The LHC gets set for more luminosity

28 September 2010

Recent work by the operations team at the LHC has focused on pushing the machine’s performance towards higher luminosity and into new territory in terms of stored beam power.

Moving to 25 bunches per beam with almost nominal bunch intensities at the beginning of August implied operation with a stored energy in each beam of more than 1 MJ (CERN Courier September 2010 p8). This corresponds to the current record for stored beam energy in existing hadron accelerators and marks an energy regime where a sudden loss of beam or operational errors can result in serious damage to equipment: an energy of 1 MJ is sufficient to melt 2 kg of copper. Extreme care and a thorough optimization of all operational procedures are therefore required in making this important transition in the machine’s performance. The work during August has included optimizing the operational procedures and the machine protection systems, with the aim of gaining experience with the reliability and reproducibility of the operation of the machine at such a high stored beam energy.

Early August also saw record results for the LHC performance in terms of delivered luminosity. For the first time the peak luminosity surpassed 4 × 1030 cm–2s–1 and the total integrated luminosity delivered to the experiments passed the milestone of 1 inverse picobarn (1 pb–1 or 1000 nb–1) over the weekend of 7–8 August. Another step towards higher luminosity occurred on 19 August, when the number of bunches in each beam was increased from 25 to 49. By the end of August the total integrated luminosity passed the threshold of 3 pb–1, about half being delivered in just one week of running with the higher number of bunches.

In parallel, the operations team has been conducting several tests for improving the LHC performance still further. The ramp speed of the magnets (the rate at which the electrical current can be changed in the LHC main dipoles) has been increased from 2 A/s to 10 A/s for the pre-cycle (without beam) of the magnet system. The ramp speed of 10 A/s has also been successfully tested for acceleration with beam, but the final implementation must wait until the LHC starts operation with bunch “trains”, in which the bunches of protons are grouped closely together, in contrast to the present operation with widely separated bunches. The faster ramp speed reduces significantly the minimum required time between two physics fills and therefore increases the overall machine performance in terms of integrated luminosity,

Operating the machine with bunch trains will open the door for increasing the total number of bunches in successive steps, so improving the LHC’s luminosity over the coming months by another factor of 10 to 100. For this the operations team is working with bunch trains with 150 ns spacing between bunches (the current minimum spacing is 1000 ns). This involves making the necessary changes throughout the injector chain, as well as in the LHC itself. In the LHC, bunch trains imply working with a defined crossing angle between the beams throughout the machine cycle, in order to avoid unwanted parasitic collisions. This means that the whole process of injection, ramp and squeeze has to be re-commissioned.

The task also includes re-commissioning all of the protection systems, both at injection and elsewhere in the cycle. This is particularly important now that the energy stored in each beam is about 3 MJ and is set to increase further in the coming weeks. Alongside these operations, the LHC teams will bring the higher-speed energy ramp (10A/s) into operation, which will reduce the time needed to fill the machine. The initial aim of this re-commissioning phase is to bring a few high-intensity bunches in trains into collision for physics and later move from 50 up to 96 bunches injected in each direction. Once again, this should result in a significant increase in the luminosity delivered to the experiments.

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