The LHC and its successors

Not too long before the first long technical stop of the LHC, engineers and physicists are already working on the next generation of accelerators: HL-LHC and LHeC. The first would push proton-proton collisions to an unprecedented luminosity rate; the second would give a second wind to electron-proton collisions.


The ring-ring configuration of the LHeC would need this type of magnets, currently being studied for possible future use.

In one year, the LHC will begin to change. During the first long shutdown, from December 2012 to late 2014, the machine will go through a first phase of major upgrades, with the objective of running at 7 TeV per beam at the beginning of 2015.

With this long technical stop and the two others that will follow (in 2018 and 2022), a new project will see the light of day. Current plans include the study of something that looks more like a new machine rather than a simple upgrade: the High Luminosity LHC (HL-LHC). Much more powerful than the current machine, the HL-LHC will aim for a very high production rate of events for the ALICE, ATLAS, LHCb and CMS detectors. “On the machine side, the HL-LHC project has been approved and should be co-funded as a study by the EU,” says Oliver Bruning, Leader of the Accelerators and Beam Physics Group (ABP) of BE Department HL-LHC Deputy Project Leader and Accelerator Study Leader of LHeC. “On the experiments’ side, teams are now working on the technical design reports. This part has not been approved yet, and the funds are still to be found.” If everything goes well, the HL-LHC could be ready to start running in around 2020.

In the meantime, engineers and physicists have begun to work on another project, which could be built in parallel to the HL-LHC. Called LHeC, it will be designed for collisions between electrons and protons. “The LHeC could be implemented in two different ways,” explains Frank Zimmermann, member of the BE/ABP Group and Deputy EuCARD Coordinator. “The first one would be to build an electron ring in the LHC tunnel, on the top of the current one - leading to what physicists call a ring-ring machine. The second solution would be to build a separate tunnel of about 9 km that would host two superconducting LINACs in a racetrack Energy Recovery LINAC (ERL) configuration where the two LINACs are connected via return arc. The electron beam would be accelerated in three passages through each LINAC, before electron-proton collisions would occur at the highest energy in the LHC.” In both cases, the electron and proton beams would cross at a unique point, where a new experiment would need to be installed.

Between now and 2040, CERN should see a whole succession of accelerator projects. The HL-LHC and the LHeC, if the latter is built, will operate in parallel for several years.

Obviously, each of these two proposals comes with its own challenges. “With the ring-ring option, the problem is that the existing LHC experiments don’t have a hole for the electron beam,” emphasizes Oliver. “So, we will have to bypass them by building an extra tunnel 1.3 km long for each experiment to permit the electron beam to follow its route without mishap.” Additional tunnel work would be necessary for the beam injection and extraction. “This option would translate into a lot of civil engineering and complicated logistics,” Oliver concludes.

On the other hand, the superconducting LINAC presents other difficulties. To reach maximum performance, the power of the beam would be so high that it would become unaffordable in terms of… the electricity bill! That's when Frank suggested using the ERL technique - recuperating the beam after the collision then decelerating it to recuperate its energy. “We can use a superconducting accelerating structure, where the field continuously oscillates between accelerating and decelerating phases. For this idea to work, the cavities need to have a very good quality factor for the fundamental resonance,” explains Frank. “Once a ‘used’ beam has been decelerated, the field resonates until the new beam arrives for acceleration. Energy is transferred continually from decelerating to accelerating bunches passing through the same LINAC. That’s very cost-effective.”

As incredible as it might seem, physicists and engineers are already looking at after 2035, over 20 years from now, when both the HL-LHC and LHeC will be switched off. Another machine could then take up the baton: the High Energy LHC (HE-LHC), the construction of which could begin in 2025. But that’s another story…

by Anaïs Schaeffer