The twists and turns of a successful year for the LHC

On 11 December, the Large Hadron Collider (LHC) is scheduled to complete its 2017 proton-physics run and go into standby for its winter shutdown and maintenance programme. With the LHC having surpassed this year’s integrated luminosity target of 45 fb^–1 to both the ATLAS and CMS experiments 19 days before the end of the run, 2017 marks another successful year for the machine. September 2017 also saw the LHC’s total integrated luminosity since 2010 pass the milestone of 100 fb^–1 per high-luminosity experiment (see panel). But the year has not been without its challenges, demonstrating once again the quirks and unprecedented complexities involved in operating the world’s highest-energy collider. The story of the LHC’s 2017 run unfolded in three main parts.

Following a longer than usual technical stop that began at the end of 2016, the LHC was cooled to its operating temperature in April and took first beam towards the end of the month, with first stable beams declared about four weeks later. Physics got off to a great start, with an impressively efficient ramp-up reaching 2556 bunches per beam and a peak luminosity of 1.6 × 10³⁴cm^–2s^-1 in very good time.

Careful examination

However, from the start of the run, for some unknown reason the beams were occasionally dumped with a particular signature of localised beam loss and the onset of a fast-beam instability. The cause of the premature dumps was traced to a region called 16L2, referring to the sixteenth LHC half-cell to the left of point 2 (each half-cell comprises three dipoles, one quadrupole and associated corrector magnets). The hypothesis was that the problems were caused by the presence of frozen gas in the beam pipes in this region; air had perhaps entered during the cool down and had become trapped on and around the beam screen. All available diagnostics were deployed and careful examination of the beam losses in the region revealed steady-state losses, which occasionally increased rapidly followed by a very fast beam instability. The issue appeared to respond positively to a non-zero field in a local orbit corrector, and this allowed the LHC teams to establish more-or-less steady operation by careful control of the corrector in question.

To ameliorate and understand the situation better, an attempt was made to flush the gas supposedly condensed on the beam screen onto the cold mass of the magnets. To this end the beam screen around 16L2 was warmed up to around 80 K with careful monitoring of the vacuum conditions. Unfortunately, the manoeuvre was not a success: the 16L2 dumps became more frequent and many subsequent fills were lost to the problem. By this stage, electron-cloud effects had been identified as a possible co-factor in driving the instability, prompting the teams to change the bunch configuration to the so-called 8b4e scheme in which gaps are introduced into the bunch configuration. This significantly reduced the rate of 16L2 losses and allowed steady and productive running to be established by late summer.

New heights

Performance was further improved by a reduction in the “beta-star” parameter following a technical stop in the middle of September. This move exploited the excellent aperture, collimation-system performance, stability, and optics understanding of the LHC and benefited from many years of experience operating the machine. Working with an optimised 8b4e scheme and beta-star of 30 cm resulted in CMS and ATLAS reaching their event pile-up limit, forcing the deployment of luminosity levelling as is already routine in LHCb and ALICE. The peak-levelled luminosity under these running conditions is around 1.5 × 10³⁴ cm^–2 s^–1, compared to more than 2 × 10³⁴ cm^–2 s^–1 without levelling. The beam availability in the latter part of the year has been truly excellent and integrated-luminosity delivery reached new heights. One day in October was also dedicated to operation with xenon beams, taking advantage of their presence in the SPS for North Area’s fixed target programme (CERN Courier November 2017 p7).

Following a period of machine development and some special physics runs, the winter maintenance break is due to begin on 11 December. The year-end technical stop will see the usual extensive programme of maintenance and consolidation for both the machine and experiments. It will also see sector 12 warmed up to room temperature to fully resolve the 16L2 issue. Then, in the spring of 2018, the LHC will begin a final 13 TeV run before a long shutdown of two years to make key preparations for its high-luminosity upgrade.