LHC Report: a break from luminosity production

The LHC has been in great shape over the last few months, delivering over 20 fb-1 of integrated luminosity before the ICHEP conference in Chicago at the beginning of August. This is not much below the 25 fb-1 target for the whole of 2016. With this success in mind, a break in luminosity production was taken for six days, starting on 26 July 2016, for a machine development period.

This year, 20 days of the LHC schedule are devoted to machine development with the aim of carrying out detailed studies of the accelerator. The 20 days are divided over five different periods, called MD blocks. They can be seen as an investment in the future, so the machine can produce collisions more efficiently in the months and years to come. A detailed programme is worked out for each MD block, whereby different specialist teams are assigned periods of four to twelve hours, depending on the topic, to perform their previously approved tests. The MD program continues 24 hours per day, as in normal physics operation.

One way of increasing the collision rate is to change the size of the beam at the interaction points in the centre of the experiments by changing the quadrupole settings on either side of a given experiment. During the first MD block, a novel way of doing this, known as ATS optics, was explored. This approach means that ever smaller beam sizes can be obtained in the future. The more the size of the proton bunches that make up the beam is reduced, the higher the chance of collisions.

Another area being studied is beam instability, a familiar operational problem. When beam intensity is increased, or a change is made to the way the accelerator is filled, the operations team has to adjust different machine parameters to prevent the beams becoming unstable. When instabilities arise, they can cause beam loss, which is automatically detected and can cause a beam dump to avoid any damage to the LHC. The relationship between the angle at which the beams collide in the centre of the experiments and beam stability is also being studied. The smaller the crossing angle, the better the chances for collisions and so higher instantaneous luminosities.

Another aspect of the current tests concerns the optimisation of the process for injecting proton bunches. This concerns both the beam instabilities at injection and the blow-up of the beam during the injection process.

After six days of study, the LHC resumed normal operation on 1 August 2016. Following the end of the MD period, the LHC resumed normal luminosity production. The last couple of weeks have, however, been interrupted by some technical problems in both the LHC and the injectors. Of note in the LHC are issues with the injection kickers and a potential inter-turn short in one of the main bending magnets. Nonetheless, good performance is being delivered, with the total number of bunches per beam now at 2220 – a high for the year.

From Monday, 22 August another four days of machine development are on the programme, to study the longitudinal behaviour of the beam, gain a deeper insight into beam stability and explore different means of increasing the luminosity.

by Jan Uythoven for the LHC team