LHC Report: Production and small angles

The last two weeks have seen steady luminosity production. The total luminosity of ATLAS and CMS exceeded 19 fb^-1, while LHCb reached 1.8 fb^-1 and ALICE, 6 pb^-1.

As reported in previous LHC reports, the continuous running with large beam intensities is resulting in beam-induced heating of certain elements, such as the synchrotron light monitor (BSRT), the ALFA detector and the injection kicker magnets. These first two elements had shown a sudden increase in temperature in the previous weeks - but only for the components that are on the counter-clockwise rotating beam. By making slight changes to the radiofrequency parameters, which affect the bunch length, the power spectrum of the beam was changed. This significantly reduced the observed heating of the BSRT and the ALFA detector.

Another improvement was recently made to the measurement process of the number of transverse oscillations of the beam in one turn, known as the “betatron tune”. The frequency of the betatron tune is one of the most important machine parameters. It is finely adjusted by a feedback loop during the energy ramp and the squeeze process of the beams. The betatron peak can sometimes be difficult to find in the noisy beam spectrum – especially since the transverse damper, used to stabilise the beam, also flattens this betatron peak. In the new development the transverse damper strength was reduced on a few bunches. The tune measurement system was set up to measure the tune on one of these bunches, as it would have had a reduced damper gain. This has resulted in a much cleaner tune signal and a more reliable functioning of its feedback system.

A 24-hour special run, in which the beams were de-squeezed to larger than normal beam sizes, was also carried out in order to allow the ALFA and TOTEM experiments to perform measurements of low-angle proton-proton scattering. Read more about this run in: The LHC, de-squeezed.

by Jan Uythoven for the LHC team