Probing LHC halo dynamics using collimator loss rates at 6.5 TeV

Abstract

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 betatroncleaning 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.

Nonlinear dynamics of proton beams with hollow electron lens in the CERN high-luminosity LHC

“…It is important to note that halo re-population mechanisms are not considered in these simulations, although they should be taken into account in future studies to provide refined predictions and an improved operational scenario. Note that a re-population time of several tens of seconds for tails above 5 σ is expected, assuming a similar diffusion coefficient as that measured recently at the LHC [16].…”

“…a Maximum total head-on tune shift of 0.02, as only two IPs will be colliding head-on One of the main motivations comes from the requirement to handle the estimated stored energy present in the beam tails for all design loss scenarios. Various measurements carried out at the LHC indicate strong overpopulated tails with respect to a typical Gaussian transverse beam distribution [14][15][16]. Scaling these observations to HL-LHC beams, under the assumption that their population scales linearly with the bunch intensity, led to the conclusion that up to about 36 MJ might be stored in the beam tails [16].…”

“…Various measurements carried out at the LHC indicate strong overpopulated tails with respect to a typical Gaussian transverse beam distribution [14][15][16]. Scaling these observations to HL-LHC beams, under the assumption that their population scales linearly with the bunch intensity, led to the conclusion that up to about 36 MJ might be stored in the beam tails [16]. The beam parameters of our interest are reported in Table 1, while a complete overview can be found in [17,18].…”

Hollow electron lenses for beam collimation at the High-Luminosity Large Hadron Collider (HL-LHC)

“…There is a solid experimental basis from measurements at the LHC that indicates consistently the presence of large tail populations [16,17]: reducing the steady-state population of tails is important to mitigate known and unknown loss effects and might even become mandatory if some fast failure scenarios cannot be excluded.…”

Electron lenses: historical overview and outlook

“…It was found that random activation of the electron beam completely removes the proton halo beyond the electron beam inner radius on a timescale 𝑂(10 s). Very important are the parallel investigations of the proton beam halo and diffusion rates in the LHC beams at CERN [78], and the halo removal rates by hollow electron lens in the RHIC [79]. For the latter study, one of the two RHIC electron lenses was changed from a Gaussian beam profile to a hollow profile.…”