The weeks following the first collisions at 7 TeV in the centre of mass have seen the LHC pass important milestones in delivering higher instantaneous luminosity to the experiments. With days dedicated mainly to beam-commissioning studies and nights given over to the preparation and delivery of collisions for experiments, the progress is clear to see.
The weekend of 23–24 April saw not only a tenfold increase in instantaneous luminosity to above 1.1 × 1028 cm–2 s–1 in all four experiments but also a record physics fill, with the machine in “stable-beam” mode for 30 hours. This allowed the experiments to more than double the total number of events recorded at 7 TeV. The successful weekend had been preceded by work to commission the “squeeze” on the beams at an energy of 3.5 TeV per beam at all four interaction points. This process, one of the most complex stages in the operation of the accelerator, was followed by a number of collimation and beam-dump tests to ensure sufficient protection of the experiments. The first physics fill with squeezed-beam optics led to a factor of five improvement in luminosity. A new bunch scheme with three bunches per beam then provided a further improvement by another factor of two.
Sunday 2 May saw another major step towards higher intensities with the first fill with two bunches per beam at the nominal (design) intensity of 1 × 1011 protons per bunch, at the injection energy of 450 GeV per beam. Within an hour of injection the team had removed the “separation bumps”, which keep the beams separated during the ramp, at all four interaction points simultaneously, thus providing collisions. After some further adjustments the operators were ready to prepare a second fill, this time with collisions in stable beam conditions, for the first time with bunches at nominal intensity.
The following two weeks saw further steps in a two-pronged approach to deliver higher luminosity to the experiments, either with more bunches or more protons per bunch. With two bunches per beam providing a total of up to 4 × 1010 protons per beam, the LHC was already delivering an instantaneous luminosity of 2 × 1028 cm–2 s–1 in some long periods of stable running at 3.5 TeV per beam on the weekend of 8–9 May.
After further tests, on the beam-dump system and aspects of machine protection, for example, on 14 May, a physics fill began with squeezed beams and four bunches per beam, giving a total of 8 × 1010 protons per beam. The next day, the first test took place to ramp a beam at nominal intensity at 3.5 TeV, with 60% of the beam surviving the ramp. This was followed by a long fill of stable beams for nearly 24 hours, now with six bunches per beam. It was then time to try ramping again with one bunch at nominal intensity and this eventually succeeded for beam 2, with 1.2 × 1011 protons, in fact, 10% above nominal and without losses.
By the end of the long weekend of 14–16 May, the LHC had doubled the integrated luminosity previously delivered since the restart in March. Then, on the evening of 17 May, both beams were successfully ramped at nominal intensity, marking the passage of another milestone in the progress towards the final targets for the year.