The HL-LHC-WP5 teams are extremely busy in LS2 because some of the key upgrades for HL-LHC will already be used in LHC Run3:

• New materials for the primary and secondary collimators of the betatron cleaning system to reduce the machine impedance;
• New dispersion suppressor collimators to cope with the ALICE luminosity upgrade (IP2) and for improved betatron cleaning (IP7).

In this challenging period, when most laboratories and companies were affected by the international crisis caused by the COVID-19, the companies involved in collimation production managed to continue working efficiently, though at a slower pace.

The new dispersion suppressor (DS) collimator, the TCLD, is a collimator that operates at room temperature and will be installed in a special cryogenic by-pass in the DS regions around IP2 and IP7. It was developed for HL-LHC and has a novel design. Around IP7, space for the TCLDs will be made available by replacing an existing 15m, 8 T LHC dipole in Cell 9 with two, shorter 11 T dipoles leaving a gap in the middle for the TCLD. For IP2, the TCLD will be installed at the location of the empty connection cryostat in Cell 11, without the need for new magnets. The TCLD design is the same for all cases. Five TCLDs have been successfully built in industry: four for installation in the tunnel and one operational spare. The production was completed in February 2020, with the last two units delivered to CERN in June (see Figure 1).

The first unit, built in 2019, was installed on the left side of IP2 in February 2020 (see Figure 14). This was an important milestone for WP5, and followed the successful installation of the new, shorter connection cryostats and cryogenic by-pass around IP2, also completed in February 2020. The second TCLD is planned for installation on the right side of IP2 in July 2020. The two units for IR7 are also essentially ready and their installation will depend on the availability of the 11 T dipoles, with the earliest possible installation date being in the first quarter of 2021.

The new, low-impedance collimators are planned for installation in IR7: 4 new primary collimators (TCPPM) made of molybdenum-graphite (MoGr), and 8 new secondary collimators (TCSPM) made of Mo-coated molybdenum-graphite. They will replace existing primary and secondary collimators or be installed in existing free slots adjacent to other collimators, with their main goal being to significantly reduce the impedance contribution of the present collimators while maintaining the same cleaning efficiency. An uncoated carbon-fibre composite (CFC) material, presenting a high impedance to the beam, is used in the present collimators, which needed to be improved to prepare the machine for the higher-brightness LIU beams. The materials used in the new design reduce the electrical resistivity of the present collimators by a factor of 5 and 100 for the un-coated and coated MoGr respectively. The industrial production of 15 new collimators, 12 for installation and 3 spares was co-funded by HL-WP5 and the Consolidation project, the latter contributing to the construction of the new TCPPM. The production of the TCCPM is complete and the first three TCSPMs have also been received and are now under acceptance tests (Figure 3). This is another important milestone for WP5 with these being the first LHC collimators based on coated materials. The first two primary collimators have already been installed in IR7 (Figure 4) and are now ready to see the LIU beams.

In addition to novel materials and designs, the new collimators also include other new features and technologies compared to the present collimators. All the new collimators feature integrated beam position monitors (BPMs), one upstream and one downstream of the jaws measuring in the collimation plane. TCPPMs and TCSPMs also have a new dual plane BPM integrated into their vacuum tank to allow measurements in the non-collimation plane. All these BPMs will allow for a precise monitoring of the beam position at each collimator, enabling faster beam-based alignment and an even more precise setup. This feature was only available for the collimators around the experiments and in the dump region during Run2 and is now extended to all new collimators. Bringing out the signal from the passing beam is quite challenging, in particular for the TCLDs that have a very compact design. New, special cables were developed for this purpose and new “tapering” pieces at the extremities of the jaw’s active surface – also made of MoGr – have been integrated in the design.

Another important upgrade of the collimation system aims to improve the passive protection of warm quadrupoles in the IR7 region. This is achieved through the installation of TCAPM absorbers. Located around the beam vacuum chamber, but not actually in-vacuum, these shield the warm magnets from long term damage due to losses that emerge from upstream collimators. Without these new absorbers, the lifetime of the magnets would be significantly reduced. Two such devices were built by the CERN main workshop, and both were installed in IR7 in June 2020 (Figure 5). Thanks to these new TCAPMs, even the most exposed warm magnets are now expected to survive until the end of the HL-LHC physics program.

Collimation production is proceeding at full steam, with nearly 60 % of the total number of collimators to be produced in LS2 already at CERN. The CERN teams are now actively working on the acceptance of these collimators and on the preparation for installation in the tunnel. The construction and installation of the new collimators is a collaborative effort across many CERN Groups in the accelerator sector, in particular: BE/ABP, BE/BI, EN/MME, EN/SMM, EN/STI, TE/VSC.