Chapter 4: RF systems

The HL-LHC beams are injected, accelerated, and stored to their nominal energy of 7 TeV by the existing 400 MHz superconducting RF system of the LHC. A novel superconducting RF system consisting of eight cavities per beam for transverse deflection (aka crab cavities) of the bunches will be used to compensate the geometric loss in luminosity due to the non-zero crossing angle and the extreme focusing of the bunches in the HL-LHC. Due to doubling of the beam currents in the HL-LHC era, an optimal detuning scheme (aka full- detuning) is required to cope with the transient beam loading effects. A modulation of the klystron and cavity phase make the phase of bunches with respect to the RF clock to progressively slip along the bunch train, but then recover during the abort gap. With this scheme the klystron power is independent of the beam current and maintained constant over one full turn at the expense of bunch-to-bunch phase modulation. This scheme was experimentally tested in 2016 and operational since then in the LHC during the acceleration ramp and flat-top. During injection of the HL-LHC beams from the SPS in to the LHC, the original half-detuning scheme to strictly preserve the bunch-to-bunch spacing is a pre-requisite. The total available voltage with HL-LHC beams is therefore limited to approximately 6 MV with the available RF power at injection. Second harmonic RF system at 800 MHz for Landau damping and lower frequency accelerating RF system at 200 MHz in conjunction with the exiting 400 MHz cavities for improved capture from the SPS for intense and longer bunches were studied but are no longer considered for the HL-LHC.