Shortly after midday on 30 March, protons circulated in the Large Hadron Collider (LHC) for the first time in 2018. Following its annual winter shutdown for maintenance and upgrades, the machine now enters its seventh year of data taking and its fourth year operating at a centre-of-mass energy of 13 TeV.

The LHC restart, which involves numerous other links in the CERN accelerator chain, went smoothly. At the beginning of March, the first protons were injected into Linac2, and then into the Proton Synchrotron (PS) Booster. On 8 March the PS received beams, followed by the Super Proton Synchrotron (SPS) one week later. In parallel, the teams had been checking all the LHC hardware and safety installations. No fewer than 1560 electrical circuits had to be powered and about 10,000 tests performed before the LHC was deemed ready to accept protons.

The first beams circulating contain just one bunch, each of which contains 20 times fewer protons than in normal operation; the energy of the beam is also limited to the SPS injection energy of 450 GeV. Further adjustments and tests were undertaken in early April to allow the energy and density of the bunches to be increased.

Bunching up

As the Courier went to press, a few bunches had been injected and accelerated at full energy for optics and collimator commissioning. The first stable beams with only a few bunches are scheduled for 23 April, but could take place earlier thanks to the good progress made so far. This will be followed by a period of gradual intensity ramp-up, during which the number of bunches will be increased stepwise. Between each step, a formal check and validation will take place. The target is to fill each ring with 2556 bunches, and the experiments will be able to undertake serious data collection as soon as the number rises above 1200 bunches – which is expected in early May.

Since early December 2017, when the CERN accelerator complex entered its end of year technical stop, numerous important activities were completed on the LHC and other accelerators. Alongside standard maintenance, the LHC injectors underwent significant preparatory work for the LHC Injector Upgrade project (LIU) foreseen for 2019 and 2020 (CERN Courier October 2017 p32). In the LHC, an important activity was the partial warm-up of sector 1-2 to solve the so-called 16L2 issue, wherein frozen air from an accidental ingress caused beam instabilities and losses during last year’s run: a total of 7 l of frozen air was removed from each beam vacuum chamber during the warm up.

The objective for the 2018 run is to accumulate more data than was collected last year, targeting an integrated luminosity of 60 fb–1 (as opposed to the 50 fb–1 recorded in 2017). While the intensity of collisions is being ramped up in the LHC, data taking is already under way at various fixed-target experiments at CERN that are served by beams from the PS Booster, PS and SPS. The first beams for physics at the n_TOF experiment and the PS East Area started on 30 March. The nuclear-physics programme at ISOLDE restarted on 9 April, followed closely by that of the SPS North Area and, later, the Antiproton Decelerator.

2018 is an important year for the main LHC experiments (ALICE, ATLAS, CMS and LHCb) because it marks the last year of Run 2. In December, the accelerator complex will be shut down for a period of two years to allow significant upgrade work for the High-Luminosity LHC, with the deployment of the LIU project and the start of civil-engineering work. Operations of the HL-LHC will begin in earnest in the mid-2020s, promising an integrated luminosity of 3000 fb–1 by circa 2035.