Crystals in the LHC
Bent crystals can be used to deflect charged particle beams. Their use in high-energy accelerators has been investigated for almost 40 years. Recently, a bent crystal was irradiated for the first time in the HiRadMat facility with an extreme particle flux, which crystals would have to withstand in the LHC. The results were very encouraging and confirmed that this technology could play a major role in increasing the beam collimation performance in future upgrades of the machine.
UA9 bent crystal tested with a laser.
Charged particles interacting with a bent crystal can be trapped in channelling states and deflected by the atomic planes of the crystal lattice (see box). The use of bent crystals for beam manipulation in particle accelerators is a concept that has been well-assessed. Over the last three decades, a large number of experimental findings have contributed to furthering our knowledge and improving our ability to control crystal-particle interactions.
In modern hadron colliders, such as the LHC, the halo of particles surrounding the beam core may produce high-power loss in sensitive areas of the accelerator, thereby endangering operation stability and machine protection. Multi-stage collimation systems are thus used to absorb them. “The UA9 experiment - supported by CERN, INFN, Imperial College, LAL, PNPI, IHEP and JINR - was set up in 2008 to investigate the advantages of using bent crystals in the collimation systems of high-energy hadron colliders,” says Walter Scandale, UA9 spokesperson. “A bent crystal replacing the primary collimator can deflect the incoming halo coherently at angles larger than can be obtained with amorphous materials. This might increase the ability of crystals to cut halos in higher intensity beams at the LHC, especially in cases of operation problems at higher energies.”
From 2009 onwards, silicon crystals were tested at the SPS and their efficiency measured with Medipix detectors. In 2011, after the successful tests performed at the SPS, the LHC Committee endorsed the LUA9 experiment to test the idea at the LHC. “When using bent crystals for collimation, the entire halo particle power is deposited in a small spot of the collimator material,” says Walter Scandale. “We knew that, at the nominal LHC intensity, crystals would have to extract a proton flux dissipating up to 500 kW short-term steady losses in a small spot of just a few square millimetres. An ad-hoc additional passive absorber system should be designed to ensure a high collimation performance.”
An additional challenge for the LUA9 collaboration consists in orienting the crystal optimally for channelling. The operation requires the use of alignment mechanisms with angular accuracy beyond the state-of-the-art. Devices to orient the crystal in the LHC are under development in partnership with various industrial companies. The results of the first tests are very encouraging. “Bent crystals with the required properties will be installed in the LHC in time for tests to be carried out after the long shut-down in close collaboration with the LHC Collimation team,” concludes Walter Scandale. “The expected result is to substantially improve the collimation efficiency, especially for lead ion beams.”
Why are particles channelled in crystals?
Charged particles have special interactions with crystals due to the anisotropy of the medium. The trajectory of a positive particle travelling at a small angle with respect to crystalline planes is strongly influenced by the repulsive potential averaged along the atomic planes. Channelling is the result of particle confinement in the potential well between neighbouring crystalline planes.
by Antonella Del Rosso