A month after restarting in February, the LHC was once again breaking records. Following a period of commissioning, the first run with stable beams for physics at 7 TeV in the centre-of-mass began on 13 March, with a modest three bunches per beam and a luminosity of 1.6 × 1030 cm–2s–1. Then, after further machine-protection tests, the way was opened for increasing numbers of bunches to be introduced in “fills” for physics, culminating with 200 bunches per beam on the evening of 22 March. This gave a peak luminosity at ATLAS and CMS of 2.5 × 1030 cm–2s–1, comfortably beating last year’s record made with 368 bunches. By 25 March, the LHC had delivered an integrated luminosity of 28 pb–1, more than half of the total delivered in 2010.
The next challenge was to have not only more bunches but also at a closer spacing; 2010 saw running with 368 bunches with 150 ns spacing, while the run with 200 bunches this March was with 75 ns spacing. However, combining small bunch spacing with a high number of bunches leads to an effect known as “electron cloud”: synchrotron radiation from the protons releases electrons at the beam-screen, which are pulled towards the protons and knock out more electrons on hitting the opposite wall.
After a brief technical stop for maintenance, the operating team began a period of “scrubbing runs”, in which a high beam current is injected at low energy to induce electron clouds under controlled conditions. The aim is to release gas molecules trapped inside the metal, to be pumped out later, and decrease the yield of electrons at the surface. These runs had already paid off by 10 April when the number of bunches per beam reached 1020, with a total of 1014 protons per beam – another record for the LHC.
