After both beams having been ramped to their full energy of 6.5 TeV, the last two weeks saw the beam commissioning process advancing on many fronts. An important milestone was achieved when operators succeeded in circulating a nominal-intensity bunch. During the operation, some sudden beam losses resulted in beam dumps at top energy, a problem that needed to be understood and resolved.
In 2015 the LHC will be circulating around 2800 bunches in each beam and each bunch will contain just over 1 x 1011 protons. Until a few days ago commissioning was taking place with single bunches of 5 x 109 protons. The first nominal bunch with an intensity of 1 x 1011 protons was injected on Tuesday, 21 April. In order to circulate such a high-intensity bunch safely, the whole protection system must be working correctly: collimators, which protect the aperture, are set at preliminary values known as coarse settings; all kicker magnets for injecting and extracting the beams are commissioned with beam and timed in properly; the aperture in critical regions is measured with beam and so on. The RF system is tuned carefully to allow loss-free capture of the injected nominal bunch. This was all done successfully and the road is now clear for determining the reference orbits for nominal intensity bunches. For the time being, operators cannot inject more than a couple of these nominal bunches, as further tests on the machine protection system are required first. High-intensity beams have sufficient energy to damage machine components if the machine protection systems are not working perfectly, hence the caution.
The other focus over the last two weeks was some unexpected beam losses for the anti-clockwise beam (beam 2) at a location in the arc between Point 1 and Point 8. The losses were similar to those caused by the UFOs (Unidentified Falling Objects) observed during LHC Run 1. At high energies (6.5 TeV), the losses were significant enough to quench the superconducting dipole located just downstream of the greatest losses. Reducing the level at which the beam-loss monitors trigger a beam dump by a factor of 2 avoided the dipole quenches in the case of these UFO-like events. However, at 6.5 TeV, beam 2 could not be circulated for much longer than 1 hour before being dumped. The LHC team decided to warm up the beam screen of the magnets in sector 8-1 to around 80 K and, once the whole system was back to normal operating conditions, it was possible to keep the beam circulating at full energy for more than 6 hours. However, subsequently an aperture restriction of some sort was measured at full energy and at injection energy, exactly at the location where the beam losses had originally appeared. Investigations are ongoing, with the aim of trying to understand this ‘moving target’. One of the next steps will be to go for multiple nominal intensity bunches and explore the behaviour of the restriction