CERN, Geneva
A first stage collimation system has been installed for the 2008 first beam commissioning of the LHC. It consists of 88 collimators distributed around the ring and the two injection lines. Each collimator has two jaws for which positions and angles must be controlled and monitored with high precision. The LHC collimation system was put into operation from July to October 2008. The installed system is described and the first results from system operation without and with beam are presented. In particular, it is shown that the LHC collimation system achieved the specified accuracy and reproducibility of jaw positioning. The next steps in collimation commissioning and the expected system evolution are described. Planned system upgrades for high LHC beam intensities are outlined.
FEL/Duke University, Durham, North Carolina
TUNL, Durham, North Carolina
A gamma-ray beam produced by Compton scattering of a laser beam with a relativistic electron beam has been used to determine the electron beam parameters. In the past, the electron beam energy and energy spread were directly fit from the high energy edge of a measured gamma beam spectrum using a gamma-ray detector. However, due to non-ideal response of the detector, the measured spectrum cannot represent the true energy distribution of the gamma-ray beam. Thus, the electron beam energy and energy spread could not be accurately determined from the measured gamma beam spectrum. In this paper, we will present a novel end-to-end spectrum reconstruction method to accurately extract the energy distribution of the gamma-ray beam from the measured gamma beam spectrum. Using this method we have accurately determined the energy and energy spread of the electron beam in Duke storage ring using a Compton gamma-ray beam from the High Intensity γ-ray Source (HIγS) facility.
Fermilab, Batavia
CERN, Geneva
IHEP Protvino, Protvino, Moscow Region
INFN-Ferrara, Ferrara
PNPI, Gatchina, Leningrad District
BNL, Upton, Long Island, New York
Università dell'Insubria & INFN Milano Bicocca, Como
Enrico Fermi Institute, University of Chicago, Chicago, Illinois
Funding: Work supported by Fermi Research Alliance, LLC, under contract No. DE-AC02-07CH11359 with the U.S. Department of Energy.
Bent-crystal channeling is a technique with a potential to increase the beam-halo collimation efficiency at high-energy colliders. First measurements at the Tevatron in 2005 have shown that using a 5-mm silicon crystal to deflect the proton beam halo onto a secondary collimator improves the system performance by reducing the machine impedance, beam losses in the collider detectors and irradiation of the superconducting magnets, all in agreement with simulations. Recent results, obtained with substantially improved goniometer and enhanced beam diagnostics, are reported showing channeling collimation of the ~1-TeV circulating proton beam halo at the Tevatron collider. Comprehensive results of computer modeling are presented which allow further developments of the T-980 experiment towards a robust system compatible with requirements to high-efficient collimation at the Tevatron and LHC hadron colliders.
CERN, Geneva
The performance reach of the LHC depends on the magnitude of beam losses and the achievable cleaning efficiency of its collimation system. The ideal performance reach for the nominal Phase 1 collimation system is reviewed. However, unavoidable imperfections affect any accelerator and can further deteriorate the collimation performance. Multiple static machine and collimator imperfections were included in the LHC tracking simulations. Error models for collimator jaw flatness, collimator setup accuracy, the LHC orbit and the LHC aperture were set up, based to the maximum extent possible on measurements and results of experimental beam tests. It is shown that combined "realistic" imperfections can reduce the LHC cleaning efficiency by about a factor 11 on average.
CERN, Geneva
The Large Hadron Collider extends the present state-of-the-art in stored beam energy by 2-3 orders of magnitude. A sophisticated system of collimators is implemented along the 27 km ring and mainly in two dedicated cleaning insertions, to intercept and absorb unavoidable beam losses which could induce quenches in the superconducting magnets. 88 collimators per beam are initially installed for the so called Phase 1. An optimized strategy for the commissioning of this considerable number of collimators has been defined. This optimized strategy maximizes cleaning efficiency and tolerances available for operation, while minimizing the required beam time for collimator setup and ensuring at all times the required passive machine protection. It is shown that operational tolerances from collimation can initially significantly relaxed.
CERN, Geneva
The different levels of LHC collimators must be set up by respecting a strict setting hierarchy in order to guarantee the required performance and protection during the different operational machine stages. The available margins are a fraction of a beam σ. Two different sub-systems establish betatron and momentum collimation for the LHC. Collimator betatronic phase space cuts are defined for a central on-momentum particle. However, due to the chromatic features of the LHC optics and energy deviations of particles, the different phase space cuts become coupled. Starting from the basic equation of the transverse beam dynamics, the influence of off-momentum beta-beat and dispersion on the effective collimator settings has been calculated. The results are presented, defining the allowed phase space regions from LHC collimation. The impacts on collimation-related setting tolerances and the choice of an optimized LHC optics are discussed.
CERN, Geneva
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
IN2P3-LAPP, Annecy-le-Vieux
DESY, Hamburg
SLAC, Menlo Park, California
The CLIC BDS is experiencing the careful revision from a large number of world wide experts. This was particularly enhanced by the successful CLIC'08 workshop held at CERN. Numerous new ideas, improvements and critical points are arising, establishing the path towards the Conceptual Design Report by 2010.
UMAN, Manchester
The LHC Collimators are designed to remove halo particles such that they do not impinge onto either detectors or other vulnerable regions of the storage ring. However, the very high 7 TeV energy means that their design is critical, as is the modelling of the absorption, scattering and wakefield effects upon the passing bunches. Existing simulations are being performed using Sixtrack and K2. We compare these simulations with results obtained using the MERLIN code, which includes a fuller description of the scattering and wakefield processes.
EPFL, Lausanne
CERN, Geneva
KIT, Karlsruhe
Due to the known limitations of Phase I LHC collimators in stable physics conditions, the LHC collimation system will be complemented by additional 30 Phase II collimators. The Phase II collimation system is designed to improve cleaning efficiency and to minimize the collimator-induced impedance with the main function of protecting the Super Conducting (SC) magnets from quenching due to beam particle losses. To fulfil these requirements, different possible innovative collimation designs were taken in consideration. Advanced jaw materials, including new composite materials (e.g. Cu–Diamond), jaw SiC insertions, coating foil, in-jaw instrumentation (e.g. BPM) and improved mechanical robustness of the jaw are the main features of these new promising Phase II collimator designs developed at CERN. The FLUKA Monte Carlo code is extensively used to evaluate the behavior of these collimators in the most radioactive areas of LHC, supporting the mechanical integration. These studies aim to identify the possible critical points along the IR7 line.
CERN, Geneva
Phase I collimators, equipped with Carbon-Carbon jaws, effectively met specifications for the early phase of LHC operation. However, the choice of carbon-based materials is expected to limit the nominal beam intensity mainly because of the high RF impedance and limited efficiency of the collimators. Moreover, C/C may be degraded by high radiation doses. To overcome these limitations, new Phase II secondary collimators will complement the existing system. Their extremely challenging requirements impose a thorough material investigation effort aiming at identifying novel materials combining very diverse properties. Relevant figures of merit have been identified to classify materials: Metal-diamonds composites look a promising choice as they combine good thermal, structural and stability properties. Molybdenum is interesting for its good thermal stability. Ceramics with non-conventional RF performances are also being evaluated. The challenges posed by the development and industrialization of these materials are addressed in a collaboration program, involving academic and industrial partners and complementing material research with an innovative design.
UPC, Barcelona
CERN, Geneva
A main concern in high intensity rings is the evaluation of uncontrolled losses and their minimization using collimation systems. A two-stage systemis foreseen for the PS2. The fundamental design strategy for the collimation design is presented, including machine apertures and collimator materials. The dependence of the collimator system efficiency on the primary scraper length and the impact parameter of the particle is evaluated for different collimator locations. Beam loss maps are finally produced displaying the detailed power load deposited around the ring.
CERN, Geneva
BNL, Upton, Long Island, New York
EPFL, Lausanne
The grazing function g is introduced – a synchrobetatron optical quantity that parametrizes the rate of change of total angle with respect to synchrotron amplitude for particles grazing a collimator or aperture. The grazing function is particularly important for crystal collimators, which have limited acceptance angles. The implications for RHIC, SPS, Tevatron and LHC crystal implementations are discussed. An analytic approximation is derived for the maximum value of g in a matched FODO cell, and is shown to be in good agreement with a realistic numerical example. The grazing function scales linearly with FODO cell bend angle, but to is independent of FODO cell length.
CERN, Geneva
EPFL, Lausanne
IHEP Protvino, Protvino, Moscow Region
The UA9 experiment at the SPS aims at testing bent crystals for usage as collimators with high energy stored proton and heavy ion beams. The experiments will try to establish crystal-based cleaning efficiency with slowly diffusing beam halo. One method for evaluating efficiency relies on Roman Pots and is described elsewhere. An alternative method relies on observing the beam loss signals around the ring. Comparisons of losses escaping from standard collimators and bent crystals will allow determination of cleaning efficiency, equivalent to the definition used for the LHC collimation design. This alternative method is described and simulations with LHC collimation tracking tools for UA9 are discussed. The predicted beam losses along the SPS ring are presented for different orientations and amorphous layer thicknesses of the crystal. The effect of different diffusion speeds for the beam are discussed.
CERN, Geneva
EPFL, Lausanne
IHEP Protvino, Protvino, Moscow Region
Bent crystals are promised to provide a path towards significant improvement of cleaning efficiency for high power collimation systems. In this paper a possible implementation of a crystal-enhanced collimation system is evaluated for the LHC. Simulation studies were performed with the same state-of the art tracking codes as used for the design of the conventional LHC collimation system. The numerical models are described and predictions for the local and global cleaning efficiency with a crystal-based LHC collimation system are presented. Open issues and further work towards a crystal collimation design for the LHC are discussed.
CERN, Geneva
A full scale prototype of the Large Hadron Collider (LHC) collimator was installed in 2004 in the CERN Super Proton synchrotron (SPS). During three years of operation the prototype has been used extensively for beam tests, for control tests and also to benchmark LHC simulation tools. This operational experience has been extremely valuable in view of the final LHC implementation as well as for estimating the LHC operational scenarios, most notably to establish procedures for the beam-based alignment of the collimators with respect to the circulating beam. This was made possible by installing in the SPS a first prototype of the LHC beam loss monitoring system. The operational experience gained at the SPS, lessons learnt for the LHC operation and various accelerator physics effects that could limit the efficiency of the collimator alignment procedures are presented.
CERN, Geneva
The UA9 experiment intends to assess the possibility of using bent silicon crystals as primary collimators to direct the beam halo onto a secondary absorber, thus reducing outscattering, beam losses in critical regions and radiation load. The experiment will be performed in the CERN-SPS in storage mode with a low intensity 120 GeV/c proton beam. The beam will be perturbed to create a diffusive halo as in the RD22 experiment. The setup consists of four stations. The crystal station contains two goniometers for crystals. The first tracking station houses silicon strip detectors for single particle tracking. The second tracking station contains the same kind of detectors for tracking. The two stations will allow to measure x-x' densities and collimation efficiencies with high precision. The TAL station, at 90 degrees phase advance,is a 600 mm long tungsten secondary collimator. The observables of the experiment are the collimation efficiencies, the measurement of the phase space and the cleaning efficiency deduced from the losses along the ring. We present here the layout of the experiment and the way we expect to collect data in 2009.
Fermilab, Batavia
Funding: Work supported by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy
The Fermilab Main Injector is moving toward providing 400 kW of 120 GeV proton beams using slip stacking injection of eleven Booster batches. Loss of 5% of the beam at or near injection energy results in 1.5 kW of beam loss. A collimation system has been implemented to localize this loss with the design emphasis on beam not captured in the accelerating rf buckets. More than 90% of these losses are captured in the collimation region. We will report on the construction, commissioning and operation of this collimation system. Commissioning studies and loss measurement tools will be discussed. Residual radiation monitoring of the Main Injector machine components since 2004 will be used to demonstrate the effectiveness and limitations of these efforts.
JAI, Oxford
We review the current status of the collimation system of the Compact Linear Collider (CLIC). New calculations are done to study the survivability of the CLIC energy spoiler in case of impact of a full bunch train considering the most recent beam parameters. The impact of the collimator wakefields on the luminosity is also studied using the updated collimator apertures, and we evaluate the beam position jitter tolerance that is required to preserve the nominal luminosity. Moreover, assuming the new collimation depths, we evaluate the collimation efficiency.
SLAC, Menlo Park, California
EPFL, Lausanne
Funding: Work supported in part by the U.S. Department of Energy contract DE-AC02-76SF00515
The Phase II upgrade to the LHC collimation system calls for complementing the 30 high robust Phase I graphite collimators with 30 high Z Phase II collimators. One option is to use metallic rotatable collimators and this design will be discussed here. The Phase II collimators must be robust in various operating conditions and accident scenarios. Design issues include: 1) Collimator jaw deflection due to heating and sagita must be small when operated in the steady state condition, 2) Collimator jaws must withstand transitory periods of high beam impaction with no permanent damage, 3) Jaws must recover from accident scenario where up to 7 full intensity beam pulses impact on the jaw surface and 4) The beam impedance contribution due to the collimators must be small to minimize coherent beam instabilities. The current design will be presented.
SLAC, Menlo Park, California
CERN, Geneva
Fermilab, Batavia
Funding: Work supported in part by the U.S. Department of Energy contract DE-AC02-76SF00515
It has been proposed to use a hollow electron lens with the LHC beam collimation system*. The hollow electron beam would be used as a beam scraper and positioned at a closer σ than the primary collimators to increase the halo particle diffusion rate striking the primaries. In this paper we use multi-turn beam tracking simulations to analyze the effectiveness of such a lens when integrated into the LHC collimation system.
*Shiltsez, V. et al. "LHC Particle Collimation by Hallow Electron Beams," Proceedings EPAC08, MOPC098 (2008)
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
JAI, Oxford
The postlinac energy collimation system of the Compact Linear Collider (CLIC) protects the machine by intercepting mis-steered beams due to possible failure modes in the linac. The collimation is based in a spoiler-absorber scheme. The mission of the spoiler is to protect the main downstream absorber by dispersing the beam, via multiple Coulomb scattering, in case of a direct hit. We present the design of energy spoilers for CLIC, considering the following requirements: spoiler survival to the deep impact of an entire bunch train, and minimisation of spoiler wakefield effects during normal operation. Different configurations of the spoiler are studied in order to achieve an optimum performance.
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
CERN, Geneva
Linac4 is a 160 MeV H- linac which will replace the existing Linac2, a 50 MeV proton linac, at CERN as a first step of the upgraded LHC proton injector chain. No collimation system is foreseen in the baseline design but it will become mandatory for opreation at highest duty cycle in order to reduce activation of the machine. Such a system will also help to reduce activation at low duty cycle. A review of different collimation options, initial studies on collimator designs capable of intercepting beam power of 10, 25 and 50 Watts at energies between 50 and 160 MeV, the activation of such designs and the downstream elements are shown in this paper.
ELETTRA, Basovizza
Funding: The work was supported in part by the Italian Ministry of University and Research under grant FIRB-RBAP045JF2.
To avoid damages on permanent magnets by the electrons, collimators will be installed in FERMI@elettra. Their dimensions and shape are defined through the beam optics and the induced wake fields while GEANT simulations are performed to determine their absorption efficiency and thermal load for both normal operating conditions and in case of miss-steering. The design, the simulations and the expected performance of the collimators are presented and discussed.
UNIFE, Ferrara
INFN-Ferrara, Ferrara
Channeling in bent crystals is a technique with high potential to steer charged-particle beams for several applications in accelerators physics. Channeling and related techniques underwent significant progress in the last years. Distinctive features of performance increase was the availability of novel ideas other than new techniques to manufacture the crystal for channeling. We show the technology to fabricate crystals through non conventional silicon micromachining techniques. Characterization of the realized crystals highlighted that the crystals are free of lattice damage induced by the preparation. The crystals were positively tested at the external line H8 of the SPS with 400 GeV protons for investigation on planar and axial channelings as well as on single and multiple volume reflection experiments by the H8-RD22 collaboration. Selected single- and multi-crystal are candidates for the experiment UA9–an experiment on beam collimation at the CERN SPS.
CERN, Geneva
SLAC, Menlo Park, California
The LHC collimation system is implemented in phases, in view of the required extrapolation by 2-3 orders of magnitude beyond Tevatron and HERA experience in stored energy. All available simulations predict that the LHC proton beam intensity with the "phase 1" collimation system may be limited by the impedance of the collimators or cleaning efficiency. Maximum efficiency requires collimator materials very close to the beam, generating the dominant resistive impedance in the LHC. Above a certain intensity the beam is unstable. On the other hand, even if collimators are set very close to the beam, the achievable cleaning efficiency is predicted to be inadequate, requiring either beam stability beyond specifications or reduced intensity. The accelerator physics concept for upgrading cleaning efficiency, for both proton and heavy ion beams, and reducing collimator-related impedance is described. Besides the "phase 2" secondary collimators, new collimators are required in a few super-conducting regions.
CERN, Geneva
Collimation and halo cleaning for the LHC beams are performed separately for betatron and momentum losses, requiring two dedicated insertions for collimation. Betatron cleaning is performed in IR7 while momentum cleaning is performed in IR3. A study has been performed to evaluate the performance reach for a combined betatron and momentum cleaning system in IR3. The results are presented.
CERN, Geneva
IHEP Protvino, Protvino, Moscow Region
We report on background studies for the LHC with detailed simulations. The simulations now include generation of beam-gas scattering in combination with multiturn tracking of protons. Low beta optics and available aperture files for this configuration have been used to generate loss maps according to the pressure distribution in the LHC.
SLAC, Menlo Park, California
CERN, Geneva
Funding: This work was supported by DOE Contract No. DE-AC02-76SF00515 and used resources of NERSC supported by DOE Contract No. DE-AC02-05CH11231, and of NCCS supported by DOE Contract No. DE-AC05-00OR22725.
A rotatable collimator is proposed for the LHC phase II collimation upgrade. When the beam crosses the collimator, trapped modes will be excited that result in beam energy loss and collimator power dissipation. Some of the trapped modes can also generate transverse kick on the beam and affect the beam operation. In this paper the parallel eigensolver code Omega3P is used to search for all the trapped modes below 2GHz in the collimator, including longitudinal modes and transverse modes. The loss factors and kick factors of the trapped modes are calculated as function of the jaw positions. The amplitude ratio between transverse and longitudinal trapped mode intensity can be used as a direct measure of the position of the beam. We present simulation results and discuss the results.
CERN, Geneva
The LHC collimators are protected against beam caused damages by measuring the secondary particle showers with beam loss monitors. Downstream of every collimator an ionisation chamber and a secondary emission monitor are installed to determine the energy deposition in the collimator. The relation between the energy deposition in the beam loss monitor and the collimator jaw is based on secondary shower simulations. To verify the FLUKA simulations the prototype LHC collimator installed in the SPS was equipped with beam loss monitors. The results of the measurements of the direct impact of the 26 GeV proton beam injected in the SPS onto the collimator are compared with the predictions of the FLUKA simulations. In addition simulation results from parameter scans and for mean and peak energy deposition with its dependencies are shown.
BNL, Upton, Long Island, New York
Fermilab, Batavia
The use of single stage bunch compressor (BC) in the International Linear Collider (ILC)* Damping Ring to the Main Linac beamline (RTML) requires new design for the extraction line (EL). The EL located downstream of the BC will be used for both an emergency abort dumping of the beam and the tune-up continuous train-by-train extraction. It must accept both compressed and uncompressed beam with energy spread of 3.54% and 0.15% respectively. In this paper we report design that allowed minimizing the length of such extraction line while offsetting the beam dumps from the main line by 5m distance required for acceptable radiation level in the service tunnel. Proposed extraction line can accommodate beams with different energy spreads at the same time providing the beam size suitable for the aluminum ball dump window.
*N. Phinney et al., “International Linear Collider Reference Design Report: Accelerator”, SLAC-R-857C
DESY, Hamburg
The post-linac collimation system should simultaneously fulfil several different functions. In first place, during routine operations, it should remove with high efficiency off-momentum and large amplitude halo particles, which could be lost inside undulator modules and become source of radiation-induced demagnetization of the undulator permanent magnets. The system also must protect the undulator modules and other downstream equipment against mis-steered and off-energy beams in the case of machine failure without being destroyed in the process. From beam dynamics point of view, the collimation section should be able to accept bunches with different energies (up to ± 1.5% from nominal energy) and transport them without deterioration not only of transverse, but also of longitudinal beam parameters. In this article we present the optics solution for the post-linac collimation system which fulfils all listed above requirements.
CERN, Geneva
As for the final focus systems of linear colliders, the chromatic aberrations induced by low-beta insertions can seriously limit the performance of circular colliders. The impact is two-fold: (1) a substantial off-momentum beta-beating wave travelling all around of the ring leading to a net reduction of the mechanical aperture of the low-beta quadrupoles but also impacting on the hierarchy of the collimator and protection devices of the machine, (2) a huge non-linear chromaticity, essentially Q’’ and Q’’’, which, when combined with the geometric non-linear imperfection of the machine could substantially reduce the momentum acceptance of the ring by sending slightly off-momentum particles towards non-linear resonances. These effects will be analyzed and illustrated in the framework of the LHC insertions upgrade and a strategy for correction will be developed, requiring a deep modification of the LHC overall optics.
CERN, Geneva
The LHC phase II collimation project requires beam shock and impact tests of materials used for beam intercepting devices. Similar tests are also of great interest for other accelerator components such as beam entrance/exit windows and protection devices. For this purpose a dedicated High Radiation Material test facility (HiRadMat) is under study. This facility may be installed at CERN at the location of a former beam line. This paper describes the associated beam line which is foreseen to deliver a 450 GeV proton beam from SPS with an intensity of up to 3·1013 protons per shot. Different beam line designs will be compared and the choice of the beam steering and diagnostic elements will be discussed, as well as operational issues.
CERN, Geneva
An LHC high-luminosity upgrade has been studied by various European and international collaborations since about 2001. Ingredients of such an LHC upgrade include the optimization of the IR layout, new high-field or large-aperture triplet quadrupoles, chromatic correction, possibly detector-integrated slim magnets, crab cavities, beam-beam compensators, operation in a regime of large Piwinski angle, luminosity leveling for reduced detector pile up, heat-load, background, and radiation damage due to the collision debris, and a renovation of the injector complex. Scenarios, decision paths, and present R&D efforts will be presented.
IF-UFRGS, Porto Alegre
Experiments and numerical simulations show that high-intensity beams composed by charged particles usually reach their final stationary state with a progressive populating of a spatial region external to its original border. This populating process occurs in such terms that beam spatial limits at equilibrium increase by an amount of two or three times its initial nominal size. This is known as halo in Beam Physics. In this way, this work intends to better understand the time scale of halo formation. The carried out investigation has shown that the time scale of halo formation in fact can be segmented in two different quantities, each one associated to distinct physical mechanisms. One is related with the initial non-homogeneity naturally present in such systems, and the other is a result of the initial beam envelope mismatch. This investigation seems to be useful to design more efficient collimation systems and/or non-linear control systems for the next generation high-power accelerators.
UMD, College Park, Maryland
Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept of Defense Office of Naval Research and Joint Technology Office.
Halo is one important limiting factor for the continuous and reliable operation of intense electron or ion beam facilities, such as FELs and spallation neutron sources. A halo population outside the core of the beam can lead to uncontrolled beam loss, electron cloud effects and activation of the beam pipe, as well as beam quality degradation. In this study, we focus on the issue of halo removal, by means of beam collimation, and subsequent halo regeneration. We compare the particle-core model of halo creation to accurate, self consistent particle-in-cell (PIC) simulations. We show that under certain conditions the halo is regenerated even after collimation. This can only be understood within the context of collective effects, particularly in the case of intense beams.
CERN, Geneva
The 2008 collimation system of the CERN Large Hadron Collider (LHC) included 80 movable collimators for a total of 316 degrees of freedom. Before beam operation, the final controls implementation was deployed and commissioned. The control system enabled remote control and appropriate diagnostics of the relevant parameters. The collimator motion is driven with time-functions, synchronized with other accelerator systems, which allows controlling the collimator jaw positions with a micrometer accuracy during all machine phases. The machine protection functionality of the system, which also relies on function-based tolerance windows, was also fully validated. The collimator control challenges are reviewed and the final system architecture is presented. The results of the remote system commissioning and the operational experience are discussed. The system tests performed for the 2009 beam operation are also reviewed.