LHC Report: serving a hot, dense soup for Christmas

With just a few days left until the end of the year, the LHC is finishing 2015 with yet another successful heavy-ion run. This has been possible thanks to a great deal of dedication and professionalism on the part of many people throughout the Organization.

As already reported in previous Bulletin articles, the last leg of the race started on 14 November with the 5.02 TeV intermediate energy proton run, the preparation for which was interleaved with the beginning of commissioning for the ion run. Early in the morning of 17 November, lead-lead collisions were produced for the first time at a new energy frontier: 5.02 TeV centre-of-mass energy per colliding nucleon pair (or 1.045 PeV total). With the proton-proton reference run safely out of the way, commissioning and validation with ions was then completed, opening the way for Stable Beams and the start of data-taking at ALICE, ATLAS, CMS and LHCb. The following days were eventful. Steady improvements were made in various accelerator chain systems to try to reach the longstanding goal of the design luminosity at ALICE of 1x10²⁷ cm^-2s^-1. 

The lead ion source was refilled on 1 December and is currently reaching 90% of the expected performance with around 22 μA now being delivered by Linac3. A charge stripper in Linac3 was replaced, giving excellent transmission to the Low Energy Ion Ring (LEIR). The SPS team has worked hard ro reduce the beam losses during the energy ramp with excellent results. All these improvements translate into average ion bunch intensities close to 2x10¹⁰equivalent proton charges at injection into the LHC, almost three times the design intensity.

The ion beams are injected into the LHC as trains of up to 24 bunches. Up to 22 SPS to LHC injections per beam are needed to fill the machine for physics, with around 400 to 500 bunches per beam. Each train is made of 12 pairs of bunches, known as batches. The two bunches are separated by 100 ns, which is the minimum spacing achievable by the PS batch compression RF gymnastics systems. Each batch was separated by 225 ns from the following batch to match the rise time of the SPS injection kicker. The bunch and batch spacing limits the total number of bunches that can be injected into the LHC.

On 4 December, an SPS injection kicker switch was replaced, allowing a faster rise time with less jitter and, as a consequence, a batch spacing of 175 ns was achieved instead of the original 225 ns. This is an unexpected bonus not originally envisaged as being possible until after LS2. This modification allowed 474 bunches per beam to be injected (up from 426). Keen to fully exploit the possibilities, the Accelerator Beam Transfer (ABT) group pursued further improvements and managed to reduce the rise time further, to 150 ns. On 9 December, they reached their target and, for the first time, 518 bunches per beam were injected into the LHC and allowed physics to begin, increasing once again the peak and integrated luminosity. The SPS and LHC transverse dampers have been closely following all these modifications to fully guarantee the required bunch stability.

ALICE’s luminosity levelled at the design luminosity of 1x10²⁷ cm^-2s^-1 for between 3 and 4 hours of a typical 5-hour fill. ATLAS and CMS enjoyed peaks of over 3x10²⁷ cm^-2s^-1. For the first time in an ion-ion run, LHCb also took data, following its participation in the proton-lead run.

Besides physics production we also managed to squeeze in other activities, such as the necessary luminosity calibration runs and the ALICE spectrometer polarity reversal. ALICE collected more than 300 μb^-1, CMS more than 500 μb^-1 and ATLAS over 700 μb^-1, putting them fully on track for the integrated Pb-Pb luminosity goal of 2015.

And last but not least, machine development activities are also a fundamental part of the run. Crystals have once more channelled high-energy beams, this time with lead ions. The test complements the initial experiment performed with proton beams in November and together they address the feasibility of bent crystals as collimators in view of the High-Luminosity LHC project.

On 8 December, the first luminosity-driven quench took place in the LHC under controlled conditions, resolving a long-standing uncertainty on quench and luminosity limits. This result finally clarifies the case for the installation of dispersion suppression collimators in the LHC, an issue that had been under discussion since 2003.

At the time of publication, the LHC has just finished brewing the hottest and densest soup ever prepared for Christmas.

by Reyes Alemany for the LHC team