The latest from the LHC
The setting-up to prepare the LHC for running with nominal ramp rates and with bunch trains instead of single-bunch injection have continued over the last two weeks. The goal is to reduce the machine filling time and increase its luminosity. Running with bunch trains requires the careful setting-up of crossing angles in order to avoid unwanted collisions on either side of the experiments.
The technical target for 2010 in terms of bunch spacing in the trains is 150 ns, which corresponds to a physical distance of about 45 m. This target will be progressively reduced next year to eventually attain the nominal value of 25 ns, equivalent to a distance of about 7.5 m between two bunches in the trains.
Following initial setting-up last week, this week tests were made to check the size of the crossing angle needed to avoid parasitic collisions when 150 ns trains are injected into the machine, and to measure the aperture with the nominal crossing angle –at injection – of 170 micro radians. The tests showed that the minimum angle needed with 150 ns spacing was 100 micro radians but that even when setting the crossing angle to the nominal value of 170 micro radians, there was more aperture than predicted in the experimental insertions.
After performing the various tests, the commissioning team decided to stick with the 170 micro radian crossing angle and to complete the machine set-up with that value. During the beam energy ramp and squeeze phases, the crossing angle will be reduced to 100 micro radians for physics runs.
Running with crossing angles means that all the protection devices (injection, dump and triplet protection) have to be set up to match the new trajectories around the machine. This has taken most of this week.
What is the crossing angle?
Particle bunches at the LHC do not actually collide exactly ‘head-on’. Instead, operators introduce the crossing angle between two colliding bunches. This is done in order to avoid too many unwanted parasitic collisions and interactions among the bunches. However, the lower the angle, the larger the probability that particles will interact with each other, and the higher the luminosity. Therefore, operators set the machine in such a way as to ensure the best compromise between high luminosity and a low number of unwanted interactions. The crossing angle is varied by setting the appropriate parameters on the magnets located just before the collision points where the experiments are installed (also called ‘interaction points’ or ‘experimental insertions’).
What is the machine aperture?
aperture of a particle accelerator is the maximum initial bunch oscillation amplitude that guarantees stable particle motion over a given number of turns. Several parameters determine the appropriate aperture, which can also be defined as the actual space that the beam can move in without producing particle losses along the path. When operators set the crossing angle at the collision points, their room for manoeuvre is represented in practice by the maximum aperture of the machine.
by CERN Bulletin