Over the years, the teams responsible for the LHC proton injector chain (Linac 4, PS Booster, PS and SPS) have developed various production schemes for the LHC beam, pushed the performance of the beam and explored its potential to enhance the collisions in the LHC. In 2023 and this year, until the end of last week, the so-called “standard LHC beam” has been used in batches of 3 x 36 bunches, provided by the SPS. On 24 May, the LHC was switched to the “BCMS (Beam Compression, Merging and Splitting) beam” mode to explore its potential to produce more collisions and to compare its performance to that of the standard beam.

In the LHC injector chain, the standard beam is produced by injecting three bunches from the PS Booster into the PS. After an initial acceleration, the PS splits each bunch longitudinally (see box) into three, resulting in nine bunches. These nine bunches are then accelerated to the maximum energy of the PS, where each bunch is split into two, and then again into two, resulting in 36 bunches, each spaced by 25 ns (see Figure 1).

The SPS receives three of these 36-bunch shots from the PS and accelerates them to an energy of 450 GeV before injecting them in the clockwise or counter-clockwise direction into the LHC. This means that one PS Booster bunch results in 12 bunches in the LHC. The number of protons per bunch (named intensity) required by the LHC is 16 x 10¹⁰. Taking the 12-fold splitting into account, this means that the number of protons per bunch which the PS Booster has to inject into the PS is 12 times higher than the LHC bunch intensity, i.e. 192 x 10¹⁰ protons per bunch.

The BCMS beam is produced by injecting six bunches into the PS: three from a first cycle and three, 1.2 seconds later, from a second cycle. After an initial acceleration, these six bunches are compressed and merged, in pairs of two, into a single bunch, resulting in three bunches, which are then each split into three bunches. The remainder of this production scheme is identical to the standard production scheme, which also results in 36 bunches spaced by 25 ns. With this scheme, six bunches are manipulated to obtain 36 bunches, which gives a splitting factor of six. Therefore, to obtain a bunch intensity of 16 x 10¹⁰ protons for the LHC, the PS Booster needs to provide only 96 x 10¹⁰ protons per bunch (see Figure 2).

The LHC has now used the BCMS beam for about a week and the first signs of improved performance compared with the standard beam have already been observed.

How is it that the BCMS beam results in more collisions in the LHC if it contains the same number of protons as a standard beam?

The BCMS beam has a greater brightness, which means that it contains the same number of protons but in a smaller beam size. This smaller beam size is the result of the lower intensity per bunch in the PS Booster.

The challenge is to preserve this increased brightness when the beam is accelerated in all the machines of the LHC injector chain and in the LHC itself. During acceleration in the LHC, the beam size seems to increase slightly more with the BCMS scheme than with the standard beam scheme. Studies of the beam behaviour and adjustments of the machine parameters may limit this growth in the future, further increasing the number of collisions.

Final adjustments will be made in the coming weeks. A fact-based comparison will allow us to decide whether to continue using the BCMS production scheme or to revert to the standard production scheme. Stay tuned!