WP2: Performance and Beam Dynamics

During a measurement campaign in 2017 at the LHC, it was clearly demonstrated for the first time the beam lifetime improvement due to a compensation of the beam-beam long range effects, using DC current baring wires. The measurements and design studies are continuing during 2018 for further refining of the hardware configuration. The success of this studies can open new optimisation strategies for improving the HL-LHC performance.

Many colliders, including LHC, use the closest quadrupole to the IP to perform optics measurements by modulating its gradient and measuring tune variations. This is typically the most accurate measurement of IP beta functions. In the HL-LHC previous baseline, the two quadrupoles closest to the IP were powered together. This configuration significantly limits the achievable beta-function accuracy. Implementing a dedicated 35 Amps circuit to power only the closest quadrupole to the IP improves the beta-function accuracy by 40%. This new circuit provides HL-LHC with a powerful tool for optics commissioning.

WP3: Magnets

The triplet program went through the third test of the short model MQXFS5, smoothly reaching required performance (see Fig. 8, left); this test is particularly significant since this magnet is the first one with PIT conductor, that will be used for one prototype and two series magnets. In the US, the first 4.0-m-long prototype was tested, reaching nominal current after few quenches. The magnet reached halfway between nominal and ultimate, and was interrupted by an electrical short; investigations are ongoing to clarify the mechanism of the short and to reduce the risk of such events in the future production.

In KEK (Japan), the winding of the second short model is ongoing; this includes a fine tuning of the cross-section to meet the requirements in terms of field quality and mechanical performance. An agreement to cover the construction of the prototype and series is under negotiation. In LASA-INFN (Italy), the third type of the corrector magnet (decapole) was successfully tested in September 2018, and the construction of the dodecapole started in the industry. In CIEMAT (Spain) the first two coils of the short model orbit corrector have been manufactured. The first coil of the short model of the recombination dipole D2 has been manufactured in the industry, within the collaboration agreement with INFN-Genova. At CERN, the first short model for the D2 correctors was successfully tested (see Fig. 1, right); this magnet is based on a canted cos theta design (also called tilted solenoid, or double helix), used at CERN for the first time. The QUACO project, planning to construct two large Q4 aperture prototypes, went through the second phase of engineering in three European companies.

WP4: Crab Cavities

A major milestone towards the crab crossing of the HL-LHC beams was achieved in 2018 with the completion of the test stand installation in the CERN SPS and first tests of the DQW Crab Cavity prototype cryostat with proton beam.

The next prototype using the RF dipole cavities for horizontal crabbing is in its initial stages of fabrication. It is anticipated to be completed by the end of 2020 for its installation in the SPS. This development is jointly carried out with the collaborations from US-AUP for the dressed cavities and UK-STFC for the cryostat. The construction of the HL-LHC series dressed cavities will be completely done under the US-AUP project.

WP5: Collimation

Important upgrades of the LHC collimation system take place already in LS2: the deployment of dispersion-suppressor (DS) collimation around IR7 (with 11T dipoles) and IR2 (without) and a first deployment of low-impedance secondary collimators. These upgrades will allow pushing the beam parameters at the LHC already in the Run III, after the LHC injector upgrade. A key milestone of the last 12 months was the signature of the main contracts for the external production of 20 new collimators (including five new primary collimators for the LHC consolidation). This follows successful tests performed in 2017 with and without beam, on two complete prototypes of the new collimators: the novel TCLD for the DS and the low-impedance secondary collimator TCSPM. The latter was installed in the LHC and tested with beam in 2017, demonstrating the impedance improvement expected from this new design.

Another important achievement for WP5 was the successful preparation of an international review on the design readiness for the hollow electron lenses (link). The panel assessed the readiness for construction of two lenses for enhanced halo collimation at the HL-LHC, following the positive outcome of a review in 2016 on the needs of such device. These lenses can provide an effective risk mitigation for the operation at the HL-LHC in presence of highly-populated beam tails. The CMAC in January 2018 also supported the insertion in the upgrade baseline that is now pending procurement of fund. A crucial role is expected from collaborators that can help with in-kind contributions.

WP6A: Cold Powering

Major milestones, achieved in 2018, confirmed the feasibility of powering the HL-LHC magnets via MgB₂ based high-current (> |100| kA) transfer lines. Sixty-meter long flexible cryostats, conceived for housing inside the MgB2 cables, were procured by CERN from industry and were in-depth characterized at CERN. The measurements confirmed the viability of using for the MgB₂ system two-wall low-loss flexible cryostats - unconceivable for Nb-Ti systems. Static loads of below 2 W/m were measured in flexible lines operated in helium gas at temperatures ranging from 5 K to 25 K. Also, after a R&D performed at CERN, hundreds of meter long high-current MgB₂ cables were for the first time produced via industrial cabling machines used for conventional power transmission lines. This has been the first industrialization of MgB₂ cables assembled from reacted MgB₂ wires. High-strength MgB₂ wire, enabling cabling after reaction, was in the past years developed by CERN in collaboration with industry. MgB₂ wire, with unit lengths exceeding 1 km, is today produced in industry for the project, and more than 300 km have been procured by CERN.

Large laboratories and universities with industry world-wide declared their interest in constructing different parts of the HL-LHC Cold-Powering system. Discussions are taking place in view of the imminent start of the series production.

WP12: Vacuum

Preparation for in-situ amorphous carbon coating of the beam screens of four standalone magnets (Q5R2, Q6R2, Q5L8, QL8) in LS2 is progressing well: coating parameters are defined, specific tooling is ready and final coating tests will be performed on a spare LHC short straight section during summer. Development of laser treatment of the beam screen surface continues as an alternative anti-multipactoring means: the CERN-STFC-Dundee University collaboration is optimising the laser parameters to minimise the impact of surface impedance of the treated surface while keeping its secondary electron yield below 1, and COLDEX MDs in SPS will resume shortly to further investigate the behavior of laser treated surface at cryogenic temperature.

During a magnet quench, the CLIQ (Coupling-Loss Induced Quench) system induces non-linear forces on the beam screen / tungsten shielding assembly. In order to cope with the induced torsion forces, the beam screen rigidity and the positioning of the tungsten shielding blocks have been enhanced with pins mounted on pedestals welded on the outer beam screen surface: first tests to validate the robustness of this configuration against quenches forces will be performed on a MQXF prototype in the coming months.

WP15: Integration and Layout

The HL-LHC project is analysing the possibility to extend the number and type of equipment that could be remotely aligned from the CERN Control Centre, leading to significant decrease of human interventions in tunnel and providing a new knob for operation optimisation. The extension of this capability to all the elements that span from the Q1 until the Q5 included, opens new possibilities to optimize the layout area between the Q4 and the Q6. The HL-LHC project has therefore moved forward in order to thoughtfully evaluate and seize the opportunity to deliver its targets with a possible most efficient lay-out and it has launched a Matching Section Optimisation study. The goal of the analysis is to prepare a coherent package of changes that could simplify the cryogenic supply, the powering scheme, the magnet lay-out maximising the re-use of existing LHC infrastructure and equipment already operational in that area. The study should be concluded by the end of 2018, in time for discussion at the next annual HL-LHC meeting at CERN.