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LHC Report: the role of the injectors

How the injector chain has contributed to increase the luminosity of the proton-lead ion run


For its last four running weeks of the year the LHC is colliding protons (p) with lead ions (Pb). This not only presents a challenge for the collider itself, but also for the six accelerators involved in producing the beams, which have to provide the bunches that will eventually collide in ALICE, ATLAS, CMS and LHCb. Two different injector chains boost and deliver the two different types of particles to the LHC. For the protons, the chain is Linac 2, the PS Booster (PSB), the Proton Synchrotron (PS) and the Super Proton Synchrotron (SPS), and for the lead ions, it’s Linac 3, the Low Energy Ion Ring (LEIR), the PS and the SPS.

The challenge for the injector complex is twofold. Firstly, the injection pattern of the protons must match with that of the lead ions, in order to maximise the number of colliding bunches in the LHC.

The ideal situation would be to have the same number of bunches and the same bunch spacing for both protons and lead ions. However, the lead ion injection technique sets a constraint. LEIR and the PS can only provide the SPS with a lead ion beam consisting of four bunches spaced by 100 nanoseconds (ns), a different pattern than in normal p-p operations where the proton beam consists of 72 bunches spaced by 25 ns.

Secondly, the intensity of the proton beam must also be reduced to correspond to that of the lead ions. For p-Pb operations, the proton bunches need to be five times less intense than usual.

The best possible match that the injector team has found is to inject a train of two batches each consisting of 18 bunches of protons and a train of seven batches consisting of four bunches of lead ions (see picture for more details).

The injector chain is already benefiting from the results of the LHC Injector Upgrade (LIU) project, which aims to upgrade the beam performance of the injectors for the future High-Luminosity LHC. Since the beginning of the run, the Pb injectors have delivered an intensity three times greater than the original design value. This was a major contributor to reaching peak luminosity about six times greater than expected when the proton-lead programme was planned just a few years ago.

At the time of writing, the LHC is still on course to achieve all the physics goals of this run, in spite of all the technical mishaps encountered, such as a power cut last week and a quench on 24 November.

When not filling the LHC, the injector chain provides beams for lots of other users, including the Antiproton Decelerator, AWAKE, HiRadMat, and the North Area.

The injection scheme for the proton beam (upper part of the picture) starts with the PSB, which first sends four bunches to the PS, followed by two more 1.2 seconds later. In the PS, the bunches are then split in three and are spaced by 100 ns. The 18 resulting bunches are accelerated to 25 GeV and delivered to the SPS. This operation is repeated and the second batch of 18 bunches is injected next to the first one in the SPS, with a spacing of 200 ns. After acceleration to 450 GeV, the 36-bunch train is transferred to the LHC. For the lead ion beam (lower half of the picture): LEIR accelerates the ion beam in two bunches to 72 MeV/nucleon, and sends it to the PS. In the PS, the bunches are split in two and their spacing is set to 100 ns using an RF technique known as “batch expansion”. The accelerated beam is extracted towards the SPS and traverses a 1-mm thick aluminium foil, which strips the ions of their 28 remaining electrons. This operation is repeated six times, accumulating the seven four-bunch trains in the SPS with a spacing of 200 ns. The 28 bunches are sent to the LHC after acceleration to 177 GeV/nucleon.Due to the different bunch spacings, only the 27 blue bunches collide in each train. Each operation is repeated 20 times to fill both rings of the LHC.