This paper is the third of a series of three on the Super Proton Synchrotron (SPS) Low Level RF (LLRF). Its focus is the upgrade concerned with the acceleration of Lead ions for injection into the LHC. Lead ions are far from relativistic at injection into the SPS. Therefore, the classic acceleration scheme at constant harmonic number (h=4620) does not work as the RF frequency swing does not fit within the cavity bandwidth. Fixed Frequency Acceleration (FFA) is therefore used. The upgraded LLRF uses a completely new implementation of the FFA, based on a Numerically Controlled Oscillator (NCO) implemented as an FPGA IP in the Controller of each cavity. In addition, the 2022 scheme for LHC ions filling calls for slip-stacking of two families of bunches, 100 ns spacing, to generate a 50 ns spacing after interleaving. The paper presents the key components for FFA and ions slip-stacking as implemented in the new system, together with successful first tests performed in Autumn 2021