Over the weekend the LHC broke two records: a record number of 2,748 proton bunches were injected into the accelerator giving a record beam intensity of around 2.7 x 1014 protons in both beams. These beams have yet to face the challenge of "ramping" to high energy.
This was made possible by a new beam configuration: the design value of 25 nanosecond spacing between proton bunches replaced - for the first time – the typical 50 nanosecond spacing. This test run was done at 450 GeV with no collisions.
Up to now, the LHC has been running with around 1,380 bunches with 50 nanoseconds between bunches. By going to 25 nanoseconds, the LHC operations team can double the number of bunches to around 2,800.
One of the main limitations for this mode of operation is the electron cloud that is strongly enhanced by the reduced spacing among bunches. The electron cloud has nasty effects on the beam (beam size increase and losses), on cryogenics (heat load on the beam pipe) and on vacuum (pressure rise). A period of beam-pipe conditioning (“scrubbing”) is therefore required before ramping the beams. During this period, the machine is operated, in a controlled way, with beams of increasingly high intensity. This improves the beam pipe surface characteristics and reduces the density of the electron cloud.
Over the last few days, the LHC operations team has injected and progressively ramped the 25 nanosecond beams from 450 GeV to 4 TeV. These intermediate steps were needed to study the behaviour of the new beam configuration at high energy. A pilot physics run at 4 TeV will be performed to give the LHC experiments some experience of running with 25 nanoseconds between bunches before this configuration is used operationally in 2015.