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LHC Report: focus on luminosity

The intensity ramp-up of the LHC beams resumed last Friday after the main powering system of the PS accelerator was put back in service


LHC Report: focus on luminosity

The image above shows the last twenty four hours of fill #4947 in the machine. The LHC operations team kept the beams of this fill in the machine for a record 35 and a half hours.

Beams are back in the LHC. On Friday, the accelerator resumed the intensity ramp-up, reaching 1752 bunches per beam last week-end. The intensity ramp-up was interrupted on 20 May because of a problem with the PS’s main power supply (see box).

A steady increase in the total number of bunches per beam is required to check out all aspects of beam operation and make sure the LHC is fully safe before the nominal number of bunches per beam can be brought into collision.

At present, four intensity steps have been completed: 313, 601, 889, and 1177 bunches per beam. The qualification of the next step with 1752 bunches is in progress. At every step, more than 20 hours in stable beams must be accumulated, as required for machine protection qualification. The last step-ups already showed signs of possible electron cloud effects, with the typical signature of blown-up bunches at the end of the trains of 72 bunches. The beam and luminosity lifetimes are, however, very good: the last LHC fill before the extended stop due to the PS powering system problem was with 1177 bunches per beam and stayed in Stable Beams for 35.5 hours. The peak luminosity at the beginning of Stable Beams was 3.6 x 1033 cm-2s-1. The integrated luminosity, 272 inverse picobarn, is around a quarter of the total luminosity delivered by the LHC up to now in 2016.

Monday, 17 and Tuesday, 18 May were dedicated to measuring the absolute scale of the luminosity at 13 TeV. The luminosity of a collider is a very important parameter because the precision obtained in measuring a given physics process’s production cross-section depends critically on the accuracy with which the luminosity is known. The luminosity is also the figure of merit used to benchmark the efficiency of the collider’s operation day by day.

Special beam optics and beam parameters are necessary to perform this task; both are tailored to get the smallest possible uncertainty in the measurement. The method is pretty simple and very old. Using a technique pioneered by Simon van der Meer in 1968 at CERN’s Intersecting Storage Rings, the inelastic proton-proton collision rate is monitored by dedicated luminosity detectors at the experiments as the beams are moved across each other, first in the horizontal and then in the vertical direction. This "VdM scan" provides a measurement of the beam-overlap area, which is proportional to the transverse beam size, the first ingredient needed to solve the luminosity equation. The second main ingredient is the simultaneous precision measurement of the bunch currents in the LHC, which is performed using different devices from the machine and the experiments. This information, combined with the total number of bunches per beam, provides a direct calibration of the experiment’s luminosity detectors at a single point in time.  

The first “VdM scan” fill, which lasted just over 9 hours, was devoted to the luminosity calibration of ALICE and then LHCb. The second fill, which lasted 7 hours, allowed the luminosity calibration of ATLAS. The luminosity calibration of CMS was completed last Friday.

With beam back from the PS last Thursday, the first step for the LHC was a couple re-qualification fills with low intensity. Following these fills, CMS’s luminosity calibration was swiftly completed, and the intensity ramp-up re-joined. At present, the LHC is working with 1752 bunches per beam which gave a storm interrupted weekend peak luminosity of 5.3 x 1033 cm-2s-1. The integrated luminosity for the year has now passed a hard-won inverse femtobarn.

The PS, POPS and the rotating machine

CERN’s PS has a big challenge powering the main magnets. The power applied to the magnets is ±40 MW, with a repetition rate of 2.4 seconds. The minus sign is important here. As these magnets are ramped down from top energy, the stored magnetic energy has to be handled somehow. The solution until 2011 was the famous rotating machine – a motor-generator set that stored the energy in a flywheel during the ramp-down and made it available again via the generator on the ramp-up. The modern solution is POPS – here large capacitor banks housed in dedicated containers provide the energy storage mechanism. It should be borne in mind that the PS executes around 15 million cycles per year!

On 27 April, POPS suffered a short circuit in one of its six capacitor containers. To bridge the repair, the PS switched swiftly to the back-up rotating machine and operated normally until 20 May when, unfortunately, a malfunctioning isolator switch rendered it inoperable for a couple of weeks. The power converter team, who were in the process of understanding, repairing and mitigating the POPS problem, have had to execute a crash programme of measures to get POPS safely back into operation as soon as possible. This was achieved on Thursday 26 May.