LHC Report: highs and wet lows

Summertime, and the livin’ is easy… not so for the LHC, which is just entering four weeks of full-on luminosity production.

In the two weeks that followed the first technical stop (7-9 June), the LHC has demonstrated once again an outstanding performance. Thanks to the excellent availability of all systems, peaking at 93% in week 24, it was possible to chain physics fill after physics fill, with 60% of the time spent in collisions.

We have now surpassed the total integrated luminosity delivered in 2015 (4.2 fb^-1). The integrated luminosity for 2016 now exceeds 6 fb^-1 for each of the two high-luminosity experiments, ATLAS and CMS. Long fills, exceeding 20 hours, are now part of regular operation, with some producing more than 0.5 fb^-1. With the summer conferences approaching, this certainly provides a good dataset for the LHC experiments to analyse and present.

Several records were broken again, namely the highest instantaneous luminosity – over 9 x 10³³ cm^-2 s^-1 on 14 June – and the largest integrated luminosity in one fill of around 550 pb^-1 delivered in 29 hours between the 20 and 21 June.  The LHC is now very close to the design luminosity value of 10³⁴ cm^-2 s^-1.

This luminosity production period was briefly interrupted for the commissioning of the high beta-star beam cycle. Contrary to what is done in the normal physics cycle, in the high beta-star cycle, the beam size at the interaction points is increased before the beams are put into collision. For this high beta-star cycle, for example, the beta functions at the interaction points 1 (IP1) and 5 (IP5) were increased to 2.5 km, while for the normal cycle these are normally squeezed to 40 cm. This results in beams with "large" transverse size - about 1 mm - and very small angular divergence - about 0.4 microradian - at the interaction point. This allows precise small angle scattering studies by the forward physics experiments AFP, ALFA and TOTEM. By comparison, the transverse beam sizes in IP1 and IP5 during normal physics cycles are of the order of 13 micrometres and the divergence, around 33 microradians. The commissioning of this high beta-star cycle has been successful and was completed in two fills, spanning a period of around 18 hours. Furthermore, the optic parameters have been measured and corrected with a remaining error of only few percent. Some validation steps are still required prior to the dedicated physics run scheduled for September.

Unfortunately, the heavy rain of the last weeks has taken a toll not only on our spirits, but also on the LHC. On Tuesday, 14 June in the morning, dedicated sensors alerted the Technical Infrastructure (TI) operators to the presence of water in the LHC at point 3. Here, the LHC tunnel crosses an underground stream descending the Jura, and in periods of heavy rain, water infiltrations inside the LHC can occur. Interventions by several teams were necessary in order to repair the damage caused by the water, the most severe being water infiltration inside electric and electronic equipment of the collimation system, grounding the LHC for almost 48 hours.

by Enrico Bravin and Stefano Redaelli for the LHC team