It’s almost two years since the team in the Cern Control Centre switched off the beams in the Large Hadron Collider, marking the end of the accelerator’s first three-year run – and now it’s preparing to switch it on again.
For the last two years, hundreds of engineers and technicians have been repairing and strengthening the laboratory’s accelerators and experiments in preparation for running the LHC at higher energy.
When the LHC restarts this year, the energy of particle collisions will be 13 TeV (or 6,5 TeV per beam) compared to 8 TeV (4 TeV per beam) in 2012. This higher energy will allow physicists to extend their searches for new particles and to check previously untestable theories.
To prepare the machine for this new energy frontier, 18 of the LHC’s 1232 superconducting dipole magnets, which steer particle beams around the accelerator, were replaced due to wear and tear.
More than 10 000 electrical interconnections between dipole magnets were fitted with splices – pieces of metal that act as an alternative path for the 11 000 amp current, saving the interconnection if there is a fault.
The machine will operate at a higher voltage to run the higher energy beams, and has been fitted with new sets of radiation-resistant electronics.
The vacuum system that keeps the beam pipe clear of stray molecules has been upgraded, and the cryogenics system for the LHC’s superconducting dipole magnets has been refurbished.
Bunches of protons in the accelerator will be separated in time by 25 nanoseconds compared to 50 nanoseconds. The LHC will thus deliver more particles per unit time, as well as more collisions, to the
The Cern IT department has bought and installed almost 60 000 new cores and more than 100 petabytes of additional disk storage to cope with the increased amount of data that is expected from the experiments during the new run. Significant upgrades have also been made to the networking infrastructure, including the installation of new uninterruptible power supplies.
Among the new features that the LHC will boast when it starts up again, is supplying argon ions to an experimental programme for the first time. The argon ions are produced at a special source and made to circulate around four accelerators before being sent to a target.
To control these particles, operators had to adapt the acceleration system of the Super Proton Synchrotron (SPS), a 7km-circumference accelerator that represents the last loop on the ions’ journey before they are ejected.
The SPS is the last accelerator in the chain before the 27km-circumference (LHC). To allow eight weeks of physics with argon ions while also sending protons to the LHC experiments, the accelerators will alternate between these two types of particles. In each cycle of 21,6 seconds, the SPS will deliver two beams of protons and one beam of argon ions.
The argon ions are destined for the NA61/Shine experiment, which is studying the phenomenon of quark-gluon plasma, a state that is thought to have existed at the very beginning of the universe and in which quarks moved around freely, unconfined by the strong force in protons and neutrons. More specifically, the experiment is studying the transitions between the phase in which quarks are confined and the phase in which they are free.
Last Thursday, the NA61/SHINE team recorded first collisions with argon: the argon ions, travelling with a momentum of 150 GeV/c per nucleon, collided with scandium nuclei.