Lumière collimatée | EPFL Graph Search

Une lumière collimatée est une lumière dont les rayonnements sont quasiment parallèles et se déploient lentement quand ils se propagent. Le mot est relatif à « colinéaire » et implique une lumière qui ne se disperse pas avec la distance (idéalement), ou qui sera très peu dispersée (dans la réalité). Un faisceau parfaitement collimaté, sans divergence, ne peut pas être créé à cause du principe de diffraction, mais la lumière ne peut qu'être approximativement collimatée par certains processus, par exemple au moyen d'un collimateur. On dit parfois de la lumière collimatée qu'elle est « portée à l'infini ». C'est ainsi que, lorsque la distance augmente à partir d'une source ponctuelle, le front d'onde sphérique devient plus plat et plus proche des ondes planes qui sont parfaitement collimatées. Étymologie Le mot « collimaté » vient du verbe latin collimare, qui est une erreur de lecture de collineare, « dirigés en ligne droite ». Bibliographie

J. Pfister, e tJ.

Experiment and Machine Protection from Fast Losses caused by Crab Cavities in the High Luminosity LHC

Andrea Santamaría García

The High Luminosity Large Hadron Collider (HL-LHC) upgrade aims for a tenfold increase in integrated luminosity compared to the nominal Large Hadron Collider (LHC), and for operation at a leveled luminosity five times higher than the nominal LHC peak luminosity. In order to compensate the geometric luminosity loss due to the increased crossing angle, crab cavities will be used to transversely rotate the beam bunches, allowing quasi head-on collisions at the experiments. Crab cavity failures can be very fast, having time constants similar to the reaction time of the machine protection system. In such a scenario the beams cannot be immediately extracted, making the protection of the machine fully rely on passive protection devices such as the collimation system. At the same time the energy stored in the HL-LHC beams will be doubled with respect to the LHC to more than 700 MJ, which increases the risk of damaging the machine and the experiments in case of failure. Crab cavity failures have the potential to displace the beam core and create considerable particle losses around the machine, posing a machine protection challenge. Any increase in failure rates will be difficult to compensate, affecting the performance of the HL-LHC and therefore the integrated luminosity goal. This is why it is important to correctly interlock the machine and make sure that certain types of failure never happen. In order to do this advanced simulations are needed well before the crab cavity prototypes can be tested in real operating conditions. This thesis analyzes different failure scenarios for crab cavities installed around the ATLAS and CMS experiments. The selected failure scenarios are later simulated with the tracking code SixTrack thanks to a newly developed functionality. The distribution of the particle losses in space and time are analyzed for the different failure cases and a quantitative estimate of the impact in the collimation system is given. The results are analyzed from a machine protection point of view, where the time for the beam abort trigger is calculated for each failure case and mitigation techniques are proposed. These results allow identifying corner cases corresponding to the most dangerous crab cavity failure scenarios, serving as input for the design of the future interlocking system.

EPFL2018

Beam-Machine Interaction Studies for the Phase II LHC Collimation System

Luisella Lari

Collimation is essential in a high Energy and Intensity hadron collider with SuperConducting magnets, like the LHC at CERN. To improve the cleaning efficiency and to reduce the LHC impedance budget in stable condition and at top Energy, additional 30 Phase II collimators are foreseen to be installed. The work of this PhD analyses the beam-machine interactions in the LHC Betatron Cleaning Insertions for its upgrade with the Phase II collimators. These studies aim at optimizing the collimation system layout from the machine protection point of view, considering different options for the Phase II collimator design and focusing on heating damage and activation problems. The outcomes of this PhD work have been used to support the Phase II design evolution and their prototype mechanical integration, and to point out possible critical points along the Betatron Cleaning Straight Section. Emphasis is on studying several configurations and material choices for nominal and failure scenarios.

EPFL2010

Monte Carlo Modeling of Crystal Channeling at High Energies

Philippe Jean Schoofs

Charged particles entering a crystal close to some preferred direction can be trapped in the electromagnetic potential well existing between consecutive planes or strings of atoms. This channeling effect can be used to extract beam particles if the crystal is bent beforehand. Crystal channeling is becoming a reliable and efficient technique for collimating beams and removing halo particles. At the European Organization for Nuclear Research (CERN), the installation of silicon crystals in the Large Hadron Collider (LHC) is under scrutiny by the UA9 collaboration with the goal of investigating if they are a viable option for the collimation system upgrade. This thesis describes a new Monte Carlo model of planar channeling which has been developed from scratch in order to be implemented in the FLUKA code simulating particle transport and interactions. Crystal channels are described through the concept of continuous potential taking into account thermal motion of the lattice atoms and using Moliere screening function. The energy of the particle transverse motion determines whether or not it is trapped between the crystal planes while single Coulomb scattering on lattice atoms can lead to dechanneling. The volume capture and reflection applying to quasi-channeled particles are also modeled. Analogously to dechanneling, single scattering is used to determine the occurrence of volume capture. The parameters of the crystals, such as torsion or miscut, are described as well. For channeled particles, the suppression of electromagnetic and nuclear collisions is implemented. It is stronger for particles oscillating close to the center of the channel and is crucial for a correct evaluation of the rate of dechanneling without having recourse to the use of a macroscopic dechanneling length. The UA9-H8 experiment conducted at CERN aims at investigating new crystal physics as well as characterizing crystals that can be of interest in view of the implementation of crystal collimation at CERN, including in the LHC. This experiment uses silicon strip detectors situated on both sides of the crystal. Putting together upstream and downstream tracks in coincidence and matching at an identical fitted location on the crystal, it yields information about the deflections given to the beam population. Several runs from the UA9-H8 experiment are analyzed and compared to the model results. Channeling and dechanneling rates, as well as angular distributions at crystal exit are shown to be in a very encouraging agreement both for strip and quasi-mosaic crystals.