[Paper Review] Beam Cleaning and Collimation Systems

Stefano Redaelli|arXiv (Cornell University)|Feb 17, 2016

Particle Accelerators and Free-Electron LasersEngineering17 citations

TL;DR

This paper presents the design, operation, and performance of the multistage beam collimation system in the CERN Large Hadron Collider (LHC), which safely manages beam losses to prevent quenching of superconducting magnets. It achieves unprecedented cleaning performance through a staged collimation system using advanced collimator materials and configurations, enabling stable high-energy operation with stored beam energies up to 150 MJ.

ABSTRACT

Collimation systems in particle accelerators are designed to dispose of unavoidable losses safely and efficiently during beam operation. Different roles are required for different types of accelerator. The present state of the art in beam collimation is exemplified in high-intensity, high-energy superconducting hadron colliders, like the CERN Large Hadron Collider (LHC), where stored beam energies reach levels up to several orders of magnitude higher than the tiny energies required to quench cold magnets. Collimation systems are essential systems for the daily operation of these modern machines. In this document, the design of a multistage collimation system is reviewed, taking the LHC as an example case study. In this case, unprecedented cleaning performance has been achieved, together with a system complexity comparable to no other accelerator. Aspects related to collimator design and operational challenges of large collimation systems are also addressed.

Motivation & Objective

• To design and implement a multistage collimation system capable of handling extreme beam energies in superconducting hadron colliders like the LHC.
• To ensure beam losses are intercepted before reaching sensitive components, particularly superconducting magnets, to prevent quenching.
• To optimize collimation performance for high-luminosity operation while minimizing radiation spread and equipment activation.
• To evaluate advanced collimation concepts such as hollow electron lenses and crystal-based collimation for future upgrades.
• To address operational challenges in maintaining collimator alignment, radiation tolerance, and system reliability under high-intensity beam conditions.

Proposed method

• Design of a multistage collimation system with primary and secondary collimators positioned at strategic locations to intercept beam halos and lossy particles.
• Use of high-absorption materials (e.g., carbon-fiber composites, graphite) in collimator jaws to efficiently absorb beam particles and minimize secondary emission.
• Implementation of active beam scraping techniques via resonant excitation of halo particles in tune space to enhance cleaning efficiency.
• Integration of hollow electron lenses to diffuse beam tails and reduce halo intensity before collimation.
• Exploration of bent crystal-based collimation to channel particles via channeling effects, reducing secondary interactions and absorber requirements.
• Simulation and beam-test validation of advanced concepts in the LHC, including crystal collimation and resonance excitation, during LHC Run II.

Experimental results

Research questions

• RQ1 How can beam losses be efficiently intercepted before reaching superconducting magnets in high-energy hadron colliders?
• RQ2 What design and operational strategies are required to achieve multistage cleaning performance at beam energies exceeding 100 MJ?
• RQ3 Can resonance-based excitation or hollow electron lenses improve halo cleaning efficiency without degrading beam core emittance?
• RQ4 What are the feasibility and performance gains of crystal-based collimation compared to conventional amorphous collimators?
• RQ5 How can collimation systems be optimized to localize radiation and reduce activation across the accelerator?

Key findings

• The LHC collimation system successfully achieved beam cleaning performance at stored beam energies up to 150 MJ, with design limits of 362 MJ, ensuring superconducting magnet quench safety.
• The multistage collimation system reduced beam loss energy deposition in magnets to below quench thresholds, even under high-intensity beam conditions.
• Resonance-based excitation techniques were validated as effective for halo particle diffusion, with potential for improved cleaning without emittance growth.
• Hollow electron lenses demonstrated potential to reduce halo intensity by 30–50%, enabling more efficient collimation and reducing load on primary collimators.
• Crystal collimation simulations indicate a potential cleaning gain of 5 to 10 times over conventional systems, with reduced secondary interactions and lower impedance.
• Beam tests in LHC Run II confirmed the feasibility of crystal collimation under real beam conditions, with sub-microradian angular stability required for reliable operation.