LHC Restarted, Researchers Eye 2,700 Bunches per Beam

The Large Hadron Collider (LHC) resumed operations on March 25 when CERN researchers opened its doors to allow particles to travel around the ring for the first time since the year-end technical stop (YETS) began in December 2015.

Several updates and interventions took place in LHC accelerators and beamlines during the 11 weeks of YETS, including maintenance of cryogenic systems and modification of vacuum chambers to allow less restricted movement of 12 LHC collimators. Eighteen magnets in the Super Proton Synchrotron (SPS) were also replaced, as were the LHC beam absorbers used to absorb the SPS beam if a problem occurs.

Bringing beams back into the LHC following the technical stop was no trivial matter. The CERN team first tested electrical circuits for the superconducting magnets, certifying readiness for operation, and then conducted some 7,000 powering tests across 12 days. The team then moved towards machine checkout, the phase of preparation before beam. All LHC systems, including magnetic circuits and collimators, were put through paces without beam during this phase. The machine checkout allowed engineers, for example, to ramp all hardware up to high-energy values and to test “squeeze,” a process that reduces beam size at an interaction point by adjusting magnet strength on either side of a given experiment, thereby increasing the collision rate.

Beams are made of “trains” of proton bunches moving at almost the speed of light around the 27 kilometer ring of the LHC. Sending more bunches around the ring allows researchers to generate more collisions, meaning more physics data for experiments. During LHC Run 2 in 2015, the machine reached a record of 2,244 bunches in each beam and a proton-proton collision energy of 13 TeV. In 2016, researchers will continue LHC’s Run 2 with the goal of reaching 2,700 bunches per beam at 6.5 TeV and with nominal 25 ns spacing.

Another goal this year is ensuring maximum availability of the machine. For this, operators will perform routine pipe scrubbing to keep the electron cloud effects under control. Operators will also 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.

LHC is scheduled for several weeks of steady standard 13 TeV operation with 2700 bunches per beam and β* = 40 cm, though the accelerator schedule for 2016 also includes a high-β* (~ 2.5 km) running period for TOTEM/ALFA dedicated to measurement of the elastic proton-proton scattering in the Coulomb-nuclear interference region and a one-month heavy-ion run. The heavy-ion run will conclude the 2016 operation of the LHC, followed by an extended year-end technical stop beginning in December and lasting until April 2017. Researchers have already planned several upgrades for this period, including installation of a new pixel system at CMS.