Probing LHC halo dynamics using collimator loss rates at 6.5 TeV

Halo diffusion measurements at the CERN Large Hadron Collider (LHC) were conducted with beams for physics at 6.5 TeV by means of collimator scans, carried out between 2016 and 2018. From the time evolution of the beam losses recorded during a collimator scan, in which collimator jaws are moved in steps toward the beam core cutting beam tails, one can extract information on the halo diffusion and its population as a function of the transverse amplitude. In this paper, results of the first scans performed at different beam intensities for both planes and both beams using the primary collimators of the betatron-cleaning system are presented. The scans were performed with squeezed optics and colliding beams after a few hours of regular physics production, during so-called end-of-fill measurements. Beam losses are measured with ionization chambers close to the collimators, which enable 1 and 100 Hz acquisitions, as well as diamond beam loss monitors, which enable turn-by-turn and bunch-by-bunch acquisitions. Parametric fits of a diffusion model are applied to the time profile of losses, for both total and individual bunch intensity. The analysis of the measurements performed in various conditions was used to estimate the diffusion coefficient as a function of the transverse amplitude and the population of LHC beam tails.