LHC Report: studies for the future

The proton run finished in the morning of Wednesday, 4 November and was followed by five days of Machine Development period, just before the start of the Technical Stop on Monday, 9 November. A lot of lessons have been learned and this opens the way to providing higher luminosity to the experiments.

During this year’s third and final Machine Development period, the different teams working on the machine were able to deepen their understanding of beam control and beam dynamics. The careful study of beam instabilities revealed a major improvement during the year. This time, stabilising the beams in the LHC required much weaker octupole magnet stabilisation than during the previous Machine Development period. This was due to the very effective electron cloud scrubbing that took place during physics fills after the summer holidays. This opens the road towards having more bunches in the machine, higher bunch charge and thus higher luminosity.

To obtain high luminosities, beam sizes need to be as small as possible at the collision points in the experiments. However, once the beam becomes unstable during the process of energy ramping and squeezing, the so-called emittance blow-up cannot be recovered. For this reason a close tracking of the beam-size evolution along the LHC cycle was performed. The study has pinned down processes that spoil the beam quality when starting with very small bunches, allowing dedicated optimisations to be considered in the future.

For the first time a bent crystal has been put in the beam halo of a full-energy 6.5 TeV proton beam, demonstrating the effectiveness of this material at deflecting the particle trajectory. A wide range of applications awaits this technology, including low-impedance collimation systems and beam injection and extraction systems.

An increased understanding of beam losses that take place when the beams are injected into the LHC was obtained by using diamond beam-loss monitors. Their fast response time on the nanosecond level enabled the source of these losses to be identified. Knowing the source, the losses can be reduced by cleaning the beam extracted from the SPS, and one can immediately measure this effect in the LHC.

Successful tests on the beta*-levelling technique were also carried out during this Machine Development period: beams were squeezed during collisions and proved to be remarkably stable. Thanks to this technique, experiments can receive constant luminosity at the beginning of a physics fill instead of having to deal with an initial pile-up that's too high. The levelling is achieved by colliding not fully squeezed beams during the first hours of the fill when the beam intensity is at its maximum and then reducing the beam size while the beam intensity reduces.

These are only a few highlights of the many tests carried out during the Machine Development days. Now it is up to the different teams to work out the wealth of measurement data and use it for future operation. The LHC team is highly grateful to all teams involved for adapting to a changing and not-always-convenient schedule.

According to the schedule, the LHC will resume operation on Wednesday, 18 November with a special proton-proton run at 2.51 TeV per beam, in preparation for the ion run that should start on Monday, 23 November.

by Jan Uythoven & Rogelio Tomás García for the LHC team