First stable beams in the LHC were declared on 23 May, just 25 days after the first beam was injected and almost three weeks ahead of schedule. Since then, interleaved with physics operation and remaining commissioning activities, the LHC teams have been busy ramping up the intensity of the beams. During this procedure, the number of proton bunches circulating the machine is increased in a stepwise manner: beginning with three bunches per beam and going up to 12, 72, 300, 600, 900, 1200, 1800, 2400 and finally 2556 bunches per beam. To ensure that all systems work as they should, each step requires a minimum of 20 hours of stable-beam operation and that the machine is filled three times. As the Courier went to press on 28 June, 2556 bunches were circulating in the machine and already the experiments had clocked an integrated luminosity of around 5 fb–1.
Another important procedure during the LHC restart is the so-called scrubbing run to condition the vacuum chamber, which took place in early June. Despite the ultra-high vacuum of the LHC beam pipe, residual gas molecules and electrons remain trapped on the walls of the chamber and can be liberated by the circulating beam, eventually heating the walls and destabilising the beam. Such “electron-cloud” effects can be reduced by repeatedly filling the LHC with closely spaced bunches, provoking intense electron clouds that gradually become less prone to produce further electrons.
The rapid and smooth restart of the LHC this year, which marks the continuation of Run 2 at a centre-of-mass energy of 13 TeV, is due to the excellent availability of the machine and its injector chain, and also the dedication of many specialists. The LHC is now ready to continue the intensity ramp for physics-data collection, with the ambitious goal of reaching an integrated luminosity of 45 fb–1 for 2017.
