LHC Report: a very productive hiatus

On Monday, 24 August, the LHC transitioned from nascent 25 ns operation to a two-week hiatus devoted to luminosity calibration (two days), machine development (five days) and a technical stop (five days).

No stopping for Sunday at the CERN Control Centre. (Image: Rogelio Tomás García)

Accurate calibration of the luminosity is vital input for many of an experiment’s measurements. The luminosity is calibrated using separation scans pioneered in 1968 by Simon van der Meer at the ISR. In these scans, carefully prepared beams are stepped across each other, horizontally and vertically, one plane at a time. Accurate measurements of the interaction rates, beam intensity, beam movement during the scan and other factors allow determination of the absolute luminosity. ATLAS, ALICE, CMS and LHCb all had dedicated time for Van der Meer scans. TOTEM and ALFA also took advantage of the special conditions to take data.

The LHC machine development (MD) period that followed consisted of a wide variety of measurement and development programmes ranging from exploring the limits of smaller beam sizes at the interaction point to collimation using bent crystals. Free from the challenges of high beam intensity, machine availability was high during this remarkably successful MD period. Highlights included the validation of a beta* of 40 cm, which effectively doubles the luminosity potential of the present set-up. Collimation experts and members of the UA9 collaboration were also able to carry out successful tests and demonstrations of the collimating potential of bent crystals.

The beams were dumped at 6 a.m. sharp on the morning of Monday, 31 August in preparation for the five-day technical stop, which has just finished. The key objectives of this technical stop were modifications to the critical Quench Protection System, the consolidation of the cooling and electrical distribution systems and important maintenance work on the cryogenics system. It involved a huge number of activities. The combined effort, coordinated by a team from EN/MEF, was very successful.

Following the technical stop, the first key step is revalidation of the LHC with beam from a machine-protection standpoint. Some dedicated system optimisation will be followed by the continuation of the intensity ramp-up with 25 ns beam and hopefully several weeks of sustained luminosity delivery with this bunch spacing.

by Mike Lamont for the LHC team