LHC Report: Preparing for a tighter squeeze

The LHC is resuming operation after a planned period of machine development followed by a technical stop. The beams returned last Friday, in the evening of 2 September, and preparations are now being made to squeeze the beams further at the collision points, aiming for new luminosity records.

To obtain as many collisions as possible in the heart of the experiments, the beams are squeezed to very small beam sizes. The beam squeezing parameter is known by experts as beta-star: the smaller the ß*, the stronger the squeezing. During the machine development period that started on 24 August, tests were made for the high-luminosity experiments ATLAS and CMS with ß* values of 1 m instead of the 1.5 m used previously. Unfortunately these tests were only partially successful, as some of the beam was lost during the squeezing process. It is thought that the beam losses were caused by the collimators, which were moved closer to the beam, and by the reduced crossing angle of the beams at the interaction points. During the same machine development period it was shown that the physical aperture of the machine is larger than assumed hitherto. Combining these two pieces of information, the decision was taken to see if ß* could be reduced to 1 m, while keeping the original collimator positions and crossing angles of the beams.

Before this could be tested a five-day technical stop took place for machine maintenance. A variety of work took place during this week, ranging from replacing an optical fibre used for the access system to upgrading software components. By 5-30 p.m. on Friday the machine was ready to resume operation, and by 10-50 p.m. the beams were circulating again. A comprehensive programme of tests was then started to verify the aperture with the smaller ß* at point 1 (ATLAS) and point 5 (CMS). In addition, on Sunday the polarity was reversed on the ALICE solenoid and dipole magnets. Although the polarity reversal itself only took about two hours, setting up the injection protection and the collimators around the ALICE experiment took up the rest of the day, as the beam trajectory is different for the different magnet polarities.

Further tests with low beam intensities are required before one can be absolutely sure that the machine is ready to again safely receive 100 megajoule beams. After these verifications the number of bunches will be increased in four steps from 264 bunches per beam to the 1380 used previously, hopefully this time with 50 % more luminosity for the ATLAS and CMS experiments.

by Jan Uythoven for the LHC Team