LHC Injection Dynamics during the HL-LHC Era

As a part of the High-Luminosity (HL) upgrade of the Large Hadron Collider (LHC) at CERN, the nominal bunch intensity will be doubled from 1.15 x 10^11 protons per bunch (p/b) to 2.3 x 10^11 p/b at injection. The increase in bunch intensity will require radio frequency (RF) power exceeding what the currently installed LHC RF system can provide. The higher RF power requirements for HL-LHC are due to two reasons. Firstly, the higher bunch charge induces significantly higher beam loading, resulting in a peak power demand beyond the 300 kW the current LHC klystrons are designed to provide. Moreover, large spreads in RF power over the 16 LHC klystrons have been observed in operation in the past. This makes projections of the ultimate performance of the LHC RF system very uncertain in terms of RF power. The second reason is that a higher total RF voltage is required to capture the longitudinal halo particles of the HL-LHC beams, which will have a larger momentum spread at injection, and to counteract debunching effects as the beams circulate at injection energy. Most of the particles that are uncaptured and the ones that leak out from the RF buckets will be lost when the magnetic field is increased to bring the beams to collision energy. If the RF voltage is insufficient, then these beam losses can cause the machine protection system to trigger an abort of the beam and send it to the beam dump. Simulation models of both the beam at extraction energy in the Super Proton Synchrotron (SPS), and at injection energy in the LHC, have been implemented to simulate both the uncaptured beam and evaluate the associated RF power requirements. The RF power available from each of the 16 klystrons is evaluated via precision measurements of the RF voltage and the loaded quality factor. The maximum RF voltage achievable with the HL-LHC bunch intensity is then evaluated using the simulation models. Measurements and studies done during the LHC Run 3 are also presented and used as a basis for projections for HL-LHC operation. Furthermore, an extensive campaign of measurements has been carried out by capturing high-intensity beams during machine development (MD) sessions. Finally, projections are made using the combination of the simulation models, the results from the calibration of the LHC RF system, the operational measurements, and the high-intensity MD sessions performed during Run 3. Based on this work, it was found that accelerating the beam with the current RF system and operational conditions will significantly impact machine availability. However, reliable HL-LHC operation will be possible with mitigation methods like the installation of higher-efficiency RF amplifiers, increasing the dump thresholds of the LHC machine protection system, and the operational optimization of the current RF system.