LS2 report: new LHC collimators


Installation of passive absorber mask TCAPM for warm magnet protection
Installation of the passive collimator absorber TCAPM in IR7, which protects the magnets from the losses produced by the interaction between the LHC beam and the IR7 collimators. In the picture: Cristina Bahamonde
(Image: CERN)

Upgrades of the LHC collimation system, which began during LS1, have continued during LS2. Sixteen new collimators have been installed in the accelerator over the last three years in preparation not only for the accelerator’s next period of operation (Run 3) but above all for the future High-Luminosity LHC (HL-LHC).

The HL-LHC, which is due to be commissioned at the end of 2027, will improve on the current LHC’s performance thanks to a tenfold increase in its integrated luminosity, i.e. the number of collisions per surface unit, thereby increasing the number of collisions inside the experiments. To achieve this, the HL-LHC’s beams of particles will be more intense, which is not without its problems.

Increasing the number of particles in circulation, and therefore the number of collisions, requires the LHC’s equipment protection systems to be reinforced. Particles that stray from their trajectory could hit sensitive components such as superconducting magnets and interfere with their operation. Protection is particularly crucial in the vicinity of the experiments and the areas of the LHC that are dedicated to beam collimation.

That’s why the HL-LHC needs a more efficient collimation system. The collimators, which are installed in two areas of the LHC (at Points 3 and 7 of the ring) and around the four big experiments (ALICE, ATLAS, CMS and LHCb), are special devices equipped with jaws – movable blocks made of heavy-duty materials – that close around the beam to clean up the stray particles. The materials used for these jaws are capable of withstanding extreme pressure and temperatures as well as high levels of radiation. Some of the collimators have fixed apertures and are there to protect the magnets from radiation.

During LS2, 16 new collimators of various types have been installed in the machine. Two TCLD (target collimator long dispersion suppressor) collimators were installed around the ALICE experiment in 2020. The majority of the new collimators was installed in Point 7, where most of the beam “cleaning” takes place. “We’ve installed no fewer than 14 colllimators around Point 7 during LS2. Some have replaced existing collimators to improve them, while others are new additions,” explains Stefano Redaelli, who heads up the collimation upgrade work package for the HL-LHC project. “I’d like to thank all the teams involved from the Accelerator and Technology sector (ATS) for their unfailing commitment – they’ve accomplished a remarkable feat!”

Three types of collimators have been installed: four primary collimators (TCPPM – target collimator primary pick-up, metallic), eight secondary collimators (TCSPM – target collimator secondary pick-up, metallic) and two fixed-aperture passive absorbers. “The primary and secondary collimators, which were manufactured with contributions by international industrial partners, have a new design,” says Stefano Redaelli. “They are based on a molybdenum-graphite compound that, thanks to its low electrical resistivity, helps to improve the stability of the planned higher-intensity beams. The secondary collimators are also coated in 6 microns of pure molybdenum, which further reduces their electrical resistivity by a factor of 20.” What’s more, these new collimators are equipped with sensors that monitor the beam position to allow the position of the jaws to be adjusted.

Two new crystal collimators, which were developed for operation with heavy ions, are also due to be installed at Point 7 at the end of this year. We’ll report back next year with more details and the results of the first tests with beam.

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