A Novel Concept for Phase Error Correction in Superconductive Undulators: Theory and Experimental Verification

Undulators are the most advanced sources for the generation of synchrotron radiation in present storage rings. The photons generated by a single electron add up coherently along the electron trajectory. Small magnetic errors can cause unwanted destructive interferences. In this thesis a novel passive shimming method for superconductive undulators based on superconductive closed loops is presented. It is shown that the system can significantly reduce the effect of the field errors

Operational results with fast automatic beam-based LHC collimator alignment

The CERN Large Hadron Collider (LHC) is the largest and highest-energy particle accelerator ever built. It is designed to collide particles at a centre-of-mass energy of 14 TeV to explore the fundamental forces and constituents of matter. Due to the potentially destructive high-energy particle beams, with a total design energy of 362 MJ, the collider is equipped with a series of machine protection systems. The beam cleaning or collimation system is designed to passively intercept and absorb particles at large amplitudes. The cleaning efficiency depends heavily on the accurate positioning of the jaws with respect to the beam trajectory. Beam-based collimator alignment is currently the only feasible technique that can be used to determine the beam centre and beam size at the collimator locations. If the alignment is performed without any automation, it can require up to 30 hours to complete for all collimators. This reduces the beam time available for physics experiments. This article provides a brief recap of the algorithms and software developed to automate and speed up the alignment procedure, and presents the operational results achieved with fast automatic beam-based alignment in the 2011-2013 LHC runs.peer-reviewe

Measured and simulated heavy-ion beam loss patterns at the CERN Large Hadron Collider

The Large Hadron Collider (LHC) at CERN pushes forward to new regimes in terms of beam energy and intensity. In view of the combination of very energetic and intense beams together with sensitive machine components, in particular the superconducting magnets, the LHC is equipped with a collimation system to provide protection and intercept uncontrolled beam losses. Beam losses could cause a superconducting magnet to quench, or in the worst case, damage the hardware. The collimation system, which is optimized to provide a good protection with proton beams, has shown a cleaning efficiency with heavy-ion beams which is worse by up to two orders of magnitude. The reason for this reduced cleaning efficiency is the fragmentation of heavy-ion beams into isotopes with a different mass to charge ratios because of the interaction with the collimator material. In order to ensure sufficient collimation performance in future ion runs, a detailed theoretical understanding of ion collimation is needed. The simulation of heavy-ion collimation must include processes in which Pb82+208 ions fragment into dozens of new isotopes. The ions and their fragments must be tracked inside the magnetic lattice of the LHC to determine their loss positions. This paper gives an overview of physical processes important for the description of heavy-ion loss patterns. Loss maps simulated by means of the two tools ICOSIM [1,2] and the newly developed STIER (SixTrack with Ion-Equivalent Rigidities) are compared with experimental data measured during LHC operation. The comparison shows that the tool STIER is in better agreement.peer-reviewe

Semi-automatic beam-based alignment algorithm for the LHC collimation system

Full beam-based alignment of the LHC collimation system was a lengthy procedure as the collimators were setup manually. A yearly alignment campaign has been sufficient for now, although in future this may lead to a decrease in the cleaning efficiency if machine parameters such as the beam orbit drift over time. Automating the collimator setup procedure can allow for more frequent alignments, therefore reducing this risk. This paper describes the design and testing of a semi-automatic algorithm as a first step towards a fully automatic setup. Its implementation in the collimator control software and future plans are described.peer-reviewe

End-of-fill study on collimator tight settings

In 2010 and 2011 the collimation system has been operated with relaxed settings, i.e. with retractions between different collimator families larger than the nominal settings that provide optimum cleaning. This configuration ensured a sufficient cleaning performance at 3.5 TeV while allowing larger tolerances on orbit control. Tighter collimator settings were proposed to push the cleaning performance and to allow larger orbit margins between TCDQ dump protection and tertiary collimators. With the same margins as with the relaxed settings, the β∗ could be reduced. After having verified with beam that the cleaning is improved as expected, the feasibility of tighter collimator settings must be addressed with high stored intensity. For this purpose, an end-of-fill study was proposed after a standard physics fill with 1380 bunches nominal bunches at 3.5 TeV, for a total stored energy of 95 MJ. During this test, primary and secondary collimators were moved to tight settings after about 8 hours of stable physics conditions in all experiments. This note summarises the operational procedure followed and the results of beam measurements during this study.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

First ion collimation commissioning results at the LHC

First commissioning of the LHC lead ion beams to 1.38 A TeV beam energy was successfully achieved in November 2010. Ion collimation has been predicted to be less efficient than for protons at the LHC, because of the complexity of the physical processes involved: nuclear fragmentation and electromagnetic dissociation in the primary collimators creating fragments with a wide range of Z/A ratios, that are not intercepted by the secondary collimators but lost in the dispersion suppressor sections of the ring. In this article we present first comparisons of measured loss maps with theoretical predictions from simulation runs with the ICOSIM code. An extrapolation to define the ultimate intensity limit for Pb beams is attempted. The scope of possible improvements in collimation efficiency coming from the installation of new collimators in the cold dispersion suppressors and combined betatron and momentum cleaning is also explored.Ministerio de Ciencia e Innovacion - Gobierno de Espana,Ayuntamiento de San Sebastian,Gobierno Vasco,Diputacion Foral de Gipuzkoa,San Sebastian Turismo - Convention Bureaupeer-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