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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., CERN Yellow Reports: Monographs, Vol. 10 (2020): High-Luminosity Large Hadron Collider (HL-LHC): Technical design report

During the filling of the Large Hadron Collider (LHC), it is desirable to keep the RF cavity voltage constant both in amplitude and phase to minimize the emittance blow-up and injection losses. To have a constant voltage and to minimize power consumption, a special beam-loading compensation scheme called half-detuning is used in the LHC, for which the cavity fundamental resonant frequency needs to be de-tuned from the RF frequency by an appropriate value. This, however, can result in fast coupled-bunch instabilities caused by the asymmetry of the fundamental cavity impedance. To mitigate them, a fast direct RF feedback and a one-turn delay feedback are presently used in the LHC. The semi-analytical model that describes the dynamics of the Low-Level RF system in the LHC shows that, depending on the mitigation scenario, the required transient RF power during injection could significantly exceed the steady-state value. This means that for High-Luminosity LHC (HL-LHC) beam intensities, one can potentially reach the limit of available RF power. In this paper, the model is described, and benchmarks with LHC measurements are presented. We also shortly revisit the damping requirements for the longitudinal coupled-bunch instability at injection energy, to find a compromise between longitudinal stability and RF power requirements for the HL-LHC beam., CERN Yellow Reports: Conference Proceedings, Vol. 9 (2020): Proceedings of the ICFA mini-Workshop on Mitigation of Coherent Beam Instabilities in Particle Accelerators, Zermatt, Switzerland, 23–27 September 2019

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.

Controlled emittance blow-up is a widely-spread method to mitigate beam instabilities in accelerators. This paper summarises the different methods used to generate and apply RF phase noise or RF phase modulation in the RF systems of the CERN synchrotrons. It also details machine by machine when and how different methods are used., CERN Yellow Reports: Conference Proceedings, Vol. 9 (2020): Proceedings of the ICFA mini-Workshop on Mitigation of Coherent Beam Instabilities in Particle Accelerators, Zermatt, Switzerland, 23–27 September 2019

Linear accelerator 4 (Linac4) is designed to accelerate negative hydrogen ions for injection into the Proton Synchrotron Booster (PSB). It will become the source of proton beams for the Large Hadron Collider (LHC) after the long shutdown in 2019–2020. Linac4 will accelerate H– ions, consisting of a hydrogen atom with an additional electron, to 160 MeV energy and then inject them into the PSB, which is part of the LHC injection chain. The new accelerator comprises an ion source and four types of accelerating structures. The particles are accelerated first to 3 MeV energy by a Radio-Frequency Quadrupole (RFQ), then to 50 MeV by three Drift Tube Linacs (DTL) tanks, then to 100 MeV by seven Cell-Coupled Drift Tube Linac (CCDTL) modules, and finally to 160 MeV by twelve Pi-Mode Structures (PIMS). A chopper line placed between the RFQ and the first DTL tank modulates the linac beam at the PSB injection frequency. Linac4 includes transfer and measurement lines up to the PSB injection, where the ions are stripped of their two electrons to leave only protons. Linac4 is 76 metres long and located 12 metres below ground. The first low-energy beams were produced in 2013 and after the commissioning of all accelerating structures the milestone energy of 160 MeV was reached in 2016. Linac4 will be connected to the PSB during the long shutdown of 2019–20, after which it will replace the 50 MeV Linac2 as source of protons for the LHC. The Linac4 is a key element in the project to increase the luminosity of the LHC during the next decade.

The Large Hadron Collider (LHC) is one of the largest scientific instruments ever built. Since opening up anew energy frontier for exploration in 2010, it has gathered a global user community of about 9000 scientists working in fundamental particle physics and the physics of hadronic matter at extreme temperature and density. To sustain and extend its discovery potential, the LHC will need a major upgrade in the 2020s. This will increase its instantaneous luminosity (rate of collisions) by a factor of five beyond the original design valueand the integrated luminosity (totalnumber of collisions) by a factor ten. The LHC is already a highly complexand exquisitely optimised machine so this upgrade must be carefully conceived and will require new infrastructures(underground and on surface)and over a decade to implement. The new configuration, known as High Luminosity LHC (HL-LHC), relies on a number of key innovations that push accelerator technology beyond its present limits. Among these are cutting-edge 11–12Tesla superconducting magnets, compact superconducting cavities for beam rotation with ultra-precise phase control, new technology and physical processes for beam collimation and 100 metre-long high-power superconducting links with negligible energy dissipation, all of which required several years of dedicated R&D; effort on a global international level. The present document describes the technologies and components that will be used to realise the projectand is intended to serve as the basis for the detailed engineering design of the HL-LHC.

The availability of the LHC ADT and RF systems in 2017 is presented in details, including high-and low-power RF and RF controls. A comparison with 2016 availability is performed. The full-detuning scheme, commissioned early this year, has been operational throughout the year and the first experience with this scheme is summarised. New operational diagnostics, implemented in 2017, are shown as well. Finally, the latest findings from beam dynamics studies and measurements, which have implications for the operation today and in the near future, are highlighted.

During the LHC Run 2, many advances have been made on the beam dynamics in the longitudinal plane. The controlled longitudinal emittance blow-up used in the acceleration ramp was improved and bunch flattening was implemented for bunch length control during collisions. In order to minimise RF power consumption, the capture voltage was optimised and the full-detuning beam-loading compensation scheme was made operational for the ramp and at top energy. Various experimental and simulation studies have helped to improve operation and prepare for the future runs at increased intensities.

The Run 2 operation and availability for the LHC RF system are presented, including year-to-year performance comparisons. In addition, the evolution of the present RF system is discussed, with improvements in diagnostics, operation, and software being highlighted. Lessons learned from the Run 2 RF performance are reviewed, with a focus both on limitations that were addressed during Run 2 and limitations that may have implications for Run 3 and beyond. Finally, an overview of operational parameters, spares management, and long-term developments is presented.

The operation of the LHC RF system and beam dynamics studies in 2016 are presented. A fault summary is given, showing a reliable operation. Power consumption and promising studies of the full-detuning scheme are discussed. Diagnostics and software improvements done or to be done are detailed. As for beam dynamics, important advancement has been achieved concerning loss of Lan- dau damping and bunch flattening in 2016. Open questions related to controlled emittance blow-up and how PS—SPS-LHC bunch—to-bueket transfer studies helped to improve LHC injection losses are shown as well. Finally, future improvements and studies are presented.

The LHC will resume operations next year with four weeks of dedicated commissioning of the machine with beam. This period will include all the measurements that are needed by the different systems in order to re-establish stable beam conditions after the end-of-year shutdown. In addition, the performance of the machine will be pushed by decreasing the beta-star. A series of measurements (optics, aperture, collimator settings, orbit, etc.) are needed in order to prepare and validate this new configuration. Before ramping up high intensities a dedicated scrubbing at injection will be scheduled. The machine protection intensity ramp up strategy for next year will be defined.

The operation of the LHC RF system in 2015 is presented. After a brief summary of the changes made during Long Shutdown 1 (LS1) and recommissioning in the beginning of 2015, operational RF parameters, new diagnos-tics, and failures related to low-power level RF and controls are detailed. Machine development studies, as well as simulation and measurement studies related to longitudinal beam stability and controlled longitudinal emittance blow-up have greatly advanced our beam dynamics knowledge about the machine. Finally, an outlook of plans for the 2016 is given.

Chapter 4 in High-Luminosity Large Hadron Collider (HL-LHC). The Large Hadron Collider (LHC) is one of the largest scientific instruments ever built. Since opening up a new energy frontier for exploration in 2010, it has gathered a global user community of about 7,000 scientists working in fundamental particle physics and the physics of hadronic matter at extreme temperature and density. To sustain and extend its discovery potential, the LHC will need a major upgrade in the 2020s. This will increase its luminosity (rate of collisions) by a factor of five beyond the original design value and the integrated luminosity (total collisions created) by a factor ten. The LHC is already a highly complex and exquisitely optimised machine so this upgrade must be carefully conceived and will require about ten years to implement. The new configuration, known as High Luminosity LHC (HL-LHC), will rely on a number of key innovations that push accelerator technology beyond its present limits. Among these are cutting-edge 11-12 tesla superconducting magnets, compact superconducting cavities for beam rotation with ultra-precise phase control, new technology and physical processes for beam collimation and 300 metre-long high-power superconducting links with negligible energy dissipation. The present document describes the technologies and components that will be used to realise the project and is intended to serve as the basis for the detailed engineering design of HL-LHC.

The paper presents the work done on the LHC RF system during Long Shutdown 1 (LS1). On the High Level side we have replaced a cryomodule (four cavities, beam 2), which could not operate reliably at the design voltage (2 MV per cavity). The upgrade of klystron collectors has been completed and new crowbar systems have been installed (solid state thyristors replacing the old thyratrons). On the Controls side, all RIO3 CPUs are beeing replaced and the new ones are now using Linux. The new FESA classes are being designed with FESA3. The consequences of the increased beam current (0.55 A DC compared to 0.35 A in 2012), the increased energy (physics planned at 6.5 TeV/c per beam), and the exotic bunch spacing (5-20 ns for the scrubbing beams) will be analyzed from an RF hardware point of view. A tracking code is being developed to understand the effect of coloured phase noise on the longitudinal bunch profile. The expected benefits are the optimization of the blow-up and the possible shaping of bunch profile (flatter bunches) to avoid beam induced heating and improve beam stability. Upgraded longitudinal bunch-by-bunch measurements are being implemented.

The LHC RF/LLRF system is currently setup for extremely stable RF voltage to minimize transient beam loading effects. The present scheme cannot be extended beyond nominal beam current since the demanded power would push the klystrons to saturation. For beam currents above nominal (and possibly earlier), the cavity phase modulation by the beam (transient beam loading) will not be corrected, but the strong RF feedback and One-Turn Delay feedback will still be active for RF loop and beam stability in physics. To achieve this, the voltage set point should be adapted for each bunch. The goal of these MDs was to test thefirmware version of an iterative algorithm that adjusts the voltage set point to achieve the optimal phase modulation for klystron forward power considerations.

The MD was performed on July 10th with the nominal RF setup. Two bunches of approximately 3e11 protons were injected in each ring, in buckets 17821 and 27541. Three fills were attempted with this intensity (2823-2825). There were issues with the longitudinal emittance blowup for Beam 1 during the first two fills. The blow-up did not work in filll 2823, whereas in fill 2824 it was started manually late in the ramp and was really active only at the beginning of flat top. The problem was traced to an RF controls issue. These issues inadvertently provided very useful information on longitudinal stability.

During the night of 12-13 September 2012 the LHC collided protons with lead nuclei for the first time, demonstrating the feasibility of hybrid collisions despite the basic two-in-one magnet design. The centre-of-mass energy was 5 TeV per colliding nucleon pair, "Stable Beams" were declared 9 hours after the first injection of Pb beams in 2012. The integrated luminosity delivered to the four large LHC experiments was sufficient to yield new physics results. Within the same fill, stable beams were declared twice more, with the collision points displaced longitudinally by ±0.5 m from their usual locations. We provide a general overview of this p-Pb pilot physics fill before focusing on beam data at injection energy and at flat-top, before stable beams for physics were declared. We monitored the beam parameters throughout the fill and present an analysis of their evolution based on a simulation of intra-beam scattering (IBS), synchrotron radiation and the consumption of the beam intensity by collisions ("luminosity burn-off"). We also present some considerations on beam-beam effects with unequal beam sizes and the pilot run is compared, in this respect, to expectations for the forthcoming physics run in January. This pilot run was a major step in the preparation of the physics run. However it was not possible to perform an additional feasibility test designed to clarify the limits to the intensity of two beams injected and ramped with unequal revolution frequencies. We describe the plan for this test and discuss the reasons why it could not be carried out.