The Future Circular electron-positron Collider, FCC-ee, is a design study for a luminosity-frontier and highest-energy e+e- collider with a 91 km circumference. In a circular machine, the interactions between the beam particles and the residual gas in the vacuum chamber may degrade the beam quality, potentially affecting the beam lifetime and the collider luminosity, and cause local beam losses. In addition, experimental backgrounds may be increased. Ideally, the vacuum system must be able to keep vacuum conditions sufficiently good so that beam-residual gas interaction effects are tolerable for collider operation. This paper presents a study of the beam loss distribution arising from beam-residual gas interactions in the FCC-ee, together with beam-gas lifetime estimates.

During 2022, a dedicated study was undertaken at CERN, together with the FCC Feasibility Study collaborators, to propose a robust configuration for the FCC-ee arc half-cell. The proposed layout considers integration aspects of the elements in the arc cross section, both for the booster and the collider, as well as aspects related to powering, cooling and ventilation, supporting and alignment, optics, instrumentation, handling and installation. The interfaces between the arc elements and the straight sections have also been analysed. This paper summarizes the main conclusions of the assessment and reports the preliminary engineering analyses performed to design the supporting system of the booster and of the collider. A proposal for a possible mock-up of the arc half-cell, to be built at CERN in the next years, is also presented.

In the FCC-ee study, it is proposed that electron and positron beams circulate at high current and high energy in a 92-km circumference twin ring. The present operational scenario foresees a first running step at an energy of 45.6 GeV and around 1.4 A current, which would generate copious amounts of synchrotron radiation (SR) power and flux. To guarantee a quick decrease of the photon desorption yields and so a fast vacuum conditioning, it has been proposed to use localized SR absorbers along the vacuum chamber, spaced about 5 m apart. This would also help contain the high-energy Compton-scattered secondaries once the beam energy is increased up to 182.5 GeV, later in the experimental program. In the preliminary design of FCC-ee vacuum chamber absorbers presented in this work, the SR thermal power is intercepted along around 100 mm of slanted surface. The temperature distribution in the adsorbers is estimated by Finite Element Analysis (FEA) and needs to be assessed to avoid any liquid-gas phase change within the water-cooling circuit. The cooling channels contain a twisted tape that increases the turbulence of water. This results in the desired heat transfer coefficient. The mechanical deformations due to the non-uniform temperature map are presented and analysed as well.

A beam gas curtain (BGC) monitor has been designed to obtain information about the relative position between the LHC proton beam and the hollow electron lens electron beam through a minimally invasive process. Its working principle relies on intersecting the path of both beams with a supersonic gas curtain, introduced transversely into the LHC beamline, to produce a fluorescence signal. As an intermediate project stage (phase II), a preliminary version of the BGC monitor has been installed into the LHC beamline. To ensure the successful integration of the monitor and subsequent operation under LHC ultrahigh vacuum conditions, a series of vacuum studies have been performed. These can be classified as follows: An off-line laboratory test campaign, to assess BGC behavior during pump down and gas injections; simulations and analytical calculations, to evaluate BGC behavior and estimate the impact of its installation and operation in the LHC. This document will briefly present the off-line tests campaign, followed by a more extensive description of the simulations performed.

In 2022, the FCC Feasibility Study management mandated a working group to analyse the best configuration of the FCC-ee tunnel in the arc regions, in view of the construction of a mock-up of the arc half -cell. One of the main and most challenging goals of the study, named FCC-ee Arc Half -Cell Mock-up Project, was to perform a preliminary investigation on the principles of supporting the Short -Straight Sections and dipoles of the half -cells, both for the booster and for the collider machines. This is an important input needed for the choice of the best configuration of the relative placement of the booster with respect to the collider. The structural stiffness, mass and stability of the supporting structures must be optimized to minimize the vibrations transmitted/transferred to the magnetic system of the accelerators by elements such as pumps, water cooling system, beam thermomechanical stresses, powering elements, etc. To perform the study, tools such as CAD software, FEM and analytical techniques were employed. This paper summarizes the preliminary design concepts and the results of the simulations performed.

Chapter 12 in High-Luminosity Large Hadron Collider (HL-LHC) : Preliminary Design Report. 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.

Modern particle accelerators, synchrotrons, and other machines requiring UHV frequently make use of NEG coatings to achieve low pressures and resulting high beam stability and lifetime. It is the nature of NEG pumps and coatings, that a limited amount of gas can be pumped before saturation is reached, at which point the NEG must be re-activated to pump efficiently again. While the pumping mechanism and absorption limits are well understood and software packages exist for simulating the steady-state vacuum systems, there is a need for simulating the temporal evolution of the pumping capability of NEG coatings in regions of high outgassing rates such as near crotch absorbers. This paper presents a simple framework, written in Python, for running time-dependent simulations of NEG-coated vacuum systems in molecular flow, by making use of the command-line interface of the already widely used MolFlow simulation software. It also compares simulation results with accelerator pressure measurements.

Modern particle accelerators, synchrotrons, and other machines requiring UHV frequently make use of NEG coatings to achieve low pressures and resulting high beam stability and lifetime.It is the nature of NEG pumps and coatings, that a limited amount of gas can be pumped before saturation is reached, at which point the NEG must be re-activated to pump efficiently again.While the pumping mechanism and absorption limits are well understood and software packages exist for simulating the steady-state vacuum systems, there is a need for simulating the temporal evolution of the pumping capability of NEG coatings in regions of high outgassing rates such as near crotch absorbers.This paper presents a simple framework, written in Python, for running time-dependent simulations of NEG-coated vacuum systems in molecular flow, by making use of the command-line interface of the already widely used MolFlow simulation software.It also compares simulation results with accelerator pressure measurements.

A minimally invasive transverse beam profile monitor based on supersonic gas curtain technology and beam-induced fluorescence has been developed and demonstrated. The concept presented can be used to measure both the profile of the proton beam in the Large Hadron Collider (LHC) and the concentricity of the electron and the proton beams in the LHC hollow electron lens. In this Letter, the performance of such a monitor for a low energy electron beam is discussed, which paves the way for its wider implementation.

Molflow+ is a Monte Carlo (MC) simulation software for ultra-high vacuum, mainly used to simulate pressure in particle accelerators. In this article, we present and discuss the design choices arising in a new implementation of its ray-tracing–based simulation unit for Nvidia RTX Graphics Processing Units (GPUs). The GPU simulation kernel was designed with Nvidia’s OptiX 7 API to make use of modern hardware-accelerated ray-tracing units, found in recent RTX series GPUs based on the Turing and Ampere architectures. Even with the challenges posed by switching to 32 bit computations, our kernel runs much faster than on comparable CPUs at the expense of a marginal drop in calculation precision.

Electron clouds forming inside the beam vacuum chamber due to photoemission and secondary emission may limit the accelerator performance. Specifically, the electron clouds can blow up the vertical emittance of a positron beam, through a head-tail-type single-bunch instability, if the central electron density exceeds a certain threshold value, that can be estimated analytically. Using the codes PyECLOUD and VSim, we carried out detailed simulations of the electron-cloud build up for the main arcs and the damping ring of the FCC-ee collider, in order to identify the effective photoemission rate and secondary emission yield required for achieving and maintaining the design emittance. To this end, we present the simulated electron density at the centre of the beam pipe for various bunch spacings, secondary emission yields, and photoemission parameters, in the damping ring and in the arcs of the collider positron ring. To gain further insight into the underlying dynamics, the obtained spatial and energy distributions of the cloud electrons are illustrated as a function of time. In addition, we compare results obtained for two different secondary emission models (“Furman–Pivi” and “ECLOUD”), thereby indicating the uncertainty inherent in this type of study, without any prototype vacuum chambers yet available. We also point out a few situations where the two secondary-emission models yield similar density values. Finally, based on our simulation results for two different design variants, we conclude that the new parameter baseline of the FCC-ee will facilitate electron-cloud mitigation.

The FCC-ee is a very challenging accelerator project from the point of view of vacuum. Apart from the sheer size of the machine, a twin-ring of 100 km circumference, the vacuum system design must be capable of dealing with the low-energy 45.6 GeV, high-current version of the machine (the Z -pole) as well as the higher energy, lower current versions. The main difficulty is related to the very much different synchrotron radiation (SR) spectra of the Z -pole vs the other energies, in particular the ttbar at 182.5 GeV. The critical energy of the SR spectrum of the Z -pole is 19.5 keV, while the ttbar exceeds 1.2 MeV. It is particularly challenging in terms of shielding the beryllium chamber in the detectors, for the Machine Detector Interface (MDI) area. We discuss the evolution of the vacuum system design for the arc sections, and some new ideas on NEG-coating, SR absorbers, and pumping system, with the aim to build prototypes soon, in the framework of the FCC Innovation Study program. The design of the vacuum hardware depends on the choices made for the magnets, and the required shielding from high-energy radiation generated by the circulating beam interacting with the residual gas and the interaction of the intense SR fans with the photon. There is also an important collaboration with the engineering integration of the vacuum system in the tunnel, particularly considering the full-energy booster injector, which is not detailed here. We also briefly describe the raytracing montecarlo modelling efforts carried out in the MDI area, and its pumping configuration.

The Future Circular Collider study program comprises several machine concepts for the future of high-energy particle physics. Among them there is a twin-ring e-e+ collider capable to run at beam energies between 45.6 and 182.5 GeV, i.e. the energies corresponding to the resonances of the Z, W, H bosons and the top quark. The conceptual design of the two 100-km rings has advanced to what is believed to be a working solution, i.e. capability to deal with low-energy (45.6 GeV) high-current (1390 mA) version as well as the high-energy (182.5 GeV) low-current (5.4 mA) one, with intermediate energy and current steps for the other 2 resonances. The limit for all the versions is given by the 50 MW/beam allotted to the synchrotron radiation (SR) losses. The paper will outline the main beam/machine parameters, the vacuum requirements, and the choices made concerning the vacuum chamber geometry, material, surface treatments, pumping system, and the related pressure profiles. The location of lumped SR photon absorbers for the generic arc cell has been determined. MACHINE AND VACUUM PARAMETERS All FCC-ee machine versions generate a copious flux of SR, in particular the low-energy high-current one. The relevant machine and vacuum parameters are listed in Table 1 [1]. The dynamic gas load is calculated assuming a customary value for the photo-desorption rate of 1·10-6 molecules/ph, and it is proportional to the photon flux. The critical energy of the SR spectrum of each machine, for the arc dipoles, is 19.5, 105.5, 356.2, and 1253.1 keV, respectively. Only for the low-energy version the critical energy is well below the Compton edge for copper or aluminium (100~200 keV), the candidate vacuum materials. The dynamic gas load Q’ in Table 1 does not include the contribution from Compton photons scattering back into the vacuum chamber and generating additional gas load. This effect had been observed in the LEP collider when the energy had been raised above 45 GeV [2]. This additional gas load will be evaluated in the future, once the design of the vacuum system is finalized, as it depends on choices related to the vacuum chamber and magnets materials and shielding arrangements for minimizing high-energy, large-angle Compton photon background scattering towards machine and tunnel equipment [3]. We also neglect any gas load contribution coming from either electron cloud (EC) in the e+ ring or ion-desorption in the ering, as we assume that these effects will be properly mitigated either via surface treatments given by low secondary electron yield (SEY) thin-films or surface textures (e.g. laserablation). We make use here of the results of the previous Horizon 2020-funded R&D program for the FCC-hh machine, where both laser-ablated and amorphous-carbon coatings were tested [4, 5]. All machine versions will have to absorb and dissipate efficiently the same amount of SR power, 50 MW. Table 1: Machine and Vacuum Parameters Beam Energy E (GeV) Beam Current I (mA) Photon Flux F’(ph/s/m) Dynamic Gas Load

A Hollow Electron Lens (HEL) is currently under development for the High-Luminosity upgrade of the Large Hadron Collider (HL-LHC). In this device, a hollow electron beam co-propagates with a central proton beam and provides active halo control in the LHC. To ensure the concentricity of the two beams, a non-invasive diagnostic instrument is currently being commissioned. This instrument is a compact version of an existing prototype that leverages beam induced fluorescence with supersonic gas curtain technology. This contribution includes the design features of this version of the monitor, recent progress, and future plans for tests at the Cockcroft Institute and the electron lens test stand at CERN.

The FCC-ee is a proposed future high-energy, high-intensity and high-precision lepton collider. Here, we present the latest development for the FCC-ee interaction regions, which shall ensure optimum conditions for the particle physics experiments. We discuss measures of background reduction and a revised interaction region layout including a low impedance compact beam chamber design. We also discuss the possible impact of the radiation generated in the interaction region including beamstrahlung.

Synchrotron radiation (SR) originated at superconducting bending magnets is known to be at the origin of several beam detrimental effects related to vacuum instabilities. One of the major challenges in the design of the vacuum beam pipes of high-energy hadron colliders is the SR coping strategy. In the case of the future circular hadron collider (FCC-hh), a Cu-coated beam screen (BS) operating in the range of 40-60 K has been designed with the aim of protecting the superconducting magnet cold bores from direct synchrotron irradiation. In order to experimentally study the FCC-hh BS vacuum and cryogenic performance, two sample prototypes were manufactured and installed in the beam screen test-bench experiment (BESTEX) at the Karlsruhe Research Accelerator (KARA) at the Karlsruhe Institute of Technology (KIT). The emitted SR has a critical energy of 6.2 keV, very similar to the 4.6 keV of FCC-hh. Irradiation at both room (RT) and cryogenic (77 K) temperatures showed a significant reduction of the molecular photostimulated desorption yields (eta) of the FCC-hh beam screen compared to those of Cu samples. A first approximation of eta and its evolution with the photon dose accumulated on the FCC-hh BS prototype at 77 K allows to estimate that a machine conditioning period of similar to 1.2 months would be needed to reduce the photostimulated molecular density at the necessary levels to ensure a 100 h beam lifetime at nominal FCC-hh operation.

The presented document if the Letter of Intent for the Gamma Factory Proof-of-Principle Experiment.

A beam profile monitor based on a supersonic gas-curtain is currently under development for transverse profile diagnostics of electron and proton beams in the High Luminosity LHC. This monitor uses a thin supersonic gas curtain that crosses the primary beam to be characterized under an angle of 45 degrees. The fluorescence caused by the interaction between the beam and gas-curtain is detected using a specially designed imaging system to determine the 2D transverse profile of the primary beam. Another prototype monitor based on beam induced ionization is installed at The Cockcroft Institute. This paper presents the design features of both the monitors, the gas-jet curtain formation and various experimental tests, including profile measurements of an electron beam, using helium, nitrogen and neon as gases. Such a non-invasive online beam profile monitor would be highly desirable also for medical LINAC’s and storage rings as it can characterize the beam without stopping machine operation. The paper discusses opportunities for simplifying the monitor design for integration into a medical accelerator and expected monitor performance.