Yesterday the Large Hadron Collider produced a record run lasting 20 hours and delivering a record 62/pb. The run ended with a programmed dump for the first time in a while. They then turned the machine round in just four hours for a new run with 1380 bunches for the first time. This is the maximum bunch number that will be used for physics runs this year.
At this significant point in the LHC commissioning process it is worth reflecting just how much of an achievement it is to run with so many bunches. For comparison, the Tevatron runs with just 36 bunches per beam. Of course the LHC is bigger so it is possible to get more bunches in, but it is only four and a bit times bigger. To get 1380 bunches in they have to pack them much closer together. In the Tevatron the bunches run about 175 meters apart on average but in the LHC they are on average 20 meters apart.
This improvement in the design of the LHC over previous hadron colliders is just as important as the increase in energy. Hadron collisions are messy processes and to get the full information out the physicists will need to look for very rare events with clear signals of unusual processes. By time the LHC has run its full length it will have collected thousands of inverse femtobarns of data to explore the Higgs sector in the best possible detail. To achieve this it has to run with lots of bunches and with high quality, low emittance beams.
You can’t just inject individual proton bunches into an accelerator very close together because there is a limit to how fast the kicker magnets can change as they inject the bunches into the ring. As the energy increases the magnets have to produce more powerful fields and it gets harder to pack the bunches together. The injection process uses a series of increasingly powerful rings to put together the bunches in trains (see my earlier post about the injection chain). The early stages have lower energy so the bunches can be slotted closer, but the rings are smaller and fill up quickly. You can build up as you go along but this is not enough to get the bunches as close together as they need them.
The trick that made this possible was invented in the 1990’s using the PS accelerator at CERN which is now part of the injection chain for the LHC. They first considered a procedure of debunching the protons in the ring, so that they could then reform new smaller bunches, but they found that this ruined the good emittance properties of the beams. The solution was to split the bunches by starting with a low-frequency RF signal in the ring and gradually boosting one of its harmonics to higher amplitude. If you raise the second harmonic the bunches split in two and if you raise the third harmonic it splits them in three. In the PS they start with 6 big bunches. These are first split in three to provide 18 bunches. The bunches are then accelerated to a higher energy before being further split into two. The 36 bunch trains are moved to the larger SPS ring and gathered into 144 bunch trains which are further accelerated before being injected into the main LHC ring. Later, possibly next year, they will split the bunches one more time in the PS to double the number of bunches again.
I’ve no idea who worked out how to do this bunch splitting but they are just some of the many unsung heroes of the LHC.