Crystals channel high-energy beams in the LHC

Bent crystals can be used to deflect particle beams, as suggested by E. Tsyganov in 1976. Experimental demonstrations have been carried out for four decades in various laboratories worldwide. In recent tests, a bent crystal inserted into the LHC beam halo successfully channelled and deflected 6.5 TeV protons into an absorber, with reduced secondary irradiation.

Quasimosaic crystal for the LHC (developed by PNPI).

Bent crystal technology was introduced at CERN and further developed for the LHC by the UA9 Collaboration. For about ten years, experts from CERN, INFN (Italy), Imperial College (UK), LAL (France), and PNPI, IHEP and JINR (Russia) have been investigating the advantages of using bent crystals in the collimation systems of high-energy hadron colliders. A bent crystal replacing the primary collimator can deflect the incoming halo deeply inside the secondary collimators, improving their absorption efficiency. “The bent crystals we have just tested at the world-record energy in the LHC were built in Russia and Italy and then meticulously optimised in the H8 line at the SPS North Area,” explains Walter Scandale, head of the UA9 collaboration. “The successful results were made possible by the strong support we received from the Accelerator Sector Management and the EN-STI group in close collaboration with the LHC collimation team.”

Strip crystal for the LHC (developed by INFN).

A full-size collimation system using a bent crystal as a primary deflector was initially installed in the SPS to evaluate the effectiveness of the technology and to provide a quantitative estimate of collimation inefficiency with proton and lead-ion beams. High-resolution goniometers built at IHEP were installed in the SPS vacuum pipe to orient the crystal planes. Loss rates were measured using detectors based on scintillation radiation built by INFN-Roma1 and installed around the beam pipe. Cherenkov radiation detectors built by PNPI, LAL, CERN and INFN-Roma1 were inserted into the vacuum pipe to intercept deflected beams and Medipix detectors were inserted into Roman pots. Goniometers with sub-microradian accuracy, required for the LHC, were developed in cooperation with the EN/STI group and industrial partners in Italy, Switzerland and Germany. “Tests with stored beams – that is, stable beams kept at high energy – demonstrated the feasibility of crystal-assisted collimation and a reduction by an order of magnitude of the very dangerous background induced by inelastic interaction in the primary collimator,” says Scandale. “After many years of intense work and important results, UA9 proved that the bent crystal technology was mature enough for investigating high-efficiency collimation in the LHC.

The LHC goniometer uses a piezoelectric rotational device.

For the test carried out recently in the LHC, two prototypes of high-accuracy goniometers, equipped with two silicon crystals, were installed in the betatron cleaning insertion (IR7) of the LHC, where the beam is “cleaned” by hardware components that absorb part of the primary beam halo and part of the secondary radiation. “At the 1990 LHC workshop in Aachen, Giuseppe Fidecaro asked me if bent crystals could be used to extract beam halo in the LHC,” recalls Walter Scandale. “My first impression at that time was that it was a dream, much beyond wishful thinking. Twenty-five years later we are closer to reality for crystal-assisted collimation, and crystal-assisted extraction may well be the next step forward in LHC beam manipulations.”

These initial tests at reduced beam intensity were extremely successful. Higher intensity tests are being prepared to clarify if bent crystals could improve the baseline LHC collimation as a further step towards the collimation upgrade in the High-Luminosity LHC project.

by CERN Bulletin