Accelerator Report: Lead ions take the stage

On 30 October at 8 a.m., the SPS delivered its final protons of the year to the North Area, marking the shift to the lead-ion physics run.

To prepare for this transition, the BE departmental safety officer (DSO), with support from various experts, conducted this morning the SPS North Area lead-ion DSO test. This test involves verifying the additional safety measures required for lead-ion operations, ensuring that no protons can accidentally be directed to the North Area. The DSO test includes evaluating fault scenarios, confirming safety protocols and validating all the measures in place. Once it was completed, the lead-ion beam was extracted and directed towards the North Area, where beamline physicists are now adjusting the beam trajectories and optics to deliver high-quality lead-ion beam to the experiments.

Approximately 30 hours after the last proton beam to the North Area was stopped, a radiation survey will be conducted in the early afternoon of 1 November. During this survey, radiation protection specialists will measure radiation levels along the machines (Linac4, PSB, PS and SPS). Annual comparisons of these measurements help track radiation level changes and identify areas where levels may have increased or decreased. Machine experts are then consulted to understand the potential causes of any changes. If any anomalies are detected, they can be investigated further and corrected for future runs. This approach helps to keep radiation levels as low as reasonably possible, minimising downtime and ensuring safe conditions for machine interventions following a predefined cooldown period.

Over the weekend of 2 and 3 November, the final adjustments will be made to the SPS lead-ion beam production and North Area beamlines before starting the four-week lead-ion physics run in the North Area on 4 November, which will be concluded at 6 a.m. on 2 December.

The lead-ion beam from the PS is also used in the PS East Area by the HEARTS (High-Energy Accelerators for Radiation Testing and Shielding) project, which will start its experiments on 11 November and will also conclude at 6 a.m. on 2 December.

The lead-ion beam for the LHC is of a unique type and has been meticulously prepared over recent weeks. This involved five dedicated 10-hour sessions during which SPS proton physics was paused in order to allow expert teams to set up the LHC lead-ion beam in the SPS. This beam requires a series of 14 injections, each containing four lead-ion bunches from the PS, resulting in 56 bunches spaced by 100 ns. After an initial acceleration, a process called slip-stacking reduces the bunch spacing to 50 ns, allowing more bunches to be injected into the LHC. The beam then undergoes a final acceleration, reaching the final flat-top, after which it is extracted and injected into the LHC in either a clockwise or counterclockwise direction.

The initial set-up of lead ions in the LHC, using only a few bunches, took place over the weekend of 26 and 27 October. Currently, the LHC is colliding protons at 2.68 TeV - compared with the usual 6.8 TeV - for the proton-proton reference run, providing calibration data for the upcoming lead-ion collisions. Final adjustments for the lead-ion beam set-up in the LHC are scheduled for the coming weekend and should be concluded on 4 November, with lead-ion collisions for physics set to begin later that day. The LHC lead-ion run will conclude at 6 a.m. on 25 November, with the injector complex remaining operational for another week to provide beam to the experimental facilities.

As mentioned in my previous report, additional enhancements to the lead-ion beam's performance as well as efforts to reduce background, particularly for the ALICE experiment, are essential. Last weekend offered the first promising signs of these improvements, and there is much anticipation for the final outcomes with full beams. Next week will bring further answers, hopefully confirming these positive developments.