This year, the SPS accelerated argon ions for the first time, delivering beam to the NA61/SHINE experiment in the North Area. NA61/SHINE explores the phenomenon of deconfinement by studying the collisions of ions of different masses, at different energies. As argon ions are much lighter than lead ions that were previously used, they will add further information in the search for the deconfinement point, a collision energy threshold above which the creation of quark-gluon plasma occurs, advancing our knowledge on matter and strong interactions.

A “super-cycle” of the SPS, featuring a proton cycle for the LHC, followed by an argon ion cycle for the North Area.

“It was a great challenge, but everything went according to plan, and we even managed to run ahead of the demanding schedule,” says Django Manglunki, who is responsible for the project.

SPS had to undergo a number of modifications in the preparations for the new beams, as it was originally designed for accelerating protons, and the revolution frequency of argon ions during acceleration differs. These preparations had to be completed in a very short time: for example, the SPS was available to the team only for two days per week during daytime (8:00–20:00) in late October, as other experiments were also using the facility. 

The source and the linear accelerator were commissioned using argon before LS1, and everything was ready before the restart of the operations. LINAC3 started with argon beams on 19 June 2014, LEIR on 30 June, PS on 25 August, and SPS on 6 October, once the North Area was ready.

The argon ion beams are extracted at six different energies following the request from NA61. As the runs are expected to take place over the course of eight weeks in 2015, argon beams have to alternate with beams of protons to avoid monopolising the SPS. This simultaneous operation involves the risk of protons escaping to the North Area, hence a new safety system was designed, and all necessary precautions were taken. This was a challenging task, requiring the effective collaboration and coordination between many different departments, but was successfully completed.

The argon runs are scheduled to end on 8 April to allow recommissioning the complex for lead ions. The lead runs will begin at the Linac on 3 May, at LEIR in early July, at the PS in mid-August, at SPS in late September, and finally at the LHC and the North Area in mid-November. The runs will continue throughout 2016 and be used for machine developments in view of Pb-Pb operations after LS2, for the upgraded ALICE detector.

The start-up of the SPS with argon ions presents a new challenge while the teams are already preparing for operation with other types of ions, lead and xenon, which will also be used by NA61/SHINE and other experiments in the North Area.