LHC Report: Start of intensity ramp-up before a short breather

The first Stable Beams on 3 June were followed, to the accompaniment of thunderstorms, by the start of a phase known as the “intensity ramp-up” which saw the LHC team deliver physics with 50 bunches per beam. Time was also taken for a special five-day run devoted principally to the LHCf experiment. This week (15-19 June) the beam-based programme of the machine and its experiments was stopped temporarily for regular maintenance work.


LHCf’s Arm1 detector.

While the first stable colliding beams were delivered with only 3 nominal bunches per beam, the aim of last week’s operations was to start the process of increasing the number of bunches in the beam with an ultimate 2015 target of ~2400 bunches per beam. The number of bunches is gradually increased in well-defined steps. At each step – 3 bunches per beam, then 13, 40 and, finally, 50 – the machine protection team requests 3 fills and around 20 hours of Stable Beams to verify that all systems are behaving properly. During each fill, checks are made of instrumentation, feedback response, beam loss through the cycle, machine protection systems, RF, beam induced heating, orbit stability, etc. A check list is completed and signed off by the machine protection panel before authorisation is given for the next step with increased intensity. Following this pattern, the LHC reached 50 on 50 bunches by the weekend of 13-14 June. 

There was an extended hiatus in the intensity ramp-up during the week for a five-day special physics run devoted primarily to LHCf – the far forward experiment situated in the LHC around 140 m left and right of the ATLAS interaction point. Low luminosity and low pile-up conditions were required by LHCf and these were delivered at 6.5 TeV with a special de-squeezed optics with relatively large beam sizes at the interaction points of all experiments. The required data were successfully delivered to LHCf  in a series of fills with up to 39 bunches per beam. ATLAS, CMS, LHCb and ALICE all took advantage of the special conditions to take data themselves.

Monday 15 June saw the start of a five-day technical stop. This is the first of three technical stops scheduled during the 2015 operating period, before a longer stop planned during the end-of-year holidays. A normal year of LHC operation includes five-day technical stops every ten weeks or so to allow the machine and the experiments to carry out maintenance work and other interventions. Following the restart this weekend, a week or so will be devoted to a scrubbing run aimed at reducing electron clouds by conditioning the surface of the beam pipes around the ring. This run will prepare the way for a three-week period of operation with 50 ns bunch spacing and an associated intensity ramp-up to the order of 1000 bunches per beam.

by Mike Lamont for the LHC team