Beams travel inside the LHC at close to the speed of light and cleaning out particles that fly too far from the main path is not an easy feat. More than 100 collimators punctuate the ring at specific locations to make sure that the particles that stray – called beam halos – are cleaned or absorbed to protect delicate accelerator equipment. This October, an advanced technique using bent crystals was proven to increase the overall performance of the cleaning process.
Bent crystals, as the name suggests, are crystals bent mechanically to create a curve at a microscopic angle. The nature of crystalline structure means that halo particles, which would otherwise scatter at random angles, can be steered and channeled directly toward absorbers installed downstream, relaxing the need for the secondary and tertiary collimators used by the LHC’s existing system. This application for beam collimation was triggered by the pioneering work started by the UA9 collaboration in 2009, and is supported by funds from the High-Luminosity LHC project. Used in several scenarios, bent crystals were a tool for collimation studies in the LHC, but had never been used in a physics run until now.
“We have two bent crystals installed on each LHC beam pipe at Point 7 for crystal collimation studies,” says Stefano Redaelli, who is in charge of the LHC collimation system and its upgrade for the High-Luminosity LHC. “In these devices, halo particles are steered by the highly pure periodical lattice structure of the crystal. At 7 TeV, channeling in bent crystals can curve the particle trajectory by amounts that are beyond the reach of the LHC superconducting magnets. For instance, a 4mm long silicon crystal, with a bending angle of 50 micro radiant, produces about the same effect as that of 10 standard LHC dipoles together!”
Beam halos are a nuisance, as they can damage sensitive equipment like superconducting magnets and also affect the quality of physics data by creating more background noise. To reduce this effect, the LHC’s beam collimation team use many different techniques. One such technique is scraping, which requires interrupting the beam to clean and diffuse the halo, causing a significant time loss per fill before data-taking can resume. With bent crystals, beam halos are steered toward dedicated absorbers, reducing this time loss and increasing the quality of data by providing cleaner backgrounds.
“We decided to use crystals to control background particles on the TOTEM and ATLAS-ALFA experiments’ Roman pot detectors. Detailed simulations were carried out before the crystals were inserted and used as primary collimators for low-intensity beams,” says Redaelli. The results measured showed less background noise and frequent scraping was not needed.
Tunnel view of the crystal goniometer that is used to adjust the crystal angle with respect to the beam, with sub-micro radiant precision. The design is developed by the EN/SMM group and installed in the LHC by the EN/STI group. (Image: courtesy of D. Mirarchi, CERN)
Tests of crystal collimation to clean beams will continue until the end of the year. Many teams have been involved in the development of crystal collimation systems installed in the LHC, including the Accelerator and Beam Physics (BE-ABP), Survey, Mechatronics and Measurements (EN-SMM), and Sources, Targets and Interactions (EN-STI) groups. There is particular interest in seeing how efficiently the system will work in cleaning the complex losses from heavy-ion beam halos. These new results will provide important input to decide if crystal collimation can become part of the upgrade plans for the HL-LHC.
* The UA9 Collaboration consists of CERN, PNPI, INFN, LAL, JINR, IHEP, Imperial College London.