Optimal injection voltage in the LHC

Finding the optimal rf injection voltage in the LHC is a trade-off between minimising capture and flat-bottom losses, which call for an increased voltage, and minimising rf power consumption as well as improving beam stability, both of which call for a reduced voltage. From the beam stability point of view, earlier particletracking simulations showed that the decrease of the injection voltage from 6 MV to 4 MV is beneficial. This reduction was performed in the LHC Run 2 over a three-week period in steps of 0.5 MV. The impact of different voltages and injection energy errors on the evolution of beam parameters such as bunch length and beam losses are analysed in this paper. Operation at 4 MV, maintained in the machine after the reduction period, is studied in more detail. The implications for the future High-Luminosity LHC operation with high-intensity beams are also discussed.