Snapshots to shed light on LHC performance

With the impressive size and unprecedented power of the LHC, it is all too easy to overlook the smaller devices that have the difficult task of monitoring the new accelerator. You don't have to stand too far back from the big picture to see examples of clever technology inside the LHC.


One of the undulators installed in the LHC tunnel can be seen on the right of the photo. From right to left, back row: Lucio Rossi (group leader, MCS), Davide Tommasini (conceptual design, MCS), Thierry Tenaglia (integration design,TS-MME), Remo Maccaferri (project leader, MCS) and Hans Kummer (MCS/ME); front row: Gilles Trachez (MCS-ME) and Bruno Meunier (FSU-AT12).

In contrast to the usual articles about the LHC's big number statistics, examples of clever problem-solving found in beam monitoring machinery show that smaller things can be beautiful too. The design of the LHC accelerator brought new challenges for monitoring the shape of the particle beam, known as the beam profile. The size of the beam shrinks as higher energies are reached, but its position inside the pipe should stay the same in every lap of the machine. In previous CERN proton accelerators, wires moving through the beams were used to monitor the profiles. However, the use of similar devices would dangerously interfere with the beam at nominal intensity inside the superconducting LHC accelerator. New instrumentation was needed to accurately monitor the size of the beam without undesirable consequences.

The answer was simple but no less ingenious. A beam of protons travelling in a straight line is invisible to the human eye; however, when its path is bent by a specific amount, visible Synchrotron Radiation (SR) light is emitted. The normal bending magnets are sufficient for this purpose at higher beam energies (around 2 TeV), but not at 'injection' (when the beam is first launched into the accelerator) where the energy is lower. Several options were explored and the solution was a device called an 'undulator'. At one tenth of the size of the dipole magnets, the two undulators produced in total will only make up about 3 metres of the 27 km LHC ring. Each 5 tesla undulator contains a series of superconducting magnets to 'wiggle' the path of the beam. The SR light emitted in the process will travel in straight lines to silicon mirrors positioned at specific points inside the vacuum chambers of the accelerator. The light is reflected from the mirrors into two telescopes equipped with cameras fast enough to capture images of the SR light head-on (see diagram). The profile of each bunch of particles in the beam and changes in its position in the beam pipe can therefore be deduced from the images. During the LHC's operation, two beams of particles will travel in opposite directions inside the accelerator before they are made to collide. One undulator will be used to wiggle each beam throughout the operation.


The layout of the beam monitoring system at point 4 of the LHC tunnel.


Although the principles may appear simple, the realisation of the technology was a challenge that required cross-departmental expertise. The project was initiated and funded by the Beam Instrumentation group (AB/BI), who also built all the necessary optics and electronics for the SR light capture, with the help of the Mechanical and Materials Engineering group (TS/MME) and the Vacuum group (AT/VAC). The design and construction of the undulators were carried out entirely within CERN under the responsibility of the Magnets, Cryostats and Superconductors group (AT/MCS), in collaboration with the Cryogenics for Accelerator (AT/ACR) group and the vacuum group. The hardware was built by the CERN main workshop (TS/MME) and the superconducting magnet workshop (AT/MCS/ME).

The two undulators were successfully installed in the LHC tunnel on 21 November at point 4 (IR4) near Echenevex in France. The undulator magnets were satisfactorily tested by the Magnet Tests and Measurements group (AT/MTM) prior to installation and are now ready for the LHC start-up.

A record for a superconducting undulator

The use of a Synchrotron Radiation Profile Monitor for the LHC has been deeply debated because the requirements of the superconducting undulator appeared too demanding. In particular, the compactness required to reproduce a sharp beam profile, associated with the large aperture required by beam optics, imposes an operation at current and magnetic field very close to the critical limit of the superconducting zone. Due to the large ratio between magnet aperture (60 mm) and period (280 mm), a magnet field of 8 teslas had to be obtained at the magnet poles: this was never obtained in the past with impregnated NbTi coils operating at 4.5 K. The challenge has been met thanks to a 'Swiss Watch' style manufacture, where all components have been built and assembled with micrometric precision and original solutions, thus making the superconducting coils very stable against possible small perturbations.