CERN Accelerating science

Thesis
Report number	CERN-THESIS-2021-022
Title	Applications of Crystal Collimation to the CERN Large Hadron Collider (LHC) and its High Luminosity Upgrade Project (HL-LHC)
Author(s)	D'Andrea, Marco (Padua U.)
Publication	144.
Thesis note	PhD : Padua U. : 2021
Thesis supervisor(s)	Zanetti, Marco ; Redaelli, Stefano ; Mirarchi, Daniele
Note	Presented 23 Feb 2021
Subject category	Accelerators and Storage Rings
Accelerator/Facility, Experiment	CERN HL-LHC
Abstract	Charged particles interacting with materials with a highly ordered structure can experience a variety of coherent processes, among which crystal channeling is of particular interest to applications to high-energy colliders. Bent crystals can be used to efficiently steer charged particles by trapping impacting particles in the potential well generated by adjacent crystalline planes. A crystal of a few millimeters can apply a deflection equivalent to the effect of a magnetic field of hundreds of Tesla, far beyond what can be achieved by state-of-the-art superconducting magnets. This impressive property stimulates accelerator physicists to make use of crystals for advanced beam manipulations. A variety of possible applications were conceived in accelerators around the world in order to exploit the specific features of this process, including beam extraction, collimation and in-vacuum fixed target physics. Applications to beam collimation were the driving motivation for this effort at CERN in the last 15 years. The crystal collimation concept relies on bent crystals to deflect beam halo particles towards, in principle, a single absorber. Compared to the standard collimation system currently deployed at the Large Hadron Collider (LHC), this advanced technique promised to improve both the cleaning performance and the impedance of the machine. While the use with high-energy proton beams requires a dedicated absorber capable of withstanding the impact of the deflected particles, this system could potentially be deployed in operations with heavy-ion beams, where a standard secondary collimator is sufficient. Throughout Run 2, this concept has been extensively studied for the High-Luminosity upgrade of the machine (HL-LHC), which will increase the total energy stored by the circulating beams and pose serious challenges to the present collimation system. Crystal channeling, however, can be applied to a variety of applications aside from beam collimation. In the context of Physics Beyond Colliders (PBC), the possibility to use the equivalent magnetic field experienced by channeled particles to measure the magnetic dipole moment of short-lived baryons has been first explored at the Fermi National Accelerator Laboratory (FNAL) in an extraction line of the Tevatron. The application of this complex setup at the LHC poses a number of challenges, but a feasible “double-crystal” layout has been conceived for the integration in the hierarchy of the LHC collimation system. Additionally, in 2018 bent crystals were deployed for the first time during a physics run with proton beams at the LHC, with the specific goal to minimize the observed background at the ALFA and TOTEM experiments. Other applications of bent crystals to particle accelerators at CERN include crystal-assisted slow-extraction schemes, explored at the Super Proton Synchrotron (SPS) and a two-crystal setup for the production of simultaneous particle beams at the NA48 experiment. In this Ph.D. work, the main results gathered with the use of bent crystals at the high-energy frontier in the CERN accelerator complex are reported, with particular focus on crystal collimation tests carried out at the LHC in 2018 with Pb ion beams. These activities allowed to demonstrate the cleaning improvement provided by this collimation scheme, and were a key ingredient in the decision to include crystal collimation in the HL-LHC baseline for the run starting in 2022. The code development effort aimed at improving the available simulation tools for crystal collimation with proton beams is also described, using as case studies two important applications beyond ion beam collimation that were explored at CERN: the double-crystal setup at the SPS and the deployment of crystals for the reduction of background on forward physics detectors.

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