Performance Evaluation of the SPS Scraping System in View of the High Luminosity LHC

Injection in the LHC is a delicate moment, since the LHC collimation system cannot offer adequate protection during beam transfer. For this reason, a complex chain of injection protection devices has been put in place. Among them, the SPS scrapers are the multi-turn cleaning system installed in the SPS aimed at halo removal immediately before injection in the LHC. The upgrade in luminosity of the LHC foresees beams brighter than those currently available in machine, posing serious problems to the performance of the existing injection protection systems. In particular, the integrity of beam-intercepting devices is challenged by unprecedented beam parameters, leading to interactions potentially destructive. In this context, a new design of scrapers has been proposed, aimed at improved robustness and performance. This thesis compares the two scraping systems, i.e. the existing one and the one proposed for upgrade. Unlike any other collimation system for regular halo cleaning, both are "fast" systems, characterised by the variation of the relative distance between the beam and the absorbing medium during cleaning, which enhances the challenge on energy deposition values. Assets / liabilities of the two systems are highlighted by means of numerical simulations and discussed, with particular emphasis on energy deposition in the absorbing medium, time evolution of the beam current during scraping and losses in the machine. Advantages of the system proposed for upgrade over the existing one are highlighted. The analysis of the existing system takes into account present operational conditions and addresses the sensitivity to settings previously not considered, updating and extending past studies. The work carried out on the upgraded system represents the first extensive characterisation of a multi-turn cleaning system based on a magnetic bump. Results have been obtained with the FLUKA-SIXTRACK coupling, developed during this PhD activity from its initial version to being a state-of-art tracking tool for cleaning studies in circular machines. Relevant contributions to the development involve the handling of time-varying impact conditions. An extensive benchmark against a test of the scraper blades with beam has been carried out, to verify the reliability of results. Effects induced in the tested blades confirm the high values of energy deposition predicted by the simulation. Moreover, the comparison with the time profile of the beam intensity measured during scraping allowed the reconstruction of the actual settings of the blades during the test. Finally, the good agreement of the quantitative benchmark against readouts of beam loss monitors finally proves the quality of the analyses and the maturity of the coupling

Beam size measurements based on movable quadrupolar pick-ups

Quadrupolar pick-ups (PU) have been widely studied as candidates for non-intercepting beam size and emittance measurements. However, their application has been proven to be limited. Two fundamental factors make quadrupolar measurements exceptionally challenging: first, the low quadrupolar sensitivity of PUs and second, the parasitic position signal incorporated into the measured quadrupolar measurement. In this paper, an alignment technique, based on movable PUs, is proposed to efficiently cancel the parasitic position signal. Tests have been performed using PUs embedded in collimators in the Large Hadron Collider. Beam measurements demonstrate promising results.peer-reviewe

Probing LHC halo dynamics using collimator loss rates at 6.5 TeV

Halo diffusion measurements at the CERN Large Hadron Collider (LHC) were conducted with beams for physics at 6.5 TeV by means of collimator scans, carried out between 2016 and 2018. From the time evolution of the beam losses recorded during a collimator scan, in which collimator jaws are moved in steps toward the beam core cutting beam tails, one can extract information on the halo diffusion and its population as a function of the transverse amplitude. In this paper, results of the first scans performed at different beam intensities for both planes and both beams using the primary collimators of the betatron cleaning system are presented. The scans were performed with squeezed optics and colliding beams after a few hours of regular physics production, during so-called end-of-fill measurements. Beam losses are measured with ionization chambers close to the collimators, which enable 1 and 100 Hz acquisitions, as well as diamond beam loss monitors, which enable turn-by-turn and bunch-by-bunch acquisitions. Parametric fits of a diffusion model are applied to the time profile of losses, for both total and individual bunch intensity. The analysis of the measurements performed in various conditions was used to estimate the diffusion coefficient as a function of the transverse amplitude and the population of LHC beam tails.peer-reviewe

Final implementation, commissioning, and performance of embedded collimator beam position monitors in the Large Hadron Collider

During Long Shutdown 1, 18 Large Hadron Collider (LHC) collimators were replaced with a new design, in which beam position monitor (BPM) pick-up buttons are embedded in the collimator jaws. The BPMs provide a direct measurement of the beam orbit at the collimators, and therefore can be used to align the collimators more quickly than using the standard technique which relies on feedback from beam losses. Online orbit measurements also allow for reducing operational margins in the collimation hierarchy placed specifically to cater for unknown orbit drifts, therefore decreasing the β and increasing the luminosity reach of the LHC. In this paper, the results from the commissioning of the embedded BPMs in the LHC are presented. The data acquisition and control software architectures are reviewed. A comparison with the standard alignment technique is provided, together with a fill-to-fill analysis of the measured orbit in different machine modes, which will also be used to determine suitable beam interlocks for a tighter collimation hierarchy.peer-reviewe

Improved aperture measurements at the LHC and results from their application in 2015

A good knowledge of the available aperture in the LHC is essential for a safe operation due to the risk of magnet quenches or even damage in case of uncontrolled beam losses. Experimental validations of the available aperture are therefore crucial and were in the past carried out by either a collimator scan combined with beam excitations or through the use of local orbit bumps. In this paper, we show a first comparison of these methods in the same machine configuration, as well as a new very fast method based on a beam-based collimator alignment and a new faster variant of the collimator scan method. The methods are applied to the LHC operational configuration for 2015 at injection and with squeezed beams and the measured apertures are presented.peer-reviewe

Beam losses, lifetime and collimator hierarchy

The aim of the LHC collimation system is to ensure a safe machine operation; it provides the LHC with passive protection and minimises the risk of magnet quenches induced by beam losses. In 2017, the LHC collimation system confirmed its excellent performance, with no magnet quenches due to losses from circulating beams while accommodating changes in machine configurations. The system availability in 2017 was also very good. The present work reviews key elements of the 2017 operation, from initial commissioning with beam to beam losses, lifetime and collimator hierarchypeer-reviewe

Roman Pot insertions in high-intensity beams for the CT-PPS project at LHC

In 2015 the Roman Pots (RPs) of the CMS-TOTEM Precision Proton Spectrometer in the LHC Interaction Region 5 successfully approached the 6.5 TeV beam in regular fills (β∗ = 0.8 m) to distances of 25 beam width sigmas at all intensity steps reached during that running season, i.e. up to 2244 bunches producing a luminosity of 4.8 × 1033 cm-2s-1. Given that earlier insertion tests at low β∗ before the Long Shutdown 1 (LS1) had suffered from impedance heating at the RPs, this first-time achievement proves the effectiveness of the impedance mitigation actions undertaken in LS1 and represents an important milestone towards physics production at distances as small as 15 sigmas. This contribution reviews the diagnostic measurements assessing debris showers and beam impedance effects, i.e. the data from Beam Loss Monitors, beam vacuum gauges and temperature sensors. The dependences of the observables on luminosity or beam current are shown. Extrapolations to higher luminosities and smaller distances to the beam do not indicate any fundamental problems. Finally the plans for 2016 are outlined.ALBA-Cells,AS,CEA Saclay,CERN,Cockcroft Institute,et al.peer-reviewe

Overview of the FLUKA code

Abstract The capabilities and physics models implemented inside the FLUKA code are briefly described, with emphasis on hadronic interaction. Examples of the performances of the code are presented including basic (thin target) and complex benchmarks, and radiation detector specific applications. In particular the ability of FLUKA in describing existing calorimeter performances and in predicting those of future ones, as well as the use of the code for neutron and mixed field radiation detectors will be demonstrated with several examples

Performance of the LHC collimation system during 2015

During the long shutdown, a total of 32 collimators were replaced or installed to improve the Run 1 system, including 18 new collimators with embedded beam position monitors, additional physics debris collimators, additional passive absorbers and the re-installation or displacement of existing collimators. The commissioning experience as well as the performance of the collimation system in 2015 is reviewed following these upgrades. In particular, the operational experience with the embedded BPM collimators is discussed, emphasizing the improvements on alignment efficiency and the strategy for better orbit monitoring. Improvements of collimation setting validation through an optimized loss map strategy are also presented. Further improvements planned for 2016 are discussed.peer-reviewe