Design and implementation of a crystal collimation test stand at the Large Hadron Collider

Abstract

Future upgrades of the CERN Large Hadron Collider (LHC) demand improved cleaning performance of its collimation system. Very efficient collimation is required during regular operations at high intensities, because even a small amount of energy deposited on superconducting magnets can cause an abrupt loss of superconducting conditions (quench). The possibility to use a crystal-based collimation system represents an option for improving both cleaning performance and impedance compared to the present system. Before relying on crystal collimation for the LHC, a demonstration under LHC conditions (energy, beam parameters, etc.) and a comparison against the present system is considered mandatory. Thus, a prototype crystal collimation system has been designed and installed in the LHC during the Long Shutdown 1 (LS1), to perform feasibility tests during the Run 2 at energies up to 6.5 TeV. The layout is suitable for operation with proton as well as heavy ion beams. In this paper, the design constraints and the solutions proposed for this test stand for feasibility demonstration of crystal collimation at the LHC are presented. The expected cleaning performance achievable with this test stand, as assessed in simulations, is presented and compared to that of the present LHC collimation system. The first experimental observation of crystal channeling in the LHC at the record beam energy of 6.5 TeV has been obtained in 2015 using the layout presented (Scandale et al., Phys Lett B 758:129, 2016). First tests to measure the cleaning performance of this test stand have been carried out in 2016 and the detailed data analysis is still on-going.

Layouts for fixed-target experiments and dipole moment measurements of short-lived baryons using bent crystals at the LHC

“…The crystal length is l Cry 1 = 1.2 cm, which was chosen to have a bending radius R = 80 m, as the present crystals installed in the LHC. This follows the parametric studies reported in [21] and ensures an optimum crystal channeling performance at LHC top energy, while keeping the nuclear interactions rate as low as possible.…”

“…It was therefore decided to design optimised layouts for the existing and already commissioned optics. The same approach was used to design the crystal collimation layout currently installed in the LHC [21], which led to the successful observation of crystal channeling with 6.5 TeV/c protons beams [5].…”

“…• Required bending of Cry 1 and angular cut performed by the absorbers, due to the local optics in proximity of IP8. = 50 µrad and length l Cry 1 = 4 mm (same parameters are used for crystals presently installed in IR7 for collimation studies [5,21]). With respect to the layout in IR8, these parameters allow to reduce the nuclear interaction probability in the first crystal from the 1.6 % to the 0.7 %.…”

First observation of ion beam channeling in bent crystals at multi-TeV energies

“…It contains a high-precision goniometer with sub-µrad accuracy [11] that is mounted on a special tank with an "O" shaped pipe, which can hide the crystal during the standard operational activity of the accelerator. This design was conceived to minimize the impedance of the device for high-intensity The layout of the LHC crystal setup is described in detail in [9]. It was optimized for the operation with proton beams, but it is fully adequate for ion beam tests as well.…”

“…In order to study the channeling at the hadron-beam energies relevant for the LHC, and to obtain a direct comparison to the performance of the present LHC collimation system [7], a crystal collimation test stand was installed in 2015 in the betatron collimation insertion [8]. It consists of two crystals, with length of 4 mm along the beam direction and bent to a target design angle of 50 µrad, that enable horizontal and vertical beam collimation tests [9]. 1 Standard LHC collimators, which are part of the LHC betatron cleaning system and are located downstream of the crystals, are used to intercept and probe the channeled halo particles.…”