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Crystal collimation

Crystal collimation is an advanced collimation technique that has been extensively studied at the CERN accelerator complex in the past decade. As opposed to the standard system routinely employed in particle accelerators such as the LHC, this scheme uses bent crystals to coherently steer beam halo particles towards, in principle, a single absorber.

Working principle

Crystals are materials with a highly organized atomic structure. Charged particles interacting with a crystal with the right impact conditions can get trapped in the potential well generated by neighboring crystalline planes, and be forced to oscillate in relatively empty space. While in channeling, the probability of inelastic interactions between the trapped particles and the atoms of the crystalline lattice is greatly reduced.

This property can be applied to collimation systems by using a bent crystal to channel beam halo particles, forcing them to follow the curvature of the crystal itself. The lower rate of inelastic interactions inside the crystal allows to significantly reduce off-momentum losses, as well as leakage caused by fragmentation and dissociation of halo particles. For this reason, crystal collimation has been considered by the HL-LHC project as an option to improve the ion collimation cleaning efficiency of the LHC for future runs. Four crystal collimators (two per each circulating beam, one per each collimation plane) have been installed in the LHC between 2014 and 2017, providing a complete setup for thorough tests.

In 2018 crystal collimators were used for the first time in a physics run at the LHC, with the specific goal of reducing the background observed by the ATLAS-ALFA and TOTEM experiments.

Updates and News

Crystal collimation is now officially part of the LHC baseline for operations with ion beams during Run 3.

Relevant Publications

R. Assmann, S. Redaelli and W. Scandale, Optics study for a possible crystal-based collimation system for the LHC, in 10th European Particle Accelerator Conference, 2006. Link

W. Scandale, F. Andrisani, G. Arduini, M. Butcher, F. Cerutti, M. Garattini et al., Observation of strong leakage reduction in crystal assisted collimation of the SPS beam, Physics Letters B, 2015. Link

D. Mirarchi, G. Hall, S. Redaelli and W. Scandale, Design and implementation of a crystal collimation test stand at the Large Hadron Collider, The European Physical Journal C, 2017. [Link] (http://dx.doi.org/10.1140/epjc/s10052-017-4985-4)

D. Mirarchi, V. Avati, R. Bruce, M. Butcher, M. D’Andrea, M. Di Castro et al., Reducing beam-related background on forward physics detectors using crystal collimation at the Large Hadron Collider, Physical Review Applied, 2020. Link

Recent Doctoral Thesis on the Topic

M. D'Andrea, Applications of Crystal Collimation to the CERN Large Hadron Collider (LHC) and its High Luminosity Upgrade Project (HL-LHC), Padua University, Italy. CERN-THESIS-2021-022, 2021. Link

R. Rossi, Experimental Assessment of Crystal Collimation at the Large Hadron Collider, University of Rome, Italy. CERN-THESIS-2017-424, 2017. Link

D. Mirarchi, Crystal Collimation for LHC, Imperial College, London, UK. CERN-THESIS-2015-099, 2015. Link

V. Previtali, Performance Evaluation of a Crystal-Enhanced Collimation System for the LHC, Ecole Polytechnique, Lausanne, Switzerland. CERN-THESIS-2010-133, 2010. Link

Other Useful References

2018 HL-LHC Crystal Collimation Day. Link