LHC Report: machine commissioning - looking good

Since first beam was injected on Friday, 25 March, the recommissioning of the LHC has made good progress.

 

An event display showing the first collisions after the 2015 year-end technical stop as seen by the CMS experiment.

At present, work is being performed with either probe bunches (around 1010 protons per bunch) or single nominal bunches (1.1 x 1011 protons per bunch). This procedure is intended to ensure safe operation before full qualification of the machine protection set-up.

Beam has been taken up the ramp to 6.5 TeV and through the squeeze to the planned 2016 operating scenario. The optics (i.e. the global focusing properties of the whole ring) have been measured and corrected to an unprecedented level: there is now excellent agreement between the machine model and the actual LHC. Some preliminary measurements of the global aperture have been performed and there are no apparent bottlenecks. The position of the ULO (Unidentified Lying Object) had been probed and it is in a similar position to where it was measured at the end of last year.

One major activity is the set-up of the extensive collimation system for all phases of the machine cycle. A number innovative developments have greatly reduced the time required for this critical activity. These measures include a fast set-up based on the use of beam loss monitors, and the use of collimators with built-in beam position monitors. The collimators are now set-up at injection, at the top of the ramp and through the squeeze. A critical point is also the set-up and verification of the injection and beam dump system and the associated protection devices, but so far everything is going according to plan.

The various system specialists are also performing beam-based commissioning of the many beam instrumentation systems, the transverse damper system and the radio-frequency system.

First collisions of a small number of nominal bunches took place last Friday, 8 April. These collisions were not for the experiments but to establish the interaction points and open the way for collimator set-up under normal physics conditions.

The next couple of weeks will see the set-up of the collimation and protection devices finalised. This will then be qualified by a series of loss maps, in which beam loss is deliberately induced. Careful checks will be made that the losses have ended up where they should and that the cold aperture of the machine is fully protected. A number of other tests will fully qualify the machine protection system and set-up in order to proceed with the injection of high-intensity 25 ns beam and finally with the preparations for the scrubbing run.

by CERN Bulletin