In about a week, lead ions will be sent from the SPS into the LHC to collide in the accelerator’s four big experiments – ALICE, ATLAS, CMS and LHCb. This is a particular highlight for the ALICE collaboration, which has been eagerly awaiting lead-ion collisions since the end of Long Shutdown 2 (LS2), when its detector was upgraded. ALICE (A Large Ion Collider Experiment) is a detector dedicated to heavy-ion physics. It is designed to study the physics of strongly interacting matter at extreme energy densities, where a phase of matter called quark-gluon plasma forms.

The following week, the SPS will also provide slow-extracted lead-ion beam pulses of 4.5 seconds per cycle to the North Area experiments. The NA61/SHINE experiment is the main user of lead ions in the North Area, but other users will also benefit from these during the short period they are available.

In the last two weeks of the 4-week 2023 run, the PS will provide lead ions to the East Area, where the CHIMERA facility irradiates electronics with high-energy heavy ions to study the effects of cosmic radiation on electronics used in the CERN accelerators and experiments, as well as for space missions and avionics.

Although the lead-ion physics period is relatively short, it is of great importance, and special care is taken by the experts and operations teams to provide high-quality beams.

The origin of lead ions and lead-ion beams
Lead ions are “born” in the source of Linac3, where a pure lead sample is evaporated: oxygen gas and lead vapour are injected into the source plasma chamber. A microwave is applied to create the plasma in which the lead and oxygen atoms are ionised. These ions are then extracted, partially stripped and accelerated. The lead-ion charge after the stripping process is 54+, meaning that 28 of the 82 electrons have been removed (a lead atom originally has 82 electrons).

These lead ions are then transported and injected into the next machine in the chain, LEIR (Low Energy Ion Ring), which can receive single or multiple pulses, depending on the beam intensities needed (the more pulses, the more lead ions accumulated and the higher the intensity).

For the LHC beam, LEIR receives seven pulses from Linac3, each of which is cooled using electron cooling to reduce the beam size. In this process, a “cold” electron beam is sent along over a distance of 2.5 m with the “hot” lead-ion beam. The exchange of energy between the two beams reduces the beam size of the lead-ion beam, leaving space to inject another pulse from Linac3 and repeat the cooling process. Finally, two bunches are accelerated and extracted towards the PS.

The PS further accelerates the two-bunch beam and performs several longitudinal beam manipulations using the radiofrequency (RF) cavities to finally obtain four bunches spaced by 100 ns. After up to 14 cycles, these four bunches of Pb⁵⁴⁺ ions are then transported to the SPS. In the transfer line between the PS and the SPS, the ions are fully stripped of their remaining electrons to become Pb⁸²⁺ ions divided into 56 bunches spaced by 100 ns.

After an initial acceleration in the SPS, the beam is slip-stacked (see box) to reduce the bunch spacing to 50 ns, thus doubling the total lead-ion beam intensity in the LHC. Following a final acceleration phase, the beam is extracted and injected into the LHC, either in a clockwise or counter-clockwise direction. The LHC will be filled with up to 1248 bunches per beam.

As I write this article, the Linac3, LEIR and PS machines are producing lead-ion beams on a routine basis. The focus is now on completing the commissioning of slip-stacking in the SPS; this process is already well advanced and it looks likely that slip-stacked ion beams will be delivered to the LHC in the coming weeks.

A new method to reduce bunch spacing for lead ion beams Over the last few years, the CERN ion injector complex has undergone a series of upgrades in preparation for a doubling of the total intensity of the lead-ion beams for the HL-LHC. In the SPS, teams began using a technique known as “momentum slip-stacking”, which involves injecting two batches of four lead-ion bunches separated by 100 nanoseconds to produce a single batch of eight lead-ion bunches separated by 50 nanoseconds. In this process, the 56 bunches injected into the SPS are divided among two RF systems, which each receive 28 bunches. As there is a small frequency difference between these two RF systems, half of the beam travels slightly faster along the SPS circumference (known as “slipping”). Once the two halves of the beam are placed so that the space between two bunches is 50 ns, the beam is interleaved (or “stacked”). This allows the total number of bunches injected into the LHC to increase from 648 in Run 2 to 1248 in Run 3 and subsequent runs.