LHC Report: First stable beams at 6.5 TeV
Give or take some important loose ends, the morning of Wednesday, 3 June saw the nominal end of an intense eight weeks of beam commissioning and the delivery of the first stable beams at 6.5 TeV. Under the gaze of the media, 3 nominal bunches per beam were taken through the full cycle into collisions. This was followed by the declaration of stable beams, marking the start of Run 2 physics data-taking.
The scenes in the CCC as stable beams at an energy of 6.5 TeV marked the start of run 2 physics at the LHC.
The final stages of preparation involved the set-up of the tertiary collimators. These are situated on the incoming beam about 120 to 140 m from the interaction point where the beams are still in separate beam pipes. As the local orbit changes in this region during the squeeze and after the collapse of the separation bumps, the tertiary collimators set-up with respect to the beam is required both at the end of the squeeze and with colliding beams. The orbit and optics at the main collimator groupings in the cleaning sections of Points 7 and 3 are kept constant during the squeeze and in collision - hence the set-up and validation of these remains valid throughout all high-energy phases.
The full collimator set-up is validated at each stage with loss maps. Under controlled conditions, beam loss is deliberately provoked and the teams carefully check that the beam losses end up where they should, thereby validating the strict collimator hierarchy. The beam losses in the transverse plane are generated using the exquisite selectivity of the transverse damper system. For each beam, in each plane, a single bunch is targeted, and by injecting noise the transverse dampers are able to "blow-up" a single bunch, provoking the necessary losses.
The eight week long beam commissioning period has seen a sustained effort by the teams involved. The essential thrusts have been the commissioning of the key beam-related systems (RF, power converters, collimators, beam dumps, injection, magnets, vacuum, transverse feedback, machine protection, magnets, beam instrumentation, beam-based feedbacks, controls, databases, high-level software), characterisation and correction of the machine characteristics (magnetic, optics, aperture), and commissioning of the various phases of the operational cycle. This has translated into teams working nights, weekends and holidays to push the programme through. Activities have ranged from optics measurement and correction, injection and beam dump set-up, collimation, wrestling with the wide range of beam instrumentation, optimisation of the magnetic model, aperture measurement, etc., with operations tackling the intricacies of the ramp, squeeze, etc. All this has been backed by full validation of the various components of the machine protection system by the groups concerned.
The execution of the programme has also relied on good machine availability and the support of the injector complex, cryogenics, survey, technical infrastructure, access, radiation protection, and close and friendly collaboration with the LHC experiments.
First stable beams was an important first set-up, but there is still a long way to go before this year’s target of around 2500 bunches per beam is reached and the LHC starts delivering some serious integrated luminosity to the experiments.
