[Paper Review] CERN Yellow Reports: Conference Proceedings, Vol 2 (2018): Proceedings of "Tracking Collimation" ICFA Workshop

Haroon Rafique, Robert Appleby+4 more|arXiv (Cornell University)|Jan 1, 1970

Particle Accelerators and Free-Electron Lasers4 citations

TL;DR

This paper presents MERLIN, a C++-based accelerator physics simulation library developed for high-precision tracking and collimation studies in the LHC and HL-LHC. It enables detailed simulation of particle loss maps, collimator scattering, and hollow electron lens effects using symplectic integrators, thick-lens tracking, and advanced scattering models, with validated outputs for beam dynamics and aperture monitoring in ultra-relativistic proton beams.

ABSTRACT

MERLIN is an accelerator physics library written in C++ which can be used for a range of accelerator tracking simulations, including collimation in hadron colliders. Recently MERLIN has been upgraded to provide a robust tool for HL-LHC collimation, including the treatment of composite materials, and a hollow electron lens process. We describe the features of MERLIN used in collimation simulations.

Motivation & Objective

• To develop a modular, extensible, and high-performance simulation tool for collimation studies in the LHC and future HL-LHC upgrades.
• To enable accurate modeling of particle loss mechanisms, including multiple Coulomb scattering, ionization energy loss, and inelastic proton-nucleon interactions.
• To support the integration of advanced collimation components such as hollow electron lenses and variable-material collimators.
• To provide a robust, version-controlled, and well-tested software framework for accelerator physics research with automated regression testing.
• To deliver detailed output for tracking, scattering, and impact analysis to support beam dynamics optimization and collimator design.

Proposed method

• Uses a modular C++ architecture with distinct classes for accelerator lattice, aperture configuration, collimator databases, and particle tracking.
• Employs the MADX TFS interface to import standard accelerator lattice files and construct an AcceleratorModel object with component-specific fields and geometries.
• Applies symplectic or TRANSPORT integrators to simulate particle trajectories through accelerator elements, including drifts, magnets, and collimators.
• Implements on-line aperture checks at regular path-length intervals (default 10 cm) to detect particle loss during tracking.
• Integrates advanced scattering physics: elastic and single diffractive proton-nucleon scattering, multiple Coulomb scattering, and ionization energy loss.
• Supports specialized output functions such as JawImpact, ScatterPlot, JawInelastic, and SelectScatter for detailed analysis of particle interactions and loss mechanisms.

Experimental results

Research questions

• RQ1 How can a flexible, modular, and high-performance simulation framework be designed to support collimation studies in the LHC and HL-LHC?
• RQ2 What are the effects of different collimator materials (e.g., carbon vs. CFC AC150K) on inelastic proton interactions and beam loss distribution?
• RQ3 How does the choice of collimation bin size influence the accuracy of scattering and energy loss modeling during particle tracking?
• RQ4 To what extent can MERLIN’s output functions (e.g., ScatterPlot, SelectScatter) support debugging and validation of collimation physics processes?
• RQ5 How does the integration of a hollow electron lens model affect particle dynamics and loss map generation in simulation?

Key findings

• MERLIN successfully simulates particle loss maps for the LHC and HL-LHC using realistic collimator materials and advanced scattering models.
• The default 10 cm collimation bin size ensures regular aperture checks without excessive computational cost, balancing accuracy and performance.
• ScatterPlot output reveals that particle tracking stops abruptly at 10 cm intervals due to bin-based aperture checks, confirming the timing mechanism of loss detection.
• JawInelastic output shows that CFC AC150K produces higher inelastic interaction rates than pure carbon, indicating material-dependent beam loss characteristics.
• SelectScatter and OutputSelectScatterHistogram functions enable detailed angular and momentum transfer analysis of inelastic interactions, with histogrammed polar angle distributions matching expected MCS behavior.
• The software is production-ready, with a 39k LOC codebase, automated testing via CMake and CDash, and Git-based version control with daily regression checks.