On 11 May, four days before the original schedule had set a target of 1200 bunches per beam, the LHC made its final intensity ramp-up step to 2400 bunches per beam.
In fact, the intensity ramp-up step to 2400 bunches does not mean that there are exactly 2400 bunches in each beam, as the precise number depends on the filling scheme used. For the fill of 11 May, the actual number of bunches was 2347, which lasted close to 11 hours and contributed, with an intensity of 1.3x1011 protons per bunch, 0.48 fb-1 to the integrated luminosity goal of 75 fb-1 for 2023.
The filling scheme is defined according to the needs of the experiments, but it also depends on the beam production scheme selected in the injectors and the needs of the LHC machine itself – such as leaving sufficient empty space for the dump kickers to rise, or choosing specific bunch patterns to reduce the production of electron clouds. Therefore, it may be adapted during the run to maximise the production of luminosity within the given constraints of the LHC and the injectors.
One of the filling schemes defined for 2023 has 2374 bunches per ring. In the left-hand bottom of the LHC page 1 the filling scheme is indicated with what looks like a cryptic code and is described as:
The first number, “25ns”, indicates the spacing between the bunches, while the second, “2374b”, is the total number of bunches per beam. The following three figures specify the number of bunches that will collide in each of the four LHC experiments: “2361” is the number of bunches out of the 2374 bunches that will collide in ATLAS (IP1) and CMS (IP5); “1730” is the number of bunches that will collide in ALICE (IP2); and “1773” is the number of bunches colliding in LHCb (IP8).
The remainder of the cryptic code is an indication of the beam production scheme used. “236bpi” indicates that the maximum bunch train length coming from the SPS and injected into the LHC contains 236 bunches, but shorter bunch trains may be injected too. “13inj” means that the LHC will inject 13 bunch trains per beam with a maximum length of 236 bunches each. The very last part of the cryptic code contains some special information: “hybrid” means that the bunch train of 236 bunches is produced in the injectors through the so-called hybrid scheme, which is a combination of different bunch patterns; “2INDIV” means that two individual or single bunches are also injected.
The hybrid scheme is produced in the injectors and provides the 236-bunch train with a pattern of seven batches, each with an “8b4e” bunch pattern of 8 bunches and 4 empty buckets (56 bunches). This is then followed by 5 batches of 36 bunches (180 bunches), resulting in the total length of the bunch train of 236 bunches. This hybrid scheme was chosen to maximise the luminosity production while keeping the heat load on the LHC beam screen, which is induced by the production of electron clouds, within acceptable limits. Leaving more gaps in the bunch train by introducing the four empty buckets will lower the total number of bunches that collide, but also limits the heat load, while leaving room to increase the number of protons per bunch from 1.3x1011 to the goal of 1.8x1011.
Today, the LHC is in full production with ~2400 bunches, and the next step is the gradual increase of the number of protons per bunch. As I write, the intensity per bunch has reached 1.6x1011 protons and the integrated luminosity in ATLAS and CMS is 10 fb-1 out of the 75 fb-1 targeted.