Comparison of LHC collimator beam-based alignment to BPM-interpolated centers

The beam centers at the Large Hadron Collider collimators are determined by beam-basedalignment, where both jaws of a collimator are moved in separately until a loss spike isdetected on a Beam Loss Monitor downstream. Orbit drifts of more than a few hundredmicrometers cannot be tolerated, as they would compromise the performance of thecollimation system. Beam Position Monitors (BPMs) are installed at various locations aroundthe LHC ring, and a linear interpolation of the orbit can be obtained at the collimatorpositions. In this paper, the results obtained from beam-based alignment are compared withthe orbit interpolated from the BPM data throughout the 2011 and 2012 LHC proton runs.Louisiana State University (LSU),U.S. Department of Energy, Office of Science,COSYLAB,DIMTEL,Muons, Inc.peer-reviewe

Halo scrapings with collimators in the LHC

Understanding the population and the shape of the beam halo is important to predict possible intensity limitations due to collimation at 7 TeV. Therefore the population of the beam halo has been measured in horizontal, vertical and skew plane, using the primary collimators of the LHC collimation system. In addition these measurements were used to calibrate the beam loss monitor signals to a particle loss rate at the primary collimators. Within this paper the halo scraping method, the measured halo distribution and the calibration factors are presented and discussed.Ministerio de Ciencia e Innovacion - Gobierno de Espana,Ayuntamiento de San Sebastian,Gobierno Vasco,Diputacion Foral de Gipuzkoa,San Sebastian Turismo - Convention Bureau.peer-reviewe

Improved robustness of the LHC collimation system by operating with a jaw-beam angle

The robustness of the Phase I collimation system could be improved playing with the angular orientation of each single jaw. A preliminary study on the asymmetric misalignment of the collimator jaws, scanning through different jaw angles and varying beam sizes and energy, have been carried out, aiming at minimizing the energy deposited on metallic collimators, following an asynchronous dump.peer-reviewe

Cleaning inefficiency of the LHC collimation system during the energy ramp : simulations and measurements

The cleaning inefficiency of the LHC collimation system for the operational scenarios in 2010-12 has already been studied in detail at injection and top energy (450 GeV and 4 TeV respectively). In this paper, results are presented for the cleaning inefficiency at intermediate energies, simulated using the SixTrack code. The first comparisons with measured provoked losses are discussed. This study helps in benchmarking the energy dependence of the simulated inefficiency and is thus important for the extrapolation to future operation at higher energies.peer-reviewe

Modeling and simulation of LHC beam-based collimator setup

In the 2011 Large Hadron Collider run, collimators were aligned for proton and heavy ion beams using a semiautomatic setup algorithm. The algorithm provided a reduction in the beam time required for setup, an elimination of beam dumps during setup and better reproducibility with respect to manual alignment. A collimator setup simulator was developed based on a Gaussian model of the beam distribution as well as a parametric model of the beam losses. A time-varying beam loss signal can be simulated for a given collimator movement into the beam. The simulation results and comparison to measurement data obtained during collimator setups and dedicated fills for beam halo scraping are presented. The simulator will then be used to develop a fully automatic collimator alignment algorithm.peer-reviewe

Simulations and measurements of beam losses on LHC collimators during beam abort failures

One of the main purposes of tracking simulations for collimation studies is to produce loss maps along the LHC ring, in order to identify the level of local beam losses during nominal and abnormal operation scenarios. The SixTrack program is the standard tracking tool used at CERN to perform these studies. Recently, it was expanded in order to evaluate the proton load on different collimators in case of fast beam failures. Simulations are compared with beam measurements at 4 TeV. Combined failures are assumed which provide worst-case scenarios of the load on tungsten tertiary collimators.peer-reviewe

Simulations and measurements of beam losses on LHC collimators during beam abort failures

One of the main purposes of tracking simulations for collimation studies is to produce loss maps along the LHC ring, in order to identify the level of local beam losses during nominal and abnormal operation scenarios. The SixTrack program is the standard tracking tool used at CERN to perform these studies. Recently, it was expanded in order to evaluate the proton load on different collimators in case of fast beam failures. Simulations are compared with beam measurements at 4 TeV. Combined failures are assumed which provide worst-case scenarios of the load on tungsten tertiary collimators.peer-reviewe

Preliminary comparison of the response of LHC tertiary collimators to proton and ion beam impacts

The CERN Large Hadron Collider is designed to bring into collision protons as well as heavy ions. Accidents involving impacts on collimators can happen for both species. The interaction of lead ions with matter differs to that of protons, thus making this scenario a new interesting case to study as it can result in different damage aspects on the collimator. This paper will present a preliminary comparison of the response of collimators to proton and ion beam impacts.peer-reviewe

Beam feasibility study of a collimator with in-jaw beam position monitors

At present, the beam-based alignment of the LHC collimators is performed by touching the beam halo with both jaws of each collimator. This method requires dedicated fills at low intensities that are done infrequently and makes this procedure time consuming. This limits the operational flexibility, in particular in the case of changes of optics and orbit configuration in the experimental regions. The performance of the LHC collimation system relies on the machine reproducibility and regular loss maps to validate the settings of the collimator jaws. To overcome these limitations and to allow a continuous monitoring of the beam position at the collimators, a design with jaw-integrated Beam Position Monitors (BPMs) was proposed and successfully tested with a prototype (mock-up) collimator in the CERN SPS. Extensive beam experiments allowed to determine the achievable accuracy of the jaw alignment for single and multi-turn operation. In this paper, the results of these experiments are discussed. The non-linear response of the BPMs is compared to the predictions from electromagnetic simulations. Finally, the measured alignment accuracy is compared to the one achieved with the present collimators in the LHC.peer-reviewe

Beam-based validation of LHC collimator settings

The collimator system provides efficient beam halo cleaning and plays an important role in passive machine protection. About 100 movable collimators are precisely aligned to the beam orbit with gaps as small as 2 mm. In order to ensure the required collimation functionality, the collimator positions need to be validated. This is done by acquiring regularly controlled loss maps in each machine configuration. During 2012, the use of the transverse damper (ADT) to excite transversally the beams in a controlled way has reduced the time to produce betatron loss maps. However, the validation of the off-momentum losses and asynchronous dumps still determines the minimum number of required fills. The experience with the loss maps in the 2010-2013 running period is reviewed and possible improvements are discussed. Aspects related to the minimum time between re-validation by loss maps, possible further improvements such as loss maps at the end of every physics fill and better online monitoring are also discussed.peer-reviewe