2017 was a tough, but very interesting year at the LHC with many transverse instabilities observed due to the degraded vacuum in the beam pipe of a magnet cell. Instability rise-times as short as 10 turns were measured and none of the usual mitigation measures (chromaticity, Landau octupoles and transverse damper) can be effective for such fast-growing instabilities.
Other mitigation measures needed to be found and one of them was to use the so-called "8b+4e beam" (with 8 bunches followed by 4 empty buckets), which reduces the electron cloud effect. This was possible thanks to the huge flexibility of the injector complex and in particular of the 58-year old Proton Synchrotron. Thanks also to other improvements, a new record peak luminosity in a collider could finally be established, with ~ 2.2E34 cm-2.s-1. However, this needs to be confirmed (after detailed luminosity calibration) as this value is just slightly above the previous record established in the KEKB lepton collider in Japan (with ~ 2.1E34 cm-2.s-1).
What is sure is that more than twice the LHC design luminosity (of 1E34 cm-2.s-1) was reached in 2017 and we can thank the fathers of the LHC (the current JUAS director, Philippe Lebrun, is one of them) for having built such an incredible machine! Following the experience of the LHC, one lesson to be learnt for the future High-Luminosity project is that back-up solutions in beam parameters are important:
- the Brout-Englert-Higgs boson was discovered in 2012 thanks to the 50 ns (bunch spacing) beam, while the nominal bunch spacing is 25 ns;
- the record peak luminosity in a collider (still to be confirmed) was established with the "8b+4e" beam.