The latest from the LHC : Training for higher intensities

Three weeks of intense machine development were brought to a satisfactory conclusion on the night of 21 September with the final validation of the machine protection systems for operation with bunch trains. The machine is now ready to accept more and more trains of bunches.


On Wednesday 22 September, the first physics fill was made using bunch trains, with 3 trains of 8 bunches per beam, providing 16 pairs of colliding bunches per experiment. This fill was used to restart operation for physics both for the machine and for the experiments. On Thursday, the number of bunches was increased to 56 per beam, providing 47 colliding pairs at Points 1, 5 and 8, and a smaller number at Point 2 to meet the requirements of ALICE. This is roughly the same intensity that we had in the machine in August. The first fill made under these conditions, fill 1366, brought an unexpected bonus. Bunches of nominal intensity were injected into the LHC with a smaller than usual transverse size, which was expected to cause lifetime problems when they were brought into collision. On the contrary, however, the beam lifetime in collision remained surprisingly high (25 hours) and the luminosity was significantly higher than expected. This opens up unexpected possibilities for the way that the machine is operated in future.

The strategy for increasing the intensity is to make an initial step of around 50 bunches, and then make 3 fills delivering over 20 hours of colliding beams under these conditions before progressing to the next step.

This first step was made on 25 September, with an increase to 104 bunches per beam, providing 93 colliding pairs at Points 1, 5 and 8. This means that the total intensity per beam now exceeds 1013 protons, and the stored energy per beam at 3.5TeV is 6MJ. With these levels of stored energy, the LHC is now operating with the highest stored beam energy of any collider, exceeding the record set by the ISR many years ago. Under these conditions the LHC is able to deliver over 1pb-1 luminosity in a 12-hour physics fill.

As the total intensity increases, many accelerator systems have to adapt to the higher intensities. This week, beam instrumentation and radio frequency systems started to show different behaviour and need to be tuned for the new conditions.

The next increase (to 152 bunches per beam) was made on Wednesday 30 September, with a further increase scheduled over the weekend. The peak luminosity to date is just under 5 1031 cm-2 s-1, which is within a factor 2 of the target for 2010.

by CERN Bulletin