Celebrations, challenges and business as usual

6 November 2012

Champagne corks popped on 13 September as the LHC confirmed its potential as a multipurpose machine and successfully switched to a new running mode with proton–ion collisions. This achievement marked the first test with colliding beams in this mode, in preparation for the planned four-week proton–ion run in 2013.

Even though the LHC does not change magnetically, proton–ion operation is a challenge for the LHC RF system and its synchronization with the Super Proton Synchrotron. The proton and ion beams are injected and ramped with different RF frequencies; they then need to be re-phased and locked to provide a stable collision point. Despite a 36-hour break to repair a vacuum leak on one of the LHC wire scanners, the tests went well and the first 4 TeV proton–lead collisions were successfully recorded by the LHC experiments – an outstanding achievement for all of the teams involved (see Successful test of proton–ion collisions in LHCb).

A day later, the machine’s repertoire was extended further to collide “unsqueezed” proton beams at a β* of 1000 m (a measure of the envelope of the beam oscillations) at Points 1 and 5. This is to allow the ALFA and TOTEM experiments, co-located with ATLAS and CMS respectively, to probe proton–proton scattering at low angles (TOTEM extends study of elastic scattering). The tests were followed by a return to routine proton–proton collisions, with the integrated luminosity for the year passing 15 fb–1 in both ATLAS and CMS.

A five-day technical stop – the third this year – began on 17 September for scheduled maintenance and consolidation of systems, but with two out-of-the-ordinary interventions. These involved the replacement of the mirrors and supports of the beam synchrotron light monitors (BSRTs) and the replacement of one of the fast-pulsed kicker magnets used to inject the beam. The BSRTs had been put out of operation because of deformations caused by beam-induced heating. The injection magnets have also suffered from this heating, and waiting for them to cool down can delay the injection process by hours.

In total there are eight injection magnets in the machine. The “hottest” of these was replaced during the technical stop with a new version of the magnet with improved measures to reduce impedance. The LHC will gain some running time from this intervention, which will also allow the new design to be checked under operational conditions. The replacement of the injection magnet was carefully planned and executed successfully in four and a half days, requiring round-the-clock work from all of the teams involved.

It is always challenging to restart after a technical stop, with debugging, testing and requalification of all critical systems. A number of technical problems affected this recovery, which was further slowed down by the need to re-establish good vacuum conditions in the newly installed injection magnet. Once the so-called vacuum “scrubbing” was complete, the normal ramp-up in the number of bunches in the machine took place and nominal conditions were re-established on 30 September.

Despite the rocky restart, the LHC made a good recovery. On 6 October, an integrated luminosity of 286 pb–1 was delivered to the ATLAS and CMS experiments in the space of only 24 hours – a new record.

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