Measurements of beam halo diffusion and population density in the Tevatron and in the Large Hadron Collider

TL;DR

This paper presents measurements of beam halo diffusion and population density in the Tevatron and LHC, highlighting their impact on accelerator performance and safety, and discusses new methods for nondestructive halo monitoring.

Contribution

It introduces experimental techniques for measuring halo diffusivities and densities, including collimator scans and discusses plans for nondestructive measurement methods.

Findings

Halo diffusivities vary with experimental conditions

Collimator scans effectively measure halo population dynamics

Plans for nondestructive halo measurement methods are discussed

Abstract

Halo dynamics influences global accelerator performance: beam lifetimes, emittance growth, dynamic aperture, and collimation efficiency. Halo monitoring and control are also critical for the operation of high-power machines. For instance, in the high-luminosity upgrade of the LHC, the energy stored in the beam tails may reach several megajoules. Fast losses can result in superconducting magnet quenches, magnet damage, or even collimator deformation. The need arises to measure the beam halo and to remove it at controllable rates. In the Tevatron and in the LHC, halo population densities and diffusivities were measured with collimator scans by observing the time evolution of losses following small inward or outward collimator steps, under different experimental conditions: with single beams and in collision, and, in the case of the Tevatron, with a hollow electron lens acting on a subset of bunches. After the LHC resumes operations, it is planned to compare measured diffusivities with the known strength of transverse damper excitations. New proposals for nondestructive halo population density measurements are also briefly discussed.