LHC Report: plumbing new heights

Following the end of the arduous 2015 proton run on 4 November, the many teams working on the LHC and its injector complex are naturally entitled to a calmer period before the well-earned end-of-year break. But that is not the way things work.

The CCC team after stable heavy-ion beams are declared in the LHC.

Instead, the subdued frenzy of setting up the accelerators for a physics run has started again, this time for heavy-ion beams, with a few additional twists of the time-pressure knob. In this year’s one-month run, the first week was devoted to colliding protons at 2.51 TeV per beam to provide reference data for the subsequent collisions of lead nuclei (the atomic number of lead is Z=82, compared to Z=1 for protons) at the unprecedented energy of 5.02 TeV in the centre of mass per nucleon pair.

The chain of specialised heavy-ion injectors, comprising the ECR ion source, Linac3 and the LEIR ring, with its elaborate bunch-forming and cooling, were re-commissioned to provide intense and dense lead bunches in the preceding weeks. Through a series of exquisite RF gymnastics, the PS and SPS assemble these into 24-bunch trains for injection into the LHC. The beam intensity delivered by the injectors is a crucial determinant of the luminosity of the collider.

Commissioning of the LHC’s 2.51 TeV proton cycle had to be interleaved with that of the new heavy-ion optics in the LHC, resulting in many adjustments to the schedule on the fly and specialist teams being summoned at short notice to the CCC. Besides the overall energy shift compared to the 6.5 TeV proton optics, there is an additional squeeze of the optics and manipulations of crossing angles and the interaction point position for the ALICE experiment. Rapid work by the LHC’s optics measurements and correction team allowed the new heavy-ion magnetic cycle to be implemented from scratch (using proton beams) over the weekend of 14-15 November. Members of the collimation team also spent many hours on careful aperture measurements. At every step, one must be mindful of the strict requirements of machine protection.

The first lead-ion beams were injected on the evening of Monday, 16 November and brought into collision in all four experiments, by a bleary-eyed team, 10 hours later in the early morning.

The proton reference run resumed that Tuesday evening. After some unnerving down time, its luminosity target was comfortably attained on Sunday morning and the ion commissioning resumed with more aperture measurements and the process of verifying the “loss maps” to confirm that errant beam particles fetch up where they can do the least harm. These are very different from those of protons because of the many ways in which the lead nuclei can fragment as they interact with the collimators. A penultimate switch of particle species provided a bonus of proton reference data to the experiments overnight.

Finally, on 23 November the lead ions had the LHC to themselves and commissioning resumed with tuning of injection, RF and feedback systems. And many more loss maps.

Stable beams for physics with 10 bunches per beam was finally declared at 10:59 on 25 November and spectacular event displays started to flow from the experiments. Further fills should increase the number of bunches beyond 400.

The remaining weeks of the run will continue to be eventful with physics production interrupted by ion-source oven refills, van der Meer scans, solenoid polarity reversals and studies of phenomena that may limit future performance. These include tests of magnet quench levels with collimation losses and the use of crystals as collimators. We also plan to test strategies for controlling the secondary beams emerging from the collision point due to ultraperipheral (“near miss”) interactions.

by John Jowett for the LHC team