The LHC is restarting

18 March 2016

The LHC, last among all of CERN’s accelerators, is resuming operation with beam while this issue goes to press. The year-end technical stop (YETS) started on 14 December 2015. During the 11 weeks of scheduled maintenance activities, several interventions have taken place in all of the accelerators and beamlines. They included the maintenance of the cryogenic system at several points; the replacement of 18 magnets in the Super Proton Synchrotron (SPS); an extensive campaign to identify and remove thousands of obsolete cables; the replacement of the LHC beam absorbers for injection (TDIs) that are used to absorb the SPS beam if a problem occurs, providing vital protection for the LHC; and 12 LHC collimators have been dismantled and reinstalled after modification of the vacuum chambers, which restricted their movement.

The YETS also gave the experiments the opportunity to carry out repairs and maintenance work in their detectors. In particular, this included fixing the ATLAS vacuum-chamber bellow and cleaning the cold box at CMS, which had caused problems for the experiment’s magnet during 2015.

Bringing beams back into the machine after a technical stop of a few weeks is no trivial matter. The Electrical Quality Assurance (ELQA) team needs to test the electrical circuits of the superconducting magnets, certifying their readiness for operation. After that, the powering tests can start, and this means about 7000 tests in 12 days – a critical task for all of the teams involved, which will rely on the availability of all of the sectors. About four weeks after the start of commissioning, the LHC is ready to receive first beams and for them to circulate for several hours in the machine (stable beams).

The goal of this second part of Run 2 is to reach 2700 bunches per beam at 6.5 TeV and with nominal 25 ns spacing. In 2015, the machine reached a record of 2244 bunches in each beam, just before the beginning of the YETS. In 2016, the focus of the operators will be on ensuring maximum availability of the machine. For this, pipe scrubbing will be performed several times to keep the electron cloud effects under control. Thanks to the experience acquired in 2015, the operators will be able to improve the injection process and to perform ramping and squeezing at the same time, therefore reducing the time needed between two successive injections.

In addition to several weeks of steady standard 13 TeV operation with 2700 bunches per beam and β* = 40 cm, the accelerator schedule for 2016 includes a high-β* (~ 2.5 km) running period for TOTEM/ALFA dedicated to the measurement of the elastic proton–proton scattering in the Coulomb–nuclear interference region. The schedule also includes one month of heavy-ion run. Although various configurations (Pb–Pb and p–Pb) are still under consideration, the period – November – has already been decided. As usual, the heavy-ion run will conclude the 2016 operation of the LHC, while the extended year-end technical stop (EYETS) will start in December and will last about five months, until April 2017. Several upgrades are already planned by the experiments during the EYETS, including installation of the new pixel system at CMS.

The goal for the second part of Run 2 is to reach 1.3 × 1034 cm–2 s–1 of luminosity, which with about 2700 bunches and 25 ns spacing is estimated to produce a pile-up of 40 events per bunch crossing. This should give an integrated luminosity of about 25 fb–1 in 2016, which should ensure a total of 100 fb–1 for Run 2 – planned to end in 2018.

bright-rec iop pub iop-science physcis connect