Abstract
The design stored beam energy in the CERN high-luminosity large hadron collider (HL-LHC) upgrade is about 700 MJ, with about 36 MJ in the beam tails, according to estimates based on scaling considerations from measurements at the LHC. Such a large amount of stored energy in the beam tails poses serious challenges on its control and safe disposal. In particular, orbit jitters can cause significant losses on primary collimators, which can lead to accidental beam dumps, magnet quenches, or even permanent damage to collimators and other accelerator elements. Thus, active control of the diffusion speed of halo particles is necessary and the use of hollow electron lenses (HELs) represents the most promising approach to handle overpopulated tails at the HL-LHC. HEL is a very powerful and advanced tool that can be used for controlled depletion of beam tails, thus enhancing the performance of beam halo collimation. For these reasons, HELs have been recently included in the HL-LHC baseline. In this paper, we present detailed beam dynamics calculations performed with the goal of defining HEL specifications and operational scenarios for HL-LHC. The prospects for effective halo control in HL-LHC are presented.
Highlights
The current LHC collimation system [1,2,3,4,5,6] has achieved an excellent performance, delivering a halo-cleaning inefficiency of about 1×10−4, which is defined as the loss leakage to sensitive equipment per impacting losses on the collimation system [7], ensuring safe operations without quenches from circulating beam losses with stored beam energies up to more than 300 MJ at 6.5 TeV [8]
The main upgrades of the LHC collimation system that are part of the present high-luminosity large hadron collider (HL-LHC) baseline [10] involve the replacement of one Nb–Ti, 8.3 T dipole in the IR7 Dispersion Suppressor with two Nb3Sn, 11 T dipoles with a collimator in between them, together with the replacement of some present collimators with new devices with jaws made of lowimpedance material
The active control of the primary-halo diffusion speed is achieved by overlapping the hollow electron beam and the circulating proton beam over a few metres, where the two beams travel in opposite direction to enhance the hollow electron lenses (HELs) performance [39]
Summary
The current LHC collimation system [1,2,3,4,5,6] has achieved an excellent performance, delivering a halo-cleaning inefficiency of about 1×10−4, which is defined as the loss leakage to sensitive equipment per impacting losses on the collimation system [7], ensuring safe operations without quenches from circulating beam losses with stored beam energies up to more than 300 MJ at 6.5 TeV [8]. The main upgrades of the LHC collimation system that are part of the present HL-LHC baseline [10] involve the replacement of one Nb–Ti, 8.3 T dipole in the IR7 Dispersion Suppressor with two Nb3Sn, 11 T dipoles with a collimator in between them, together with the replacement of some present collimators with new devices with jaws made of lowimpedance material Their aim is to improve the cleaning performance of the overall system, while reducing its contribution to the resistive-wall impedance budget of the ring. These upgrades do not allow for an active control on overpopulated beam tails and their safe disposal, for which the introduction of a HEL is deemed necessary
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